MIDDLE CRANIAL FOSSA (figs. 327, 328)

The middle cranial fossa is deeper than the anterior; and it is more extensive on each side than in the median plane, and bears some resemblance to a butterfly with outspread wings. In front it is bounded by the posterior borders of the lesser wings, the anterior clinoid processes and the anterior margin of the optic groove; behind by the superior borders of the petrous parts of the temporal bones and the dorsum sellae of the sphenoid bone; laterally by the temporal squamae, the anterior inferior angles of the parietal bones and the greater wings of the sphenoid.

Figure 328
The internal surface of the left half of the base of the skull structures labeled - Figure 328
In the median area the floor is formed by the body of the sphenoid. In front, the optic groove (sulcus chiasmatis) leads on each side into the optic foramen. The sulcus does not lodge the optic chiasma, which lies above and behind it. The optic foramen is placed between the two roots of the lesser wing and is bounded medially by the body of the sphenoid. It is directed forwards, laterally and somewhat downwards and transmits the optic nerve and the ophthalmic artery. Immediately behind the groove the upper surface of the body of the sphenoid is shaped like a Turkish saddle and is often termed the sella turcica. