The hind-brain or rhombencephalon occupies the posterior
fossa of the cranial cavity and lies below a fold of dura mater, the
tentorium cerebelli. It consists of (a) the myelencephalon,
comprising the medulla oblongata and the lower part of the fourth
ventricle; (b) the metencephalon, consisting of the
pons, cerebellum, and the intermediate part of the fourth ventricle;
and (c) the isthmus rhombencephali, a constricted portion
immediately adjoining the mid-brain and including the superior peduncles
of the cerebellum, the anterior medullary velum, and the upper part
of the fourth ventricle.
The Medulla Oblongata (spinal bulb).—The medulla
oblongata extends from the lower margin of the pons to a plane passing
transversely below the pyramidal decussation and above the first pair
of cervical nerves; this plane corresponds with the upper border of
the atlas behind, and the middle of the odontoid process of the axis
in front; at this level the medulla oblongata is continuous with the
medulla spinalis. Its anterior surface is separated from the basilar
part of the occipital bone and the upper part of the odontoid process
by the membranes of the brain and the occipitoaxial ligaments. Its
posterior surface is received into the fossa between the hemispheres
of the cerebellum, and the upper portion of it forms the lower part
of the floor of the fourth ventricle.
  The medulla oblongata is pyramidal in shape, its broad
extremity being directed upward toward the pons, while its narrow,
lower end is continuous with the medulla spinalis. It measures about
3 cm. in length, about 2 cm. in breadth at its widest part, and about
1.25 cm. in thickness. The central canal of the medulla spinalis is
prolonged into its lower half, and then opens into the cavity of the
fourth ventricle; the medulla oblongata may therefore be divided into
a lower closed part containing the central canal, and an upper
open part corresponding with the lower portion of the fourth
ventricle.
  The Anterior Median Fissure (fissura mediana
anterior; ventral or ventromedian fissure
) contains a fold of
pia mater, and extends along the entire length of the medulla oblongata:
it ends at the lower border of the pons in a small triangular expansion,
termed the foramen cecum. Its lower part is interrupted by
bundles of fibers which cross obliquely from one side to the other,
and constitute the pyramidal decussation. Some fibers, termed
the anterior external arcuate fibers, emerge from the fissure
above this decussation and curve lateralward and upward over the surface
of the medulla oblongata to join the inferior peduncle.
  The Posterior Median Fissure (fissura mediana
posterior; dorsal or dorsomedian fissure
) is a narrow groove;
and exists only in the closed part of the medulla oblongata; it becomes
gradually shallower from below upward, and finally ends about the
middle of the medulla oblongata, where the central canal expands into
the cavity of the fourth ventricle.
  These two fissures divide the closed part of the medulla
oblongata into symmetrical halves, each presenting elongated eminences
which, on surface view, are continuous with the funiculi of the medulla
spinalis. In the open part the halves are separated by the anterior
median fissure, and by a median raphé which

extends from the bottom of the fissure to the floor of the fourth
ventricle. Further, certain of the cranial nerves pass through the
substance of the medulla oblongata, and are attached to its surface
in series with the roots of the spinal nerves; thus, the fibers of
the hypoglossal nerve represent the upward continuation of the anterior
nerve roots, and emerge in linear series from a furrow termed the
antero-lateral sulcus. Similarly, the accessory, vagus, and
glossopharyngeal nerves correspond with the posterior nerve roots,
and are attached to the bottom of a sulcus named the postero-lateral
sulcus.
Advantage is taken of this arrangement to subdivide each
half of the medulla oblongata into three districts, anterior, middle,
and posterior. Although these three districts appear to be
directly continuous with the corresponding funiculi of the medulla
spinalis, they do not necessarily contain the same fibers, since some
of the fasciculi of the medulla spinalis end in the medulla oblongata,
while others alter their course in passing through it.
  The anterior district (Fig. 90679) is named the pyramid (pyramis medullæ oblongatæ)
and lies between the anterior median fissure and the antero-lateral
sulcus. Its upper end is slightly constricted, and between
it and the pons the fibers of the abducent nerve emerge; a little
below the pons it becomes enlarged and prominent, and finally tapers
into the anterior funiculus of the medulla spinalis, with which, at
first sight, it appears to be directly continuous.


FIG. 90679– Medulla
oblongata and pons. Anterior surface.


FIG. 90680– Decussation
of pyramids. Scheme showing passage of various fasciculi from medulla
spinalis to medulla oblongata. a. Pons. b. Medulla
oblongata. c. Decussation of the pyramids. d. Section
of cervical part of medulla spinalis. 1. Anterior cerebrospinal
fasciculus (in red). 2. Lateral cerebrospinal fasciculus (in red).
3. Sensory tract (fasciculi gracilis et cuneatus) (in blue). 3’.
Gracile and cuneate nuclei. 4. Antero-lateral proper fasciculus
(in dotted line). 5. Pyramid. 6. Lemniscus. 7. Medial longitudinal
fasciculus. 8. Ventral spinocerebellar fasciculus (in blue). 9.
Dorsal spinocerebellar fasciculus (in yellow). (Testut.)
  The two pyramids contain the motor fibers which pass
from the brain to the medulla oblongata and medulla spinalis, corticobulbar
and corticospinal fibers. When these pyramidal fibers are traced downward
it is found that some two-thirds or more of them leave the pyramids
in successive bundles, and decussate in the anterior median fissure,
forming what is termed the pyramidal decussation. Having crossed
the middle line, they pass down in the posterior part of the lateral
funiculus as the lateral cerebrospinal fasciculus.

The remaining fibers—i. e., those which occupy the lateral
part of the pyramid—do not cross the middle line, but are carried
downward as the anterior cerebrospinal fasciculus (Fig. 90680) into the anterior funiculus of the same side.
  The greater part of the anterior proper fasciculus of
the medulla spinalis is continued upward through the medulla oblongata
under the name of the medial longitudinal fasciculus.


FIG. 90681– Hind-
and mid-brains; postero-lateral view.
  The lateral district (Fig. 90681) is limited in front by the antero-lateral sulcus and the
roots of the hypoglossal nerve, and behind by the postero-lateral
sulcus and the roots of the accessory, vagus, and glossopharyngeal
nerves. Its upper part consists of a prominent oval mass which is
named the olive, while its lower part is of the same width
as the lateral funiculus of the medulla spinalis, and appears on the
surface to be a direct continuation of it. As a matter of fact, only
a portion of the lateral funiculus is continued upward into this district,
for the lateral cerebrospinal fasciculus passes into the pyramid of
the opposite side, and the dorsal spinocerebellar fasciculus is carried
into the inferior peduncle in the posterior district. The ventral
spinocerebellar fasciculus is continued upward on the lateral surface
of the medulla oblongata in the same relative position it occupies
in the spinal cord until it passes under cover of the external arcuate
fibers. It passes beneath these fibers just dorsal to the olive and
ventral to the roots of the vagus and glossopharyngeal nerves; it
continues upward through the pons along the dorso-lateral edge of
the lateral lemniscus. The remainder of the lateral funiculus consists
chiefly of the lateral proper fasciculus. Most of these fibers dip
beneath the olive and disappear from the surface; but a small strand
remains superficial to the olive. In a depression at the upper end
of this strand is the acoustic nerve.


FIG. 90682– Superficial
dissection of brain-stem. Lateral view.


FIG. 90683– Dissection
of brain-stem. Lateral view.


FIG. 90684– Deep
dissection of brain-stem. Lateral view.


FIG. 90685– Deep
dissection of brain-stem. Lateral view.
  The olive (oliva; olivary body) is situated
lateral to the pyramid, from which it is separated by the antero-lateral
sulcus, and the fibers of the hypoglossal nerve. Behind, it is separated
from the postero-lateral sulcus by the ventral spinocerebellar fasciculus.
In the depression between the upper end of the olive and the pons
lies the acoustic nerve. It measures about 1.25 cm. in length, and
between its upper





end and the pons there is a slight depression to which the roots of
the facial nerve are attached. The external arcuate fibers wind across
the lower part of the pyramid and olive and enter the inferior peduncle.
  The posterior district (Fig. 90686) lies behind the postero-lateral sulcus and the roots of the
accessory, vagus, and the glossopharyngeal nerves, and, like the lateral
district, is divisible into a lower and an upper portion.


FIG. 90686– Upper
part of medulla spinalis and hind- and mid-brains; posterior aspect,
exposed in situ.
  The lower part is limited behind by the posterior
median fissure, and consists of the fasciculus gracilis and
the fasciculus cuneatus. The fasciculus gracilis is placed
parallel to and along the side of the posterior median fissure, and
separated from the fasciculus cuneatus by the postero-intermediate
sulcus and septum. The gracile and cuneate fasciculi are at first
vertical in direction; but at the lower part of the rhomboid fossa
they diverge from the middle line in a V-shaped manner, and each presents
an elongated swelling. That on the fasciculus gracilis is named the
clava, and is produced by a subjacent nucleus of gray matter,
the nucleus gracilis; that on the fasciculus cuneatus is termed
the cuneate tubercle, and is likewise caused by a gray nucleus,
named the nucleus cuneatus. The fibers of these fasciculi terminate
by arborizing around the cells in their respective nuclei. A third
elevation, produced by the substantia gelatinosa of Rolando, is present
in the lower part of the posterior district of the medulla oblongata.
It lies on the

lateral aspect of the fasciculus cuneatus, and is separated from the
surface of the medulla oblongata by a band of nerve fibers which form
the spinal tract (spinal root) of the trigeminal nerve. Narrow below,
this elevation gradually expands above, and ends, about 1.25 cm. below
the pons, in a tubercle, the tubercle of Rolando (tuber
cinereum
).
  The upper part of the posterior district of the
medulla oblongata is occupied by the inferior peduncle, a thick
rope-like strand situated between the lower part of the fourth ventricle
and the roots of the glossopharyngeal and vagus nerves. The inferior
peduncles connect the medulla spinalis and medulla oblongata with
the cerebellum, and are sometimes named the restiform bodies.
As they pass upward, they diverge from each other, and assist in forming
the lower part of the lateral boundaries of the fourth ventricle;
higher up, they are directed backward, each passing to the corresponding
cerebellar hemisphere. Near their entrance, into the cerebellum they
are crossed by several strands of fibers, which run to the median
sulcus of the rhomboid fossa, and are named the striæ medullares.
The inferior peduncle appears to be the upward continuation of the
fasciculus gracilis and fasciculus cuneatus; this, however, is not
so, as the fibers of these fasciculi end in the gracile and cuneate
nuclei. The constitution of the inferior peduncle will be subsequently
discussed.
  Caudal to the striæ medullares the inferior peduncle
is partly covered by the corpus pontobulbare (Essick 120),
a thin mass of cells and fibers extending from the pons between the
origin of the VII and VIII cranial nerves.
Internal Structure of the Medulla Oblongata.—Although
the external form of the medulla oblongata bears a certain resemblance
to that of the upper part of the medulla spinalis, its internal structure
differs widely from that of the latter, and this for the following
principal reasons: (1) certain fasciculi which extend from the medulla
spinalis to the brain, and vice versa, undergo a rearrangement
in their passage through the medulla oblongata; (2) others which exist
in the medulla spinalis end in the medulla oblongata; (3) new fasciculi
originate in the gray substance of the medulla oblongata and pass
to different parts of the brain; (4) the gray substance, which in
the medulla spinalis forms a continuous H-shaped column, becomes greatly
modified and subdivided in the medulla oblongata, where also new masses
of gray substance are added; (5) on account of the opening out of
the central canal of the medulla spinalis, certain parts of the gray
substance, which in the medulla spinalis were more or less centrally
situated, are displayed in the rhomboid fossa; (6) the medulla oblongata
is intimately associated with many of the cranial nerves, some arising
form, and others ending in, nuclei within its substance.
The Cerebrospinal Fasciculi.—The downward course of
these fasciculi from the pyramids of the medulla oblongata and their
partial decussation have already been described (page 761). In crossing
to reach the lateral funiculus of the opposite side, the fibers of
the lateral cerebrospinal fasciculi extend backward through the anterior
columns, and separate the head of each of these columns from its base
(Figs. 90687, 90688).
The base retains its position in relation to the ventral aspect of
the central canal, and, when the latter opens into the fourth ventricle,
appears in the rhomboid fossa close to the middle line, where it forms
the nuclei of the hypoglossal and abducent nerves; while above the
level of the ventricle it exists as the nuclei of the trochlear and
oculomotor nerves in relation to the floor of the cerebral aqueduct.
The head of the column is pushed lateralward and forms the nucleus
ambiguus,
which gives origin from below upward to the cranial
part of the accessory and the motor fibers of the vagus and glossopharyngeal,
and still higher to the motor fibers of the facial and trigeminal
nerves.
  The fasciculus gracilis and fasciculus cuneatus
constitute the posterior sensory fasciculi of the medulla spinalis;
they are prolonged upward into the lower part of the medulla oblongata,
where they end respectively in the nucleus gracilis and nucleus cuneatus.
These two nuclei are continuous with the central gray substance

of the medulla spinalis, and may be regarded as dorsal projections
of this, each being covered superficially by the fibers of the corresponding
fasciculus. On transverse section (Fig. 90694)
the nucleus gracilis appears as a single, more or less quadrangular
mass, while the nucleus cuneatus consists of two parts: a larger,
somewhat triangular, medial nucleus, composed of small or medium-sized
cells, and a smaller lateral nucleus containing large cells.


FIG. 90687– Section
of the medulla oblongata through the lower part of the decussation
of the pyramids. (Testut.) 1. Anterior median fissure. 2. Posterior
median sulcus. 3. Anterior column (in red), with 3’, anterior
root. 4. Posterior column (in blue), with 4’, posterior roots.
5. Lateral cerebrospinal fasciculus. 6. Posterior funiculus. The
red arrow, a, a’, indicates the course the lateral cerebrospinal
fasciculus takes at the level of the decussation of the pyramids;
the blue arrow, b, b’, indicates the course which the
sensory fibers take.


FIG. 90688– Section
of the medulla oblongata at the level of the decussation of the
pyramids. (Testut.) 1. Anterior median fissure. 2. Posterior median
sulcus. 3. Motor roots. 4. Sensory roots. 5. Base of the anterior
column, from which the head (5’) has been detached by the lateral
cerebrospinal fasciculus. 6. Decussation of the lateral cerebrospinal
fasciculus. 7. Posterior columns (in blue). 8. Gracile nucleus.


FIG. 90689– Superficial
dissection of brain-stem. Ventral view.


FIG. 90690– Deep
dissection of brain-stem. Ventral view.
  The fibers of the fasciculus gracilis and fasciculus
cuneatus end by arborizing around the cells of these nuclei (Fig. 90692). From the cells of the nuclei new fibers arise; some of these
are continued as the posterior external arcuate fibers into the inferior
peduncle, and through it to the cerebellum, but most of them pass
forward through the neck of the posterior column, thus cutting off
its head from its base (Fig. 90693). Curving
forward, they decussate in the middle line with the corresponding
fibers of the opposite side, and run upward immediately behind the
cerebrospinal fibers, as a flattened band, named the lemniscus
or fillet. The decussation of these sensory fibers is situated
above that of the motor fibers, and is named the decussation of
the lemniscus
or sensory decussation. The lemniscus
is joined by the spinothalamic fasciculus (page 792), the fibers of
which are derived from the cells of the gray substance of the opposite
side of the medulla spinalis.
  The base of the posterior column at first lies on the
dorsal aspect of the central canal, but when the latter opens into
the fourth ventricle, it appears in the lateral part of the rhomboid
fossa. It forms the terminal nuclei of the sensory fibers of

the vagus and glossopharyngeal nerves, and is associated with the
vestibular part of the acoustic nerve and the sensory root of the
facial nerve. Still higher, it forms a mass of pigmented cells—the
locus cæruleus—in which some of the sensory fibers
of the trigeminal nerve appear to end. The head of the posterior column
forms a long nucleus, in which the fibers of the spinal tract of the
trigeminal nerve largely end.


FIG. 90691– Dissection
of brain-stem. Dorsal view. The nuclear masses of the medulla are
taken from model by Weed, Carnegie Publication, No. 19.
  The dorsal spinocerebellar fasciculus (fasciculus
cerebellospinalis; direct cerebellar tract
) leaves the lateral
district of the medulla oblongata; most of its fibers are carried
backward into the inferior peduncle of the same side, and through
it are conveyed to the cerebellum; but some run upward with the fibers
of the lemniscus, and, reaching the inferior colliculus, undergo decussation,
and are carried to the cerebellum through the superior peduncle.
  The proper fasciculi (basis bundles) of
the anterior and lateral funiculi largely consist of intersegmental
fibers, which link together the different segments of the medulla
spinalis; they assist in the production of the formatio reticularis
of the medulla oblongata, and many of them are accumulated into a
fasciculus which runs up close to the median raphé between the
lemniscus and the rhomboid fossa;

this strand is named the medial longitudinal fasciculus, and
will be again referred to.
Gray Substance of the Medulla Oblongata (Figs.
90694, 90695).
—In addition to
the gracile and cuneate nuclei, there are several other nuclei to
be considered. Some of these are traceable from the gray substance
of the medulla spinalis, while others are unrepresented in it.
  1. The hypoglossal nucleus is derived from the
base of the anterior column; in the lower closed part of the medulla
oblongata it is situated on the ventrolateral aspect of the central
canal; but in the upper part it approaches the rhomboid fossa, where
it lies close to the middle line, under an eminence named the trigonum
hypoglossi
(Fig. 90709). Numerous fibers
connect the two nuclei, both nuclei send long dendrons across the
midline to the opposite nucleus; commissure fibers also connect them.
The nucleus measures about 2 cm. in length, and consists of large
multipolar nerve cells, similar to those in the anterior column of
the spinal cord, whose axons constitute the roots of the hypoglossal
nerve. These nerve roots leave the ventral side of the nucleus, pass
forward between the white reticular formation and the gray reticular
formation, some between the inferior olivary nucleus and the medial
accessory olivary nucleus, and emerge from the antero-lateral sulcus.


