The temporal bones are situated at the sides and base of the skull. Each consists of five parts, viz. the squamous, mastoid, petrous, and tympanic parts, and the styloid process. These represent four morphologically distinct elements which have become fused with one another. The squamous part is a dermal bone, developed to assist in the protection of the cerebrum. The petromastoid portion is preformed in cartilage as a protecting capsule for the membranous labyrinth. The tympanic part, formed in membrane, is homologous with the angular bone, which constitutes a part of the composite lower jaw of many reptilians and bony fishes ; it has become incorporated in the skull and adapted to play a part in the provision of a satisfactory mechanism for the transmission of sound-waves in an air medium. The styloid process represents the dorsal end of the skeletal element of the hyoid arch.

The squamous part of the temporal bone forms the anterior and upper part of the bone, and is scale-like, thin and translucent. Its temporal or external surface (fig. 352) is smooth and slightly convex ; it forms part of the temporal fossa and gives origin to the temporal muscle; above the opening of the external auditory meatus it is marked by a vertical groove for the middle temporal artery. A curved line, termed the supramastoid crest, courses backwards and upwards across its posterior part; it serves for the attachment of the temporal fascia and limits the origin of the temporal muscle.

The boundary between the squamous and mastoid portions of the bone lies about 1-5 cm. below the supramastoid crest and is frequently indicated by traces of the original squamomastoid suture; the external surface of this lower part is convex, and from its anterior part the auricularis posterior takes origin. Between the anterior end of the supra-mastoid crest and the posterosuperior sector of the opening of the external auditory meatus there is a depression, termed the suprameatal triangle ; this triangle is an important landmark for the tympanic antrum, which lies media l to it, at a depth of about 1.25 cm. The anterior part of the depression is usually marked by a small projection, termed the suprameatal spine.

A long, arched process termed the zygomatic process, or zygoma, projects from the lower portion of the squamous part. The posterior part of this process is triangular in shape and springs from a broad base; it is directed laterally, and its surfaces are superior and inferior. The process is then twisted forwards and medially and runs forwards, and the surfaces of this anterior portion are therefore medial and lateral. The superior surface of the posterior part is concave, and continuous with the temporal surface of the squamous part ; the inferior surface is bounded by two roots, a posterior and an anterior, which converge as they approach the anterior part of the process. At the meeting point of the two roots the tubercle of the root of the zygoma gives attachment to the temporomandibular ligament. The posterior root is prolonged forwards from the surface of the squamous part immediately above the opening of the external auditory meatus ; its upper border is continuous behind with the supramastoid crest. The anterior root juts almost horizontally from the side of the squamous part; its inferior surface, convex from before backwards, is smooth for articulation with the articular disc of the mandibular joint, and the whole root presents the form of a short semicylindrical bar, named the articular eminence (tubereulum articulare). The articular eminence forms the anterior boundary of the articular (mandibular) fossa. Very rarely the squamous part is perforated just above the posterior root of the zygoma. When present, this squamosal foramen transmits the petrosquamous sinus.

Figure 352
Temporal bone external view - Figure 352
The anterior part of the zygomatic process is thin and flat. The superior border, long and thin, gives attachment to the temporal fascia; the inferior, short and arched, gives origin to some Fibres of the masseter muscle. The lateral surface is convex and subcutaneous; the medial is concave and gives origin to part of the masseter. The anterior end is deeply serrated and cut obliquely at the expense of the lower border; it articulates with the temporal process of the zygomatic bone. In front of the articular eminence a small triangular area forms apart of the roof of the infratemporal fossa and is separated from the temporal surface of the squamous part by a ridge ; this ridge is continuous behind with the anterior root of the zygomatic process, and in front, in the articulated skull, with the infratemporal crest on the greater wing of the sphenoid bone. The mandibular fossa is bounded in front by the articular eminence ; it consists of an anterior, articular portion, formed by the squamous part, and a posterior, non-articular portion, formed by the tympanic part of the temporal bone. The articular portion, smooth, oval and deeply concave, articulates with the articular disc of the mandibular joint; the non-articular portion sometimes lodges a small part of the parotid gland. A small, somewhat conical, eminence, termed the postglenoid tubercle, separates the lateral part of the articular portion from the anterior margin of the tympanic part of the bone, and is the representative of a prominent tubercle which, in some mammals, descends behind the condyle of the mandible and prevents its backward displacement; the posterior glenoid tubercle is sometimes described as the third root of the zygomatic process. The medial part of the articular portion of the fossa is separated from the tympanic part of the bone by the squamotympanic fissure, into which projects the lower edge of the down-turned anterolateral part of the tegmen tympani of the petrous part of the bone; between this plate and the tympanic part is the petrotympanic fissure (Glaserian fissure). This fissure leads into the middle ear or tympanic cavity ; it lodges the anterior ligament of the malleus and transmits the anterior tympanic branch of the maxillary (internal maxillary) artery. The medial end of the fissure presents the anterior opening of the anterior canaliculus for the chorda tympani. Very rarely a postglenoid foramen is present just in front of the external auditory meatus and in the line of fusion of the squamous and tympanic portions of the bone. It replaces the squamosal foramen, already mentioned, and transmits the petrosquamous sinus.

Figure 353
Temporal bone internal view - Figure 353
The cerebral or internal surface of the squama (fig. 353) is concave; it presents depressions corresponding to the convolutions of the temporal lobe of the brain, and grooves for the branches of the middle meningeal vessels; its lower border is united to the anterior surface of the petrous portion by the petrosquamosal suture, traces of which are frequently seen in the adult bone.

The superior border is thin, beveled at the expense of the internal surface, and overlaps the inferior border of the parietal bone, forming with it the squamosal suture. Posteriorly the superior border forms an angle (B.N.A. parietal notch) with the mastoid portion of the bone. The antero-inferior border, thin above and thick below, articulates with the greater wing of the sphenoid bone; its upper part is beveled at the expense of the inner, its lower at the expense of the outer, surface.

The mastoid portion of the temporal bone forms the posterior part of the bone. Its outer surface (fig. 352) is rough and gives attachment to the occipital belly of the occipitofrontalis, and the auricularis posterior muscle. It is frequently perforated near its posterior border by the mastoid foramen, which gives exit to a vein from the sigmoid sinus and entrance to a small branch of the occipital artery to the dura mater, the position and size of this foramen are very variable ; it may be situated in the occipital bone, or in the suture between the temporal and the occipital bones. The mastoid portion is continued below into a conical projection, named the mastoid process, the size and form of which vary somewhat; it is larger in the male than in the female. The lateral surface of this process gives insertion to the sternomastoid, splenius capitis, and longissimus capitis; on its medial side there is a deep groove, termed the mastoid notch for the attachment of the posterior belly of the digastric muscle; medial to this notch the shallow occipital groove lodges the occipital artery.

Figure 354
Temporal bone coronal section anterior view - Figure 354
On the inner surface of the mastoid portion (fig. 353) there is a deep, curved groove, termed the sigmoid sulcus, which lodges the sigmoid sinus ; near its posterior border it receives the opening of the mastoid foramen. The sigmoid sulcus is separated from the innermost of the mastoid air-cells by a thin lamina of bone which may be partly deficient. The superior border of the mastoid portion is thick and serrated for articulation with the postero-inferior angle of the parietal bone. The posterior border, also serrated, articulates with the inferior border of the occipital bone between the lateral angle and jugular process. The mastoid portion is fused with the descending process of the squamous part above; below, it enters into the formation of the posterior wall of the tympanic cavity.