FIG. 90692– Superior
terminations of the posterior fasciculi of the medulla spinalis.
1. Posterior median sulcus. 2. Fasciculus gracilis. 3. Fasciculus
cuneatus. 4. Gracile nucleus. 5. Cuneate nucleus. 6, 6’, 6’’.
Sensory fibers forming the lemniscus. 7. Sensory decussation. 8.
Cerebellar fibers uncrossed (in black). 9. Cerebellar fibers crossed
(in black). (Testut.)


FIG. 90693– Transverse
section passing through the sensory decussation. (Schematic.) 1.
Anterior median fissure. 2. Posterior median sulcus. 3, 3. Head
and base of anterior column (in red). 4. Hypoglossal nerve. 5. Bases
of posterior columns. 6. Gracile nucleus. 7. Cuneate nucleus. 8,
8. Lemniscus. 9. Sensory decussation. 10. Cerebrospinal fasciculus.
(Testut.)
  2. The nucleus ambiguus (Figs.
90696, 90697), the somatic motor nucleus
of the glossopharyngeal, vagus and cranial portion of the accessory
nerves, is the continuation into the medulla oblongata of the dorso-lateral
cell group of the anterior column of the spinal cord. Its large multipolar
cells are like those in the anterior column of the cord; they form
a slender column in the deep part of the formatio reticularis grisea
about midway between the dorsal accessory olive and the nucleus of
the spinal tract of the trigeminal. It extends from the level of the
decussation of the median fillet to the upper end of the medulla oblongata.
Its fibers first pass backward toward the floor of the fourth ventricle
and then curve rather abruptly lateralward and ventrally to join the
fibers from the dorsal nucleus.
  3. The dorsal nucleus (Figs.
90696, 90698), nucleus ala cinerea, often
called the sensory nucleus or the terminal nucleus of the sensory
fibers of the glossopharyngeal and vagus nerves, is probably a mixed
nucleus and contains not only the terminations of the sympathetic
afferent or sensory fibers and the cells connected with them but contains
also cells which give rise to sympathetic efferent or preganglionic
fibers.

These preganglionic fibers terminate in sympathetic ganglia from which
the impulses are carried by other neurons. The cells of the dorsal
nucleus are spindle-shaped, like those of the posterior column of
the spinal cord, and the nucleus is usually considered as representing
the base of the posterior column. It measures about 2 cm. in length,
and in the lower, closed part of the medulla oblongata is situated
behind the hypoglossal nucleus; whereas in the upper, open part it
lies lateral to that nucleus, and corresponds to an eminence, named
the ala cinerea (trigonum vagi), in the rhomboid fossa.


FIG. 90694– Section
of the medulla oblongata at about the middle of the olive. (Schwalbe.)


FIG. 90695– Transverse
section of medulla oblongata below the middle of the olive.
  4. The nuclei of the cochlear and vestibular nerves
are described on page 788.


FIG. 90696– The
cranial nerve nuclei schematically represented; dorsal view. Motor
nuclei in red; sensory in blue. (The olfactory and optic centers
are not represented.)
  5. The olivary nuclei (Fig. 90694) are three in number on either side of the middle line, viz.,
the inferior olivary nucleus, and the medial and dorsal accessory
olivary nuclei; they consist of small, round, yellowish cells and
numerous fine nerve fibers. (a) The inferior olivary nucleus
is the largest, and is situated within the olive. It consists of a
gray folded lamina arranged in the form of an incomplete capsule,
opening medially by an aperture called the hilum emerging from
the hilum are numerous fibers which collectively constitute the peduncle
of the olive.
The axons, olivocerebellar fibers, which
leave the olivary nucleus pass out through the hilum and decussate
with those from the opposite olive in the raphé, then as internal
arcuate fibers they pass partly through and partly around the opposite
olive and enter the inferior peduncle to be distributed to the cerebellar
hemisphere of the opposite side from which they arise. The fibers
are smaller than the internal arcuate fibers connected with the median
lemniscus. Fibers passing in the opposite direction from the cerebellum
to the olivary nucleus are often described but their existence is
doubtful. Much uncertainty also exists in regard to the connections
of the olive and the spinal cord. Important connections between the
cerebrum and the olive of the same side exist but the exact pathway
is unknown. Many collaterals from the reticular formation and from
the pyramids enter the inferior olivary nucleus. Removal of one cerebellar
hemisphere is followed by atrophy of the opposite olivary nucleus.
(b) The medial accessory olivary nucleus lies between
the inferior olivary nucleus and the pyramid, and forms a curved lamina,
the concavity of which is directed laterally. The fibers of the hypoglossal
nerve, as they

traverse the medulla, pass between the medial accessory and the inferior
olivary nuclei. (c) The dorsal accessory olivary nucleus
is the smallest, and appears on transverse section as a curved lamina
behind the inferior olivary nucleus.
  6. The nucleus arcuatus is described below with
the anterior external arcuate fibers.
Inferior Peduncle (restiform body).—The position
of the inferior peduncles has already been described (page 775). Each
comprises:
  (1) Fibers from the dorsal spinocerebellar fasciculus,
which ascends from the lateral funiculus of the medulla spinalis.
  (2) The olivocerebellar fibers from the opposite
olivary nucleus.
  (3) Internal arcuate fibers from the gracile
and cuneate nuclei of the opposite side; these fibers form the deeper
and larger part of the inferior peduncle.


FIG. 90697– Nuclei
of origin of cranial motor nerves schematically represented; lateral
view.
  (4) The anterior external arcuate fibers vary
as to their prominence in different cases: in some they form an almost
continuous layer covering the pyramid and olive, while in others they
are barely visible on the surface. They arise from the cells of the
gracile and cuneate nuclei, and passing forward through the formatio
reticularis, decussate in the middle line. Most of them reach the
surface by way of the anterior median fissure, and arch backward over
the pyramid. Reinforced by others which emerge between the pyramid
and olive, they pass backward over the olive and lateral district
of the medulla oblongata, and enter the inferior peduncle. They thus
connect the cerebellum with the gracile and cuneate nuclei of the
opposite side. As the fibers arch across the pyramid, they enclose
a small nucleus which lies in front of and medial to the pyramid.
This is named the nucleus arcuatus, and is serially continuous
above with the nuclei pontis in the pons; it contains small fusiform
cells, around which some of the arcuate fibers end, and from which
others arise.


FIG. 90698– Primary
terminal nuclei of the afferent (sensory) cranial nerves schematically
represented; lateral view. The olfactory and optic centers are not
represented.


FIG. 90699– Diagram
showing the course of the arcuate fibers. (Testut.) 1. Medulla oblongata
anterior surface. 2. Anterior median fissure. 3. Fourth ventricle.
4. Inferior olivary nucleus, with the accessory olivary nuclei.
5. Gracile nucleus. 6. Cuneate nucleus. 7. Trigeminal. 8. Inferior
peduncles, seen from in front. 9. Posterior external arcuate fibers.
10. Anterior external arcuate fibers. 11. Internal arcuate fibers.
12. Peduncle of inferior olivary nucleus. 13. Nucleus arcuatus.
14. Vagus. 15. Hypoglossal.
  (5) The posterior external arcuate fibers also
take origin in the gracile and cuneate nuclei; they pass to the inferior
peduncle of the same side. It is uncertain whether

fibers are continued directly from the gracile and cuneate fasciculi
into the inferior peduncle.
  (6) Fibers from the terminal sensory nuclei of
the cranial nerves, especially the vestibular. Some of the fibers
of the vestibular nerve are thought to continue directly into the
cerebellum.
  (7) Fibers from the ventral spinocerebellar fasciculus.  (8) The existence of fibers from the cerebellum
(cerebellobulbar, cerebelloölivary, and cerebellospinal) to the
medulla and spinal cord is very uncertain.


FIG. 90700– The
formatio reticularis of the medulla oblongata, shown by a transverse
section passing through the middle of the olive. (Testut.) 1. Anterior
median fissure. 2. Fourth ventricle. 3. Formatio reticularis, with
3’, its internal part (reticularis alba), and 3’’,
its external part (reticularis grisea). 4. Raphé. 5. Pyramid.
6. Lemniscus. 7. Inferior olivary nucleus with the two accessory
olivary nuclei. 8. Hypoglossal nerve, with 8’, its nucleus
of origin. 9. Vagus nerve, with 9’, its nucleus of termination.
10. Lateral dorsal acoustic nucleus. 11. Nucleus ambiguus (nucleus
of origin of motor fibers of glossopharyngeal, vagus, and cerebral
portion of spinal accessory). 12. Gracile nucleus. 13. Cuneate nucleus.
14. Head of posterior column, with 14’, the lower sensory root
of trigeminal nerve. 15. Fasciculus solitarius. 16. Anterior external
arcuate fibers, with 16’, the nucleus arcuatus. 17. Nucleus
lateralis 18. Nucleus of fasciculus teres. 19. Ligula.
Formatio Reticularis (Fig. 90700).—This
term is applied to the coarse reticulum which occupies the anterior
and lateral districts of the medulla oblongata. It is situated behind
the pyramid and olive, extending laterally as far as the inferior
peduncles, and dorsally to within a short distance of the rhomboid
fossa. The reticulum is caused by the intersection of bundles of fibers
running at right angles to each other, some being longitudinal, others
more or less transverse in direction. The formatio reticularis presents
a different appearance in the anterior district from what it does
in the lateral; in the former, there is an almost entire absence of
nerve cells, and hence this part is known as the reticularis alba;
whereas in the lateral district nerve cells are numerous, and as a
consequence it presents a gray appearance, and is termed the reticularis
grisea.
  In the substance of the formatio reticularis are two
small nuclei of gray matter: one, the inferior central nucleus
(nucleus of Roller), near the dorsal aspect of the hilus of
the inferior olivary nucleus; the other, the nucleus lateralis,
between the olive and the spinal tract of the trigeminal nerve.
  In the reticularis alba the longitudinal fibers form
two well-defined fasciculi, viz.: (1) the lemniscus, which
lies close to the raphé, immediately behind the fibers of the
pyramid; and (2) the medial longitudinal fasciculus, which
is continued upward from the anterior and lateral proper fasciculi
of the medulla spinalis, and,

in the upper part of the medulla oblongata, lies between the lemniscus
and the gray substance of the rhomboid fossa. The longitudinal fibers
in the reticularis grisea are derived from the lateral funiculus of
the medulla spinalis after the lateral cerebrospinal fasciculus has
passed over to the opposite side, and the dorsal spinocerebellar fasciculus
has entered the inferior peduncle. They form indeterminate fibers,
with the exception of a bundle named the fasciculus solitarius,
which is made up of descending fibers of the vagus and glossopharyngeal
nerves. The transverse fibers of the formatio reticularis are the
arcuate fibers already described (page 782).
The Pons (pons Varoli).—The pons or forepart
of the hind-brain is situated in front of the cerebellum. From its
superior surface the cerebral peduncles emerge, one on either side
of the middle line. Curving around each peduncle, close to the upper
surface of the pons, a thin white band, the tænia pontis,
is frequently seen; it enters the cerebellum between the middle and
superior peduncles. Behind and below, the pons is continuous with
the medulla oblongata, but is separated from it in front by a furrow
in which the abducent, facial, and acoustic nerves appear.
  Its ventral or anterior surface (pars
basilaris pontis
) is very prominent, markedly convex from side
to side, less so from above downward. It consists of transverse fibers
arched like a bridge across the middle line, and gathered on either
side into a compact mass which forms the middle peduncle. It
rests upon the clivus of the sphenoidal bone, and is limited above
and below by well-defined borders. In the middle line is the sulcus
basilaris
for the lodgement of the basilar artery; this sulcus
is bounded on either side by an eminence caused by the descent of
the cerebrospinal fibers through the substance of the pons. Outside
these eminences, near the upper border of the pons, the trigeminal
nerves make their exit, each consisting of a smaller, medial, motor
root, and a larger, lateral, sensory root; vertical lines drawn immediately
beyond the trigeminal nerves, may be taken as the boundaries between
the ventral surface of the pons and the middle cerebellar peduncle.
  Its dorsal or posterior surface (pars
dorsalis pontis
), triangular in shape, is hidden by the cerebellum,
and is bounded laterally by the superior peduncle; it forms the upper
part of the rhomboid fossa, with which it will be described.
Structure (Fig. 90701).—Transverse
sections of the pons show it to be composed of two parts which differ
in appearance and structure: thus, the basilar or ventral portion
consists for the most part of fibers arranged in transverse and longitudinal
bundles, together with a small amount of gray substance; while the
dorsal tegmental portion is a continuation of the reticular formation
of the medulla oblongata, and most of its constituents are continued
into the tegmenta of the cerebral peduncles.
  The basilar part of the pons consists of—(a)
superficial and deep transverse fibers, (b) longitudinal fasciculi,
and (c) some small nuclei of gray substance, termed the nuclei
pontis which give rise to the transverse fibers.
  The superficial transverse fibers (fibræ
pontis superficiales
) constitute a rather thick layer on the ventral
surface of the pons, and are collected into a large rounded bundle
on either side of the middle line. This bundle, with the addition
of some transverse fibers from the deeper part of the pons, forms
the greater part of the brachium pontis.
  The deep transverse fibers (fibræ pontis
profundæ
) partly intersect and partly lie on the dorsal aspect
of the cerebrospinal fibers. They course to the lateral border of
the pons, and form part of the middle peduncle; the further connections
of this brachium will be discussed with the anatomy of the cerebellum.
  The longitudinal fasciculi (fasciculi longitudinales)
are derived from the cerebral peduncles, and enter the upper surface
of the pons. They stream downward on either side of the middle line
in larger or smaller bundles, separated from each other by the deep
transverse fibers; these longitudinal bundles cause a forward

projection of the superficial transverse fibers, and thus give rise
to the eminences on the anterior surface. Some of these fibers end
in, or give off collateral to, the nuclei pontis. An important pathway
is thus formed between the cerebral cortex and the cerebellum, the
first neuron having its cell body in the cortex and sending its axon
through the internal capsule and cerebral peduncle to form synapses
either by terminals or collaterals with cell bodies situated in the
nuclei pontis. Axons from these cells form the transverse fibers which
pass through the middle peduncle into the cerebellum. Others after
decussating, terminate either directly or indirectly in the motor
nuclei of the trigeminal, abducent, facial, and hypoglossal nerves;
but most of them are carried through the pons, and at its lower surface
are collected into the pyramids of the medulla. The fibers which end
in the motor nuclei of the cranial nerves are derived from the cells
of the cerebral cortex, and bear the same relation to the motor cells
of the cranial nerves that the cerebrospinal fibers bear to the motor
cells in the anterior column of the medulla spinalis. Probably none
of the collaterals or terminals of the cerebrospinal and cerebrobulbar
fibers end directly in the motor nuclei of the spinal and cranial
nerves, one or more association neurons are probably interpolated
in the pathway.


FIG. 90701– Coronal
section of the pons, at its upper part.
  The nuclei pontis are serially continuous with
the arcuate nuclei in the medulla, and consist of small groups of
multipolar nerve cells which are scattered between the bundles of
transverse fibers.
  The dorsal or tegmental part of the pons
is chiefly composed of an upward continuation of the reticular formation
and gray substance of the medulla oblongata. It consists of transverse
and longitudinal fibers and also contains important gray nuclei, and
is subdivided by a median raphé, which, however, does not extend
into the basilar part, being obliterated by the transverse fibers.
The transverse fibers in the lower part of the pons are collected
into a distinct strand, named the