A section of the mastoid process (fig. 354) exhibits a number of spaces, termed the mastoid air cells, which vary greatly in size and number. At the upper and front part of the process they are large and irregular, but towards the lower part they diminish in size, while those at the apex of the process are frequently quite small; occasionally they are entirely absent, and the mastoid is then solid throughout. In addition, a large irregular air-sinus, termed the tympanic antrum, is situated at the upper and front part of the mastoid portion of the bone; it is lined with a prolongation of the raucous membrane of the tympanic cavity. It is bounded above by a thin plate of bone, termed the tegmen tympani, which separates it from the middle fossa of the base of the skull, laterally by the portion of the squamous part which lies below the supramastoid crest ; its medial wall is related to the lateral semicircular canal of the internal ear. Below and behind, the tympanic antrum communicates with the mastoid air cells ; in front, it opens into that portion of the tympanic cavity which is known as the epitympanic recess. The tympanic antrum is a cavity of some considerable size at the time of birth; the mastoid air cells originate as diverticula from the antrum, and begin to appear at or before birth; by the fifth year they are of considerable size, but their development is not completed until puberty.

The petrous portion (or pyramid) of the temporal bone is wedged between the sphenoid and occipital bones at the base of the skull (figs. 322, 327). It is directed medially, forwards, and a little upwards, it has a base, an apex, three surfaces and three margins. The essential parts of the organs of hearing and equilibration are placed within it.

The base is fused with the squamous and mastoid portions, and is partially separated from them by the tympanic antrum.

The apex, rough and uneven, is received into the angular interval between the posterior border of the greater wing of the sphenoid bone and the basilar part of the occipital bone; it is pierced by the anterior orifice of the carotid canal and forms the posterolateral boundary of the foramen lacerum.

The anterior surface helps to form the floor of the middle fossa of the skull and is continuous with the inner surface of the squamous part, to which it is united by the pelrosquamosal suture. Remains of this suture are often distinct even at a late period of life.

The whole surface is marked by impressions for the gyri of the inferior surface of the temporal lobe of the brain. Immediately behind the apex a slight hollow lodges the trigeminal (semilunar) ganglion and it is termed the trigeminal impression. The bone anterior arid slightly lateral to the impression forms the roof of the anterior part of the carotid canal; it is often deficient in this situation. An irregular ridge separates the trigeminal impression posteriorly from a second hollow, which forms part of the roof of the internal auditory meatus and covers the cochlea. This concavity is limited behind by an elevation, termed the arcuate eminence (fig. 353), which is raised by the superior semicircular canal and, in its lateral part, roofs in the vestibule and the beginning of the facial canal. Between the squamous part on the lateral side and the arcuate eminence and the hollows just described on the medial side the surface is formed by the tegmen tympani (fig. 327). This thin plate of bone forms the roof of the tympanic antrum behind and extends forwards above the tympanic cavity and the canal for the tensor tympani muscle. Its lateral margin meets the squamous part at the petrosquamosal suture and turns downwards in front to form the lateral wall of the canal for the tensor tympani and the bony part of the pharyngotympanic (auditory) tube; the lower edge of this down-turned portion has already been observed in the floor of the squamotympanic fissure. Anteriorly the tegmen tympani presents a narrow groove, which runs backwards and laterally and enters the bone through an opening placed in front of the lateral part of the arcuate eminence. This hiatus transmits the greater superficial petrosal nerve, which runs forwards to the foramen lacerum. A second groove may mark the bone on the lateral side of the one just described; it transmits the lesser superficial petrosal nerve from the tympanic plexus. The posterior slope of the arcuate eminence covers the posterior and lateral semicircular canals, and lateral to it the posterior part of the tegmen tympani roofs in the posterior part of the tympanic antrum.

The posterior surface (fig. 353) forms the anterior part of the posterior fossa of the skull and is continuous with the inner surface of the mastoid portion. Near the centre of this surface is an orifice of varying size which leads into the internal auditory meatus, a canal about 1 cm in length, which runs laterally. It transmits the facial and auditory nerves and the internal auditory artery. The lateral end of the internal auditory meatus is separated from the internal ear by a vertical plate, which is divided by a horizontal crest, termed the transverse crest, into two unequal portions (fig. 355). Below the posterior part of the crest, and situated in the inferior veslibular area, are several openings for the transmission of the nerves to the saccule ; below and behind this area is the foramen singulare, which gives passage to the nerve to the posterior semicircular duct. Below the anterior part of the transverse crest is the tractus spiralis foraminosus, consisting of a number of small spirally arranged openings, which encircle the central canal of the cochlea; these openings together with the central canal transmit the nerves to the cochlea. The portion above the transverse crest displays, behind, the superior vestibular area, pierced by a series of small openings for the passage of the nerves to the utricle and the superior and lateral semicircular ducts, and in front, the facial area, with one large opening which is the commencement of the facial canal and transmits the facial nerve. Behind the opening of the meatus there is a small slit almost hidden by a thin plate of bone ; it leads to a canal, named the aquwduct of the vestibule, which contains the ductus endolymphaticus together with a small artery and vein. Above and between these two openings an irregular depression, termed the subarcuate fossa, lodges a process of the dura mater and transmits a small vein; in the infant this depression is represented by a large fossa which extends as a short, blind tunnel under the superior semicircular canal.

The inferior surface (fig. 356), rough and irregular, forms part of the external surface of the base of the skull. It presents the following parts for examination. (1) near the apex there is a quadrilateral rough surface, which serves partly for the attachment of the levator palati (levator veli palatini) muscle and the cartilaginous portion of the pharygotympanic (auditory) tube, and partly for connection with the basilar part of the occipital bone through the intervention of some dense fibrous tissue; (2) behind this a large, nearly circular aperture, leads into the carotid canal ; this canal runs at first vertically, and then, making a bend, is directed horizontally forwards and medially: it transmits into the cranium the internal carotid artery and the carotid plexus of nerves ; (3) behind this opening there is a deep depression, termed the jugular fossa, of variable depth and size in different skulls ; it lodges the superior bulb of the internal jugular vein; (4) in front of the medial part of the jugular fossa and directly below the internal auditory meatus, the hone is marked by a, triangular depression, which lodges the inferior (petrous) ganglion of the glossopharyngeal nerve ; at the apex of this notch a small opening leads into the cochlear canaliculus, which lodges the aqueduct of the cochlea and a tubular prolongation of the dura mater and transmits a vein from the cochlea to join the internal jugular vein; through the aqueduct the perilymph of the labyrinth is enabled to drain away into the subarachnoid space; (5) in the bony ridge dividing the carotid canal from the jugular fossa is the small canaliculus for the tympanic nerve, which is derived from the glossopharyngeal nerve; (6) in the lateral part of the jugular fossa the bone is pierced by the mastoid canaliculus for the entrance of the auricular branch of the vagus nerve; (7) behind the jugular fossa the jugular surface forms a rough quadrilateral area covered with cartilage in the recent state and articulating with the jugular process of the occipital bone; (8) extending laterally from the carotid canal is the sharp lower border of the tympanic part of the bone ; the lateral part of this border splits to ensheath the root of the styloid process and is therefore named the sheath of the styloid process (vagina processes slyloidei) ; (9) emerging from its sheath the styloid process, which is about 2.5 cm in length, is directed downwards and forwards; (10) between the styloid and mastoid processes the stylomastoid foramen forms the lower end of the facial canal and transmits the facial nerve and stylomastoid artery.

Figure 355
Internal auditory meatus lateral view - Figure 355
The superior margin, the longest, is grooved for the superior petrosal sinus and gives attachment to the tentorium cerebelli, except at its medial extremity where it is crossed by the roots of the trigeminal nerve. The posterior margin is intermediate in length between the superior and the anterior. Its medial part is marked by a sulcus, which forms, with a corresponding sulcus on the occipital bone, the channel for the inferior petrosal sinus. Behind this is the jugular fossa, which, with the jugular notch on the occipital bone, forms the jugular foramen; it presents the notch for the glossopharyngeal nerve, and either or both of the extremities of the notch may meet the occipital bone and divide the foramen in two, sometimes three, parts. The anterior margin is divided into two parts- a lateral, joined to the squamous part at the petrosquamosal suture; a medial, free, for articulation with the greater wing of the sphenoid bone.