trapezoid body. This consists of fibers which arise from the
cells of the cochlear nucleus, and will be referred to in connection
with the cochlear division of the acoustic nerve. In the substance
of the trapezoid body is a collection of nerve cells, which constitutes
the trapezoid nucleus. The longitudinal fibers, which are continuous
with those of the medulla oblongata, are mostly collected into two
fasciculi on either side. One of these lies between the trapezoid
body and the reticular formation, and forms the upward prolongation
of the lemniscus; the second is situated near the floor of the fourth
ventricle, and is the medial longitudinal fasciculus. Other
longitudinal fibers, more diffusely distributed, arise from the cells
of the gray substance of the pons.
  The rest of the dorsal part of the pons is a continuation
upward of the formatio reticularis of the medulla oblongata, and,
like it, presents the appearance of a network, in the meshes of which
are numerous nerve cells. Besides these scattered nerve cells, there
are some larger masses of gray substance, viz., the superior olivary
nucleus and the nuclei of the trigeminal, abducent, facial, and acoustic
nerves (Fig. 90696).
  1. The superior olivary nucleus (nucleus olivaris
superior
) is a small mass of gray substance situated on the dorsal
surface of the lateral part of the trapezoid body. Rudimentary in
man, but well developed in certain animals, it exhibits the same structure
as the inferior olivary nucleus, and is situated immediately above
it. Some of the fibers of the trapezoid body end by arborizing around
the cells of this nucleus, while others arise from these cells.
  2. The nuclei of the trigeminal nerve (nuclei
n. trigemini
) in the pons are two in number: a motor and a sensory.
The motor nucleus is situated in the upper part of the pons,
close to its posterior surface and along the line of the lateral margin
of the fourth ventricle. It is serially homologous with the nucleus
ambiguus and the dorso-lateral cell group of the anterior column of
the spinal cord. The axis-cylinder processes of its cells form the
motor root of the trigeminal nerve. The mesencephalic root
arises from the gray substance of the floor of the cerebral aqueduct,
joins the motor root and probably conveys fibers of muscle sense from
the temporal, masseter and pterygoid muscles. It is not altogether
clear whether the mesencephalic root is motor or sensory. The sensory
nucleus
is lateral to the motor one, and beneath the superior
peduncle. Some of the sensory fibers of the trigeminal nerve end in
this nucleus; but the greater number descend, under the name of the
spinal tract of the trigeminal nerve, to end in the substantia gelatinosa
of Rolando. The roots, motor and sensory, of the trigeminal nerve
pass through the substance of the pons and emerge near the upper margin
of its anterior surface.
  3. The nucleus of the abducent nerve (nucleus
n. abducentis
) is a circular mass of gray substance situated close
to the floor of the fourth ventricle, above the striæ medullares
and subjacent to the medial eminence: it lies a little lateral to
the ascending part of the facial nerve. The fibers of the abducent
nerve pass forward through the entire thickness of the pons on the
medial side of the superior olivary nucleus, and between the lateral
fasciculi of the cerebrospinal fibers, and emerge in the furrow between
the lower border of the pons and the pyramid of the medulla oblongata.
  4. The nucleus of the facial nerve (nucleus
n. fascialis
) is situated deeply in the reticular formation of
the pons, on the dorsal aspect of the superior olivary nucleus, and
the roots of the nerve derived from it pursue a remarkably tortuous
course in the substance of the pons. At first they pass backward and
medialward until they reach the rhomboid fossa, close to the median
sulcus, where they are collected into a round bundle; this passes
upward and forward, producing an elevation, the colliculus facialis,
in the rhomboid fossa, and then takes a sharp bend, and arches lateralward
through the substance of the pons to emerge at its lower border in
the interval between the olive and the inferior peduncle of the medulla
oblongata.
  5. The nucleus of the cochlear nerve consists
of: (a) the lateral cochlear nucleus, corresponding
to the tuberculum acusticum on the dorso-lateral surface of the inferior
peduncle; and (b) the ventral or accessory cochlear
nucleus,
placed between the two divisions of the nerve, on the
ventral aspect of the inferior peduncle.
  The nuclei of the vestibular nerve. (a)
The medial (dorsal or chief vestibular nucleus), corresponding
to the lower part of the area acustica in the rhomboid fossa; the
caudal end of this nucleus is sometimes termed the descending
or spinal vestibular nucleus. (b) The lateral
or nucleus of Deiters, consisting of large cells and situated
in the lateral angle of the rhomboid fossa; the dorso-lateral part
of this nucleus is sometimes termed the nucleus of Bechterew.
  The fibers of the vestibular nerve enter the medulla
oblongata on the medial side of those of the cochlear, and pass between
the inferior peduncle and the spinal tract of the trigeminal. They
then divide into ascending and descending fibers. The latter end by
arborizing around the cells of the medial nucleus, which is
situated in the area acustica of the rhomboid fossa. The ascending
fibers either end in the same manner or in the lateral nucleus,
which is situated lateral to the area acustica and farther from the
ventricular floor. Some of the axons of the cells of the lateral nucleus,
and possibly also of the medial nucleus, are continued upward through
the inferior peduncle to the roof nuclei of the opposite side of the
cerebellum, to which also other fibers of the vestibular root are
prolonged without interruption in the nuclei of the medulla oblongata.
A second set of fibers from the medial and lateral nuclei end partly
in the tegmentum, while the remainder ascend in the medial longitudinal
fasciculus to arborize around the cells of the nuclei of the oculomotor
nerve.
The Cerebellum.—The cerebellum constitutes the largest
part of the hindbrain. It lies behind the pons and medulla oblongata;
between its central portion and these structures is the cavity of
the fourth ventricle. It rests on the inferior occipital fossæ,
while above it is the tentorium cerebelli, a fold of dura mater which
separates it from the tentorial surface of the cerebrum. It is somewhat
oval in form, but constricted medially and flattened from above downward,
its greatest diameter being from side. Its surface is not convoluted
like that of the cerebrum, but is traversed by numerous curved furrows
or sulci, which vary in depth at different parts, and separate the
laminæ of which it is composed. Its average weight in the male
is about 150 gms. In the adult the proportion between the cerebellum
and cerebrum is about 1 to 8, in the infant about 1 to 20.
Lobes of the Cerebellum.—The cerebellum consists of
three parts, a median and two lateral, which are continuous with each
other, and are substantially the same in structure. The median portion
is constricted, and is called the vermis, from its annulated
appearance which it owes to the transverse ridges and furrows upon
it; the lateral expanded portions are named the hemispheres.
On the upper surface of the cerebellum the vermis is elevated above
the level of the hemispheres, but on the under surface it is sunk
almost out of sight in the bottom of a deep depression between them;
this depression is called the vallecula cerebelli, and lodges
the posterior part of the medulla oblongata. The part of the vermis
on the upper surface of the cerebellum is named the superior vermis;
that on the lower surface, the inferior vermis. The hemispheres
are separated below and behind by a deep notch, the posterior cerebellar
notch,
and in front by a broader shallower notch, the anterior
cerebellar notch.
The anterior notch lies close to the pons and
upper part of the medullas, and its superior edge encircles the inferior
colliculi and the superior cerebellar peduncle. The posterior notch
contains the upper part of the falx cerebelli, a fold of dura mater.
  The cerebellum is characterized by a laminated or foliated
appearance; it is marked by deep, somewhat curved fissures, which
extend for a considerable distance

into its substance, and divide it into a series of layers or leaves.
The largest and deepest fissure is named the horizontal sulcus.
It commences in front of the pons, and passes horizontally around
the free margin of the hemisphere to the middle line behind, and divides
the cerebellum into an upper and a lower portion. Several secondary
but deep fissures separate the cerebellum into lobes, and these are
further subdivided by shallower sulci, which separate the individual
folia or laminæ from each other. Sections across the laminæ
show that the folia, though differing in appearance from the convolutions
of the cerebrum, are analogous to them, inasmuch as they consist of
central white substance covered by gray substance.
  The cerebellum is connected to the cerebrum, pons, and
medulla oblongata; to the cerebrum by the superior peduncle, to the
pons by the middle peduncle, and to the medulla oblongata by the inferior
peduncles.


FIG. 90702– Upper
surface of the cerebellum. (Schäfer.)
  The upper surface of the cerebellum (Fig. 90702) is elevated in the middle and sloped toward the circumference,
the hemispheres being connected together by the superior vermis, which
assumes the form of a raised median ridge, most prominent in front,
but not sharply defined from the hemispheres. The superior vermis
is subdivided from before backward into the lingula, the lobulus centralis,
the monticulus and the folium vermis, and each of these, with the
exception of the lingula, is continuous with the corresponding parts
of the hemispheres—the lobulus centralis with the alæ, the
monticulus with the quadrangular lobules, and the folium vermis with
the superior semilunar lobules.
  The lingula (lingula cerebelli) is a small
tongue-shaped process, consisting of four or five folia; it lies in
front of the lobulus centralis, and is concealed by it. Anteriorly,
it rests on the dorsal surface of the anterior medullary velum, and
its white substance is continuous with that of the velum.
  The Lobulus Centralis and Alæ.—The
lobulus centralis is a small square lobule, situated in the
anterior cerebellar notch. It overlaps the lingula, from which it
is separated by the precentral fissure; laterally, it extends
along the upper and anterior part of each hemisphere, where it forms
a wing-like prolongation, the ala lobuli centralis.
  The Monticulus and Quadrangular Lobules.—The
monticulus is the largest part of the superior vermis. Anteriorly,
it overlaps the lobulus centralis, from which it is separated by the
postcentral fissure; laterally, it is continuous with the quadrangular
lobule
in the hemispheres. It is divided by the preclival fissure
into an

anterior, raised part, the culmen or summit, and a posterior
sloped part, the clivus; the quadrangular lobule is similarly
divided. The culmen and the anterior parts of the quadrangular lobules
form the lobus culminis; the clivus and the posterior parts,
the lobus clivi.
The Folium Vermis and Superior Semilunar Lobule.—The
folium vermis (folium cacuminis; cacuminal lobe) is
a short, narrow, concealed band at the posterior extremity of the
vermis, consisting apparently of a single folium, but in reality marked
on its upper and under surfaces by secondary fissures. Laterally,
it expands in either hemisphere into a considerable lobule, the superior
semilunar lobule
(lobulus semilunaris superior; postero-superior
lobules
), which occupies the posterior third of the upper surface
of the hemisphere, and is bounded below by the horizontal sulcus.
The superior semilunar lobules and the folium vermis form the lobus
semilunaris.