At the angle of junction of the petrous and squamous parts two canals are placed one above the other, and separated by a thin plate of bone. Both canals lead into the tympanic cavity; the upper transmits the tensor tympani ; the lower forms the canal of the pharyngotympanic (auditory) tube,

The tympanic part of the temporal bone (fig. 356) is a curved plate lying below the squamous part and in front of the mastoid process. Internally, it is fused with the petrous portion, and appears in the angle between it and the squamous part, where it lies below and lateral to the orifice of the pharyngotympanic (auditory) tube. Behind, it fuses with squamous part and the mastoid process and forms the anterior boundary of the tympanomastoid fissure. Its posterior surface is concave and forms the anterior wall, the floor, and a part of the posterior wall of the bony external auditory meatus ; at the medial end of this surface there is a narrow furrow termed the tympanic sulcus, for the attachment of the circumference of the tympanic membrane. Its anterior surface, quadrilateral and slightly concave, constitutes the posterior wall of the articular fossa and is sometimes in contact with a part of the parotid gland. Its lateral border, free and rough, forms a large part of the margin of the opening of the external auditory meatus and gives attachment to the cartilaginous part of the meatus. The lateral part of the upper border is fused with the back of the postglenoid tubercle; its medial part forms the posterior boundary of the petrotympanic fissure. The lower border is sharp; its lateral part splits to enclose the root of the styloid process and is therefore named the sheath of the styloid process. The central portion of the tympanic part of the temporal bone is thin, and in a considerable percentage of skulls is perforated by a foramen.

Figure 356
Temporal bone inferior view - Figure 356
The external auditory meatus, about 16 mm. long, is directed inwards anal slightly forwards and downwards, and the floor of the meatus is convex upwards. On parasagittal section the meatus is oval or elliptical in shape with the long axis directed downwards and slightly backwards. Its anterior wall and floor and the lower part of its posterior wall are formed by the tympanic part of the bone, the roof and the upper part of the posterior wall by the squamous part. Its inner end is closed in the recent state by the tympanic membrane; its outer end is bounded above by the posterior root of the zygomatic process, below which the small suprameatal spine is sometimes seen at the upper and posterior part of the orifice.

The styloid process of the temporal bone, slender: pointed, and averaging about 2.5 cm in length, projects downwards and forwards, from the under surface of the bone. Its proximal part (tympanohyal) is surrounded by a bony sheath, derived from the tympanic plate and best marked on its anterolateral aspect, while its distal part (stylohyal) gives attachment to the stylohyoid and stylomandibular ligaments, and to the styloglossus, stylobyoid and stylopharyngeus muscles. The process is covered by the parotid gland, and the facial nerve crosses its base and the external carotid artery its tip, as they lie within the gland. On its deep aspect the process is separated from the commencement of the internal jugular vein by the origin of the stylopharyngeus muscle.

Structure.-The structure of the squamous part is like that of the other cranial. bones the mastoid portion is spongy, and the petrous portion dense and hard.

Ossfication.-The temporal bone is ossified from eight centres (exclusive of those for the internal ear and the tympanic ossicles)-viz. one each for the squamous and the tympanic part, four for the petrous and mastoid parts, and two for the styloid process. Just before the close of fetal life the bone consists of three principal parts, viz. : the squamous, the petromastoid part, and the tympanic ring (fig. 357). The squamous part is ossified in membrane from a single centre which appears in the region of the root of the zygomatic process about the seventh or eighth week of fetal life and soon extends in an upward direction. The petromastoid part is developed from four centres, which make their appearance in the cartilaginous ear-capsule about the fifth or sixth month of fetal life.