FIG. 90703– Under
surface of the cerebellum. (Schäfer.)
  The under surface of the cerebellum (Fig. 90703) presents, in the middle line, the inferior vermis,
buried in the vallecula, and separated from the hemisphere on either
side by a deep groove, the sulcus valleculæ. Here, as
on the upper surface, there are deep fissures, dividing it into separate
segments or lobules; but the arrangement is more complicated, and
the relation of the segments of the vermis to those of the hemispheres
is less clearly marked. The inferior vermis is subdivided from before
backward, into (1) the nodule, (2) the uvula, (3) the
pyramid, and (4) the tuber vermis; the corresponding
parts on the hemispheres are (1) the flocculus, (2) the tonsilla
cerebelli,
(3) the biventral lobule, and (4) the inferior
semilunar lobule.
The three main fissures are (1) the postnodular
fissure,
which runs transversely across the vermis, between the
nodule and the uvula. In the hemispheres this fissure passes in front
of the tonsil, crosses between the flocculus in front and the biventral
lobule behind, and joins the anterior end of the horizontal sulcus.
(2) The prepyramidal fissure crosses the vermis between the
uvula in front and the pyramid behind, then curves forward between
the tonsil and the biventral lobe, to join the postnodular fissure.
(3) The postpyramidal fissure passes across the vermis between
the pyramid and the tuber vermis, and, in the hemispheres, courses
behind the tonsil and biventral lobules, and then along the lateral
border of the biventral lobule to the postnodular sulcus; in the hemisphere
it forms the anterior boundary of the inferior semilunar lobule.
  The Nodule and Flocculus.—The nodule
(nodulus vermis; nodular lobe), or anterior end of the inferior
vermis, abuts against the roof of the fourth ventricle, and can

only be distinctly seen after the cerebellum has been separated from
the medulla oblongata and pons. On either side of the nodule is a
thin layer of white substance, named the posterior medullary velum.
It is semilunar in form, its convex border being continuous with the
white substance of the cerebellum; it extends on either side as far
as the flocculus. The flocculus is a prominent, irregular lobule,
situated in front of the biventral lobule, between it and the middle
cerebellar peduncle. It is subdivided into a few small laminæ,
and is connected to the inferior medullary velum by its central white
core. The flocculi, together with the posterior medullary velum and
nodule, constitute the lobus noduli.
The Uvula and Tonsilla.—The uvula (uvula vermis;
uvular lobe
) forms a considerable portion of the inferior vermis;
it is separated on either side from the tonsil by the sulcus valleculæ,
at the bottom of which it is connected to the tonsil by a ridge of
gray matter, indented on its surface by shallow furrows, and hence
called the furrowed band. The tonsilla (tonsilla
cerebelli; amygdaline nucleus
) is a rounded mass, situated in
the hemispheres. Each lies in a deep fossa, termed the bird’s
nest
(nidus avis), between the uvula and the biventral
lobule. The uvula and tonsillæ form the lobus uvulæ.
  The Pyramid and Biventral lobules constitute
the lobus pyramidis. The pyramid is a conical projection,
forming the largest prominence of the inferior vermis. It is separated
from the hemispheres by the sulcus valleculæ, across which it
is connected to the biventral lobule by an indistinct gray band, analogous
to the furrowed band already described. The biventral lobule
is triangular in shape; its apex points backward, and is joined by
the gray band to the pyramid. The lateral border is separated from
the inferior semilunar lobule by the postpyramidal fissure. The base
is directed forward, and is on a line with the anterior border of
the tonsil, and is separated from the flocculus by the postnodular
fissure.
  The Tuber Vermis (tuber valvulæ)
and the Inferior Semilunar Lobule (lobulus semilunaris inferior;
postero-superior lobule
) collectively form the lobus tuberus
(tuberæ lobe). The tuber vermis, the most posterior
division of the inferior vermis, is of small size, and laterally spreads
out into the large inferior semilunar lobules, which comprise at least
two-thirds of the inferior surface of the hemisphere.
Internal Structure of the Cerebellum.—The cerebellum
consists of white and gray substance.
White Substance.—If a sagittal section (Fig. 90704) be made through either hemisphere, the interior will be found
to consist of a central stem of white substance, in the middle of
which is a gray mass, the dentate nucleus. From the surface
of this central white stem a series of plates is prolonged; these
are covered with gray substance and form the laminæ. In consequence
of the main branches from the central stem dividing and subdividing,
a characteristic appearance, named the arbor vitæ, is
presented. If the sagittal section be made through the middle of the
vermis, it will be found that the central stem divides into a vertical
and a horizontal branch. The vertical branch passes upward
to the culmen monticuli, where it subdivides freely, one of its ramifications
passing forward and upward to the central lobule. The horizontal
branch passes backward to the folium vermis, greatly diminished in
size in consequence of having given off large secondary branches;
one, from its upper surface, ascends to the clivus monticuli; the
others descend, and enter the lobes in the inferior vermis, viz.,
the tuber vermis, the pyramid, the uvula, and the nodule.
  The white substance of the cerebellum includes two sets
of nerve fibers: (1) projection fibers, (2) fibræ propriæ.
Projection Fibers.—The cerebellum is connected to the
other parts of the brain by three large bundles of projection fibers,
viz., to the cerebrum by the superior peduncle, to the pons by the
middle peduncle, and to the medulla oblongata by the inferior peduncles
(Fig. 90705).
  The superior cerebellar peduncles (brachia
conjunctiva
), two in number, emerge from the upper and medial
part of the white substance of the hemispheres and are placed under
cover of the upper part of the cerebellum. They are joined to each
other across the middle line by the anterior medullary velum, and
can be followed upward as far as the inferior colliculi, under which
they disappear. Below, they form the upper lateral boundaries of the
fourth ventricle, but as they ascend they converge on the dorsal aspect
of the ventricle and thus assist in roofing it in.


FIG. 90704– Sagittal
section of the cerebellum, near the junction of the vermis with
the hemisphere. (Schäfer.)


FIG. 90705– Dissection
showing the projection fibers of the cerebellum. (After E. B. Jamieson.)
  The fibers of the superior peduncle are mainly derived
from the cells of the

dentate nucleus of the cerebellum and emerge from the hilus of this
nucleus; a few arise from the cells of the smaller gray nuclei in
the cerebellar white substance, and others from the cells of the cerebellar
cortex. They are continued upward beneath the corpora quadrigemina,
and the fibers of the two peduncles undergo a complete decussation
ventral to the Sylvian aqueduct. Having crossed the middle line they
divide into ascending and descending groups of fibers, the former
ending in the red nucleus, the thalamus, and the nucleus of the oculomotor
nerve, while the descending fibers can be traced as far as the dorsal
part of the pons; Cajal believes them to be continued into the anterior
funiculus of the medulla spinalis.
  As already stated (page 762), the majority of the fibers
of the ventral spinocerebellar fasciculus of the medulla spinalis
pass to the cerebellum, which they reach by way of the superior peduncle.
  The middle cerebellar peduncles (brachia pontis)
(Fig. 90705) are composed entirely of centripetal
fibers, which arise from the cells of the nuclei pontis of the opposite
side and end in the cerebellar cortex; the fibers are arranged in
three fasciculi, superior, inferior, and deep. The superior fasciculus,
the most superficial, is derived from the upper transverse fibers
of the pons; it is directed backward and lateralward superficial to
the other two fasciculi, and is distributed mainly to the lobules
on the inferior surface of the cerebellar hemisphere and to the parts
of the superior surface adjoining the posterior and lateral margins.
The inferior fasciculus is formed by the lowest transverse
fibers of the pons; it passes under cover of the superior fasciculus
and is continued downward and backward more or less parallel with
it, to be distributed to the folia on the under surface close to the
vermis.
  The deep fasciculus comprises most of the deep
transverse fibers of the pons. It is at first covered by the superior
and inferior fasciculi, but crosses obliquely and appears on the medial
side of the superior, from which it receives a bundle; its fibers
spread out and pass to the upper anterior cerebellar folia. The fibers
of this fasciculus cover those of the restiform body. 121
  The inferior cerebellar peduncles (restiform
bodies
) pass at first upward and lateralward, forming part of
the lateral walls of the fourth ventricle, and then bend abruptly
backward to enter the cerebellum between the superior and middle peduncles.
Each contains the following fasciculi: (1) the dorsal spinocerebellar
fasciculus of the medulla spinalis, which ends mainly in the superior
vermis; (2) fibers from the gracile and cuneate nuclei of the same
and of the opposite sides; (3) fibers from the opposite olivary nuclei;
(4) crossed and uncrossed fibers from the reticular formation of the
medulla oblongata; (5) vestibular fibers, derived partly from the
vestibular division of the acoustic nerve and partly from the nuclei
in which this division ends—these fibers occupy the medial segment
of the inferior peduncle and divide into ascending and descending
groups of fibers, the ascending fibers partly end in the roof nucleus
of the opposite side of the cerebellum; (6) cerebellobulbar fibers
which come from the opposite roof nucleus and probably from the dentate
nucleus, and are said to end in the nucleus of Deiters and in the
formatio reticularis of the medulla oblongata; (7) some fibers from
the ventral spinocerebellar fasciculus are said to join the dorsal
spinocerebellar fasciculus.
  The anterior medullary velum (velum medullare
anterius; valve of Vieussens; superior medullary velum
) is a thin,
transparent lamina of white substance, which stretches between the
superior peduncle; on the dorsal surface of its lower half the folia
and lingula are prolonged. It forms, together with the superior peduncle,
the roof of the upper part of the fourth ventricle; it is narrow above,
where it passes beneath the inferior colliculi, and broader below,
where it is continuous with the white substance of the superior vermis.
A slightly elevated ridge, the frænulum