Figure 357
Three parts of the temporal bone at birth - Figure 357
One (pro-otic) appears in the neighbourhood of the arcuate eminence, spreads in front of and above the internal auditory meatus and extends to the apex of the bone ; it covers part of the cochlea, vestibule, superior semicircular duct, and medial wall of the tympanic cavity. A second (opisthotic) appears at the promontory on the medial wall of the tympanic cavity and surrounds the fenestra cochleae; it forms the floor of the tympanic cavity and vestibule, surrounds the carotid canal, invests the lateral and lower parts of the cochlea., and spreads below the internal auditory meatus. A third (pterotic) forms the roof of the tympanic cavity and antrum ; while the fourth (epiotic) appears near the posterior semicircular duet and extends to form the mastoid process (Vrolik). The tympanic ring is an incomplete circle, deficient above, the concavity of which is grooved by the tympanic sulcus, for the attachment of the circumference of the tympanic membrane.* The tympanic ring expands to form the tympanic part of the bone and is ossified in membrane, from a single centre which appears about the third month. The styloid process is developed from the cranial end of the cartilage of the second visceral or hyoid arch (p.100) by two centres: one for the proximal part of the process, the tympanohyal, appears before birth ; the other, for the distal part of the process, the stylohyal, does not appear until after birth. The tympanic ring unites with the squamous part shortly before birth; the petromastoid fuses with the squamous part during the first year. The two crests, the superior and inferior tympanic crests, run obliquely downwards and forwards across the anterior part of the inner surface of the tympanic ring. Between them an obliquely directed furrow, termed the sulcus malleolaris, lodges the anterior process of the malleus, the anterior tympanic artery and the chords tympani nerve tympanohyal portion of the styloid process about the same time (figs. 319, 320). The stylohyal does not unite with the rest of the bone until after puberty, and in some skulls never at all.

Figure 358
Temporal bone at birth external view - Figure 358
The chief subsequent changes in the temporal bone apart from increase in size are (1) The tympanic ring grows laterally and backwards to form the tympanic part of the bone. This growth does not, however, take place at an equal rate all round the ring, but occurs most rapidly on its anterior and posterior portions, and these outgrowths meet and blend, and thus, for a time, there exists in the floor of the meatus a foramen, named the foramen of Huschke ; this foramen is usually closed about the fifth year, but may persist throughout life. (2) The articular fossa is at first extremely shallow and looks more laterally than downwards ; it becomes deeper and is ultimately directed downwards. Its change in direction is accounted for as follows. The portion of the squamous part which forms the fossa lies at first below the level of the zygomatic process and is nearly vertical, but in consequence of the subsequent increase in the width of the base of the skull this portion of the squamous part comes to be directed horizontally inwards, and its surfaces therefore upwards and downwards ; the attached portion of the zygomatic process also becomes everted and projects like a shelf at right angles to the squamous. The postero-inferior portion of the squamous part grows downwards internal auditory meatus behind the tympanic ring and forms the lateral bony wall of the tympanic antrum. (3) The mastoid portion is at first flat, and the stylomastoid foramen and rudimentary styloid process lie immediately behind the tympanic ring. With the development of the mastoid air cells the lateral part of the mastoid portion grows downwa’rd’s and forwards to form the mastoid process, and the styloid process and stylomastoid foramen come to lie on the under surface of the bone. The descent of the stylomastoid foramen is necessarily accompanied by a corresponding increase in the length of the canal for the facial nerve. It is not until the latter part of the second year that the mastoid process forms a definite elevation on the surface of the skull. (4) The subarcuate fossa on the posterior surface of the petrous portion is gradually filled and almost obliterated.

Figure 359
Temporal bone at birth internal view - Figure 359
Applied Anatomy.—The external auditory meatus is relatively as long in the child as in the adult, but in the child the canal is cartilaginous, whereas in the adult the inner two-thirds of it are osseous. When it is necessary to open the tympanum for suppuration; it is approached through the tympanic antrum. In the child only a thin scale of bone requires to be removed from the suprameatal triangle to open into the tympanic antrum.

 


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