veli,
descends upon its upper part from between the inferior colliculi,
and on either side of this the trochlear nerve emerges.
  The posterior medullary velum (velum medullare
posterius; inferior medullary velum
) is a thin layer of white
substance, prolonged from the white center of the cerebellum, above
and on either side of the nodule; it forms a part of the roof of the
fourth ventricle. Somewhat semilunar in shape, its convex edge is
continuous with the white substance of the cerebellum, while its thin
concave margin is apparently free; in reality, however, it is continuous
with the epithelium of the ventricle, which is prolonged downward
from the posterior medullary velum to the ligulæ.
  The two medullary vela are in contact with each other
along their line of emergence from the white substance of the cerebellum;
and this line of contact forms the summit of the roof of the fourth
ventricle, which, in a vertical section through the cavity, appears
as a pointed angle.
  The Fibræ Propriæ of the cerebellum
are of two kinds: (1) commissural fibers, which cross the middle
line at the anterior and posterior parts of the vermis and connect
the opposite halves of the cerebellum; (2) arcuate or association
fibers,
which connect adjacent laminæ with each other.
Gray Substance.—The gray substance of the cerebellum
is found in two situations: (1) on the surface, forming the cortex;
(2) as independent masses in the anterior.
  (1) The gray substance of the cortex presents
a characteristic foliated appearance, due to the series of laminæ
which are given off from the central white substance; these in their
turn give off secondary laminæ, which are covered by gray substance.
Externally, the cortex is covered by pia mater; internally is the
medullary center, consisting mainly of nerve fibers.
Microscopic Appearance of the Cortex (Fig. 90706).—The cortex consists of two layers, viz., an external
gray molecular layer, and an internal rust-colored nuclear layer;
between these is an incomplete stratum of cells which are characteristic
of the cerebellum, viz., the cells of Purkinje.
  The external gray or molecular layer consists
of fibers and cells. The nerve fibers are delicate fibrillæ,
and are derived from the following sources: (a) the dendrites
and axon collaterals of Purkinje’s cells; (b) fibers from
cells in the nuclear layer; (c) fibers from the central white
substance of the cerebellum; (d) fibers derived from cells
in the molecular layer itself. In addition to these are other fibers,
which have a vertical direction, and are the processes of large neuroglia
cells, situated in the nuclear layer. They pass outward to the periphery
of the gray matter, where they expand into little conical enlargements
which form a sort of limiting membrane beneath the pia mater, analogous
to the membrana limitans interna in the retina, formed by the sustentacular
fibers of Müller.
  The cells of the molecular layer are small, and
are arranged in two strata, an outer and an inner. They all possess
branched axons; those of the inner layer are termed basket cells;
they run for some distance parallel with the surface of the folium—giving
off collaterals which pass in a vertical direction toward the bodies
of Purkinje’s cells, around which they become enlarged, and form
basket-like net-works.
  The cells of Purkinje form a single stratum of
large, flask-shaped cells at the junction of the molecular and nuclear
layers, their bases resting against the latter; in fishes and reptiles
they are arranged in several layers. The cells are flattened in a
direction transverse to the long axis of the folium, and thus appear
broad in sections carried across the folium, and fusiform in sections
parallel to the long axis of the folium. From the neck of the flask
one or more dendrites arise and pass into the molecular layer, where
they subdivide and form an extremely rich arborescence, the various
subdivisions of the dendrites being covered by lateral spinelike processes.
This arborescence is not circular, but, like the cell, is flattened
at right angles to the long axis of the folium; in other words, it
does not resemble

a round bush, but has been aptly compared by Obersteiner to the branches
of a fruit tree trained against a trellis or a wall. Hence, in sections
carried across the folium the arborescence is broad and expanded;
whereas in those which are parallel to the long axis of the folium,
the arborescence, like the cell itself, is seen in profile, and is
limited to a narrow area.
  From the bottom of the flask-shaped cell the axon arises;
this passes through the nuclear layer, and, becoming medullated, is
continued as a nerve fiber in the subjacent white substance. As this
axon traverses the granular layer it gives off fine collaterals, some
of which run back into the molecular layer.


FIG. 90706– Transverse
section of a cerebellar folium. (Diagrammatic, after Cajal and Kölliker.)
  The internal rust-colored or nuclear layer
(Fig. 90706) is characterized by containing
numerous small nerve cells of a reddish-brown color, together with
many nerve fibrils. Most of the cells are nearly spherical and provided
with short dendrites which spread out in a spider-like manner in the
nuclear layer. Their axons pass outward into the molecular layer,
and, bifurcating at right angles, run for some distance parallel with
the surface. In the outer part of the nuclear layer are some larger
cells, of the type II of Golgi. Their axons undergo frequent division
as soon as they leave the nerve cells, and pass into the nuclear layer;
while their dendrites ramify chiefly in the molecular layer.
  Finally, in the gray substance of the cerebellar cortex
there are fibers which come from the white center and penetrate the
cortex. The cell-origin of these fibers is unknown, though it is believed
that it is probably in the gray substance of the medulla spinalis.
Some of these fibers end in the nuclear layer by dividing into numerous
branches, on which are to be seen peculiar moss-like appendages; hence
they have been termed by Ramón y Cajal the moss fibers;
they form an arborescence around the cells of the nuclear layer and
are said to come from fibers in the inferior peduncle. Other fibers,
the clinging or tendril fibers, derived from the medullary
center can be traced into the molecular layer, where their branches
cling around the dendrites of Purkinje’s cells. They are said
to come from fibers of the middle peduncle.
  (2) The independent centers of gray substance
in the cerebellum are four in number on either side: one is of large
size, and is known as the nucleus dentatus; the other three,
much smaller, are situated near the middle of the cerebellum, and
are known as the nucleus emboliformis, nucleus globosus, and
nucleus fastigii.


FIG. 90707– Sagittal
section through right cerebellar hemisphere. The right olive has
also been cut sagitally.
  The nucleus dentatus (Fig. 90707) is situated a little to the medial side of the center of
the stem of the white substance of the hemisphere. It consists of
an irregularly folded lamina, of a grayish-yellow color, containing
white fibers, and presenting on its antero-medial aspect an opening,
the hilus, from which most of the fibers of the superior peduncle
emerge (page 792).
  The nucleus emboliformis lies immediately to
the medial side of the nucleus dentatus, and partly covering its hilus.
The nucleus globosus is an elongated mass, directed antero-posteriorly,
and placed medial to preceding. The nucleus fastigii is somewhat
larger than the other two, and is situated close to the middle line
at the anterior end of the superior vermis, and immediately over the
roof of the fourth ventricle, from which it is separated by a thin
layer of white substance.
  The cerebellum is concerned with the coördination
of movements necessary in equilibration, locomotion and prehension.
In it terminate pathways conducting impulses of muscle sense, tendon
sense, joint sense and equilibratory disturbances. With the exception
of the ventral spinocerebellar fasciculus these impulses enter through
the inferior peduncle. The reflex arc is completed by fibers in the
superior peduncle which pass to the red nucleus and the thalamus and
thence by additional neurons (rubrospinal tract) to the motor centers.
The exact functions of its different parts are still quite uncertain,
owing to the contradictory nature of the evidence furnished by (1)
ablation experiments upon animals, and (2) clinical observations in
man of the effects produced by abscesses or tumors affecting different
portions of the organ.
The Fourth Ventricle (ventriculus quartus).—The
fourth ventricle, or cavity of the hind-brain, is situated in front
of the cerebellum and behind the pons and upper half of the medulla
oblongata. Developmentally considered, the fourth ventricle consists
of three parts: a superior belonging to the isthmus rhombencephali,
an intermediate, to the metencephalon, and an inferior,
to the myelencephalon. It is lined by ciliated epithelium, and is
continuous below with the central canal of the medulla oblongata; 122
above, it communicates, by means of a passage termed the cerebral
aqueduct, with the cavity of the third ventricle. It presents four
angles, and possesses a roof or dorsal wall, a floor
or ventral wall, and lateral boundaries.
Angles.—The superior angle is on a level with
the upper border of the pons, and is continuous with the lower end
of the cerebral aqueduct. The inferior angle is on a level
with the lower end of the olive, and opens into the central canal
of the medulla oblongata. Each lateral angle corresponds with
the point of meeting of the brachia and inferior peduncle. A little
below the lateral angles, on a level with the striæ medullares,
the ventricular cavity is prolonged outward in the form of two narrow
lateral recesses, one on either side; these are situated between
the inferior peduncles and the flocculi, and reach as far as the attachments
of the glossopharyngeal and vagus nerves.
Lateral Boundaries.—The lower part of each lateral boundary
is constituted by the clava, the fasciculus cuneatus, and the inferior
peduncle; the upper part by the middle and the superior peduncle.
Roof or Dorsal Wall (Fig. 90708).—The
upper portion of the roof is formed by the superior peduncle and the
anterior medullary velum; the lower portion, by the posterior medullary
velum, the epithelial lining of the ventricle covered by the tela
chorioidea inferior, the tæniæ of the fourth ventricle,
and the obex.
  The superior peduncle (page 792), on emerging
from the central white substance of the cerebellum, pass upward and
forward, forming at first the lateral boundaries of the upper part
of the cavity; on approaching the inferior colliculi, they converge,
and their medial portions overlap the cavity and form part of its
roof.
  The anterior medullary velum (page 793) fills
in the angular interval between the superior peduncle, and is continuous
behind with the central white substance of the cerebellum; it is covered
on its dorsal surface by the lingula of the superior vermis.
  The posterior medullary velum (page 794) is continued
downward and forward from the central white substance of the cerebellum
in front of the nodule and tonsils, and ends inferiorly in a thin,
concave, somewhat ragged margin. Below this margin the roof is devoid
of nervous matter except in the immediate vicinity of the lower lateral
boundaries of the ventricle, where two narrow white bands, the tæniæ
of the fourth ventricle
(ligulæ), appear; these bands
meet over the inferior angle of the ventricle in a thin triangular
lamina, the obex. The non-nervous part of the roof is formed
by the epithelial lining of the ventricle, which is prolonged
downward as a thin membrane, from the deep surface of the posterior
medullary velum to the corresponding surface of the obex and tæniæ,
and thence on to the floor of the ventricular cavity; it is covered
and strengthened by a portion of the pia mater, which is named the
tela chorioidea of the fourth ventricle.
  The tæniæ of the fourth ventricle (tænia
ventriculi quarti; ligula
) are two narrow bands of white matter,
one on either side, which complete the lower part of the roof of the
cavity. Each consists of a vertical and a horizontal part. The vertical
part is continuous below the obex with the clava, to which it is adherent
by its lateral

border. The horizontal portion extends transversely across the inferior
peduncle, below the striæ medullares, and roofs in the lower
and posterior part of the lateral recess; it is attached by its lower
margin to the inferior peduncle, and partly encloses the choroid plexus,
which, however, projects beyond it like a cluster of grapes; and hence
this part of the tænia has been termed the cornucopia
(Bochdalek). The obex is a thin, triangular, gray lamina, which
roofs in the lower angle of the ventricle and is attached by its lateral
margins to the clavæ. The tela chorioidea of the fourth ventricle
is the name applied to the triangular fold of pia mater which is carried
upward between the cerebellum and the medulla oblongata. It consists
of two layers, which are continuous with each other in front, and
are more or less adherent throughout. The posterior layer covers the
antero-inferior surface of the cerebellum, while the anterior is applied
to the structures which form the lower part of the roof of the ventricle,
and is continuous inferiorly with the pia mater on the inferior peduncles
and closed part of the medulla.


FIG. 90708– Scheme
of roof of fourth ventricle. The arrow is in the foramen of Majendie.
Choroid Plexuses.—These consist of two highly vascular
inflexions of the tela chorioidea, which invaginate the lower part
of the roof of the ventricle and are everywhere covered by the epithelial
lining of the cavity. Each consists of a vertical and a horizontal
portion: the former lies close to the middle line, and the latter
passes into the lateral recess and projects beyond its apex. The vertical
parts of the plexuses are distinct from each other, but the horizontal
portions are joined in the middle line; and hence the entire structure
presents the form of the letter T, the vertical limb of which, however,
is double.
Openings in the Roof.—In the roof of the fourth ventricle
there are three openings, a medial and two lateral: the medial
aperture
(foramen Majendii), is situated immediately above
the inferior angle of the ventricle; the lateral apertures,
(foramina of Luschka are found at the extremities of the lateral
recesses. By means of these three openings the ventricle communicates
with the subarachnoid cavity, and the cerebrospinal fluid can circulate
from the one to the other.
Rhomboid Fossa (fossa rhomboidea; “floor” of the
fourth ventricle
) (Fig. 90709).
—The
anterior part of the fourth ventricle is named, from its shape, the
rhomboid fossa, and its anterior wall, formed by the back of
the pons and medulla oblongata, constitutes the floor of the fourth
ventricle. It is covered by a thin layer of gray

substance continuous with that of the medulla spinalis; superficial
to this is a thin lamina of neuroglia which constitutes the ependyma
of the ventricle and supports a layer of ciliated epithelium. The
fossa consists of three parts, superior, intermediate, and inferior.
The superior part is triangular in shape and limited laterally
by the superior cerebellar peduncle; its apex, directed upward, is
continuous with the cerebral aqueduct; its base it represented by
an imaginary line at the level of the upper ends of the superior foveæ.
The intermediate part extends from this level to that of the
horizontal portions of the tæniæ of the ventricle; it is
narrow above where it is limited laterally by the middle peduncle,
but widens below and is prolonged into the lateral recesses of the
ventricle. The inferior part is triangular, and its downwardly
directed apex, named the calamus scriptorius, is continuous
with the central canal of the closed part of the medulla oblongata.


FIG. 90709– Rhomboid
fossa. (See enlarged image)
  The rhomboid fossa is divided into symmetrical halves
by a median sulcus which reaches from the upper to the lower
angles of the fossa and is deeper below than above. On either side
of this sulcus is an elevation, the medial eminence, bounded
laterally by a sulcus, the sulcus limitans. In the superior
part of the fossa the medial eminence has a width equal to that of
the corresponding half of the fossa, but opposite the superior fovea
it forms an elongated swelling, the colliculus facialis, which
overlies the nucleus of the abducent nerve, and is, in part at least,
produced by the ascending portion of the root of the facial nerve.
In the inferior part of the fossa the medial eminence assumes the
form of a triangular area, the trigonum hypoglossi. When examined
under water with a lens this trigone is seen to consist of a medial
and a lateral area separated by a series of oblique furrows; the medial
area corresponds with the upper part of the nucleus of the hypoglossal
nerve, the lateral with a small nucleus, the nucleus intercalatus.
  The sulcus limitans forms the lateral boundary
of the medial eminence. In the superior part of the rhomboid fossa
it corresponds with the lateral limit of the

fossa and presents a bluish-gray area, the locus cæruleus,
which owes its color to an underlying patch of deeply pigmented nerve
cells, termed the substantia ferruginea. At the level of the
colliculus facialis the sulcus limitans widens into a flattened depression,
the superior fovea, and in the inferior part of the fossa appears
as a distinct dimple, the inferior fovea. Lateral to the foveæ
is a rounded elevation named the area acustica, which extends
into the lateral recess and there forms a feebly marked swelling,
the tuberculum acusticum. Winding around the inferior peduncle
and crossing the area acustica and the medial eminence are a number
of white strands, the striæ medullares, which form a portion
of the cochlear division of the acoustic nerve and disappear into
the median sulcus. Below the inferior fovea, and between the trigonum
hypoglossi and the lower part of the area acustica is a triangular
dark field, the ala cinerea, which corresponds to the sensory
nucleus of the vagus and glossopharyngeal nerves. The lower end of
the ala cinerea is crossed by a narrow translucent ridge, the funiculus
separans,
and between this funiculus and the clava, is a small
tongue-shaped area, the area postrema. On section it is seen
that the funiculus separans is formed by a strip of thickened ependyma,
and the area postrema by loose, highly vascular, neuroglial tissue
containing nerve cells of moderate size.

Note 120.  Essick, Am. Jour. Anat., 1907. [back]
Note 121.  See article by E. B. Jamieson, Journal of Anatomy and Physiology, vol. xliv. [back]
Note 122.  J. T. Wilson (Journal of Anatomy and Physiology, vol. xl) has pointed out that the central cana of the medulla oblongata, immediately below its entrance into the fourth ventricle, retains the cleft-like form presented by the fetal medulla spinalis, and that it is marked by dorso- and ventro-lateral sulci. [back]

 


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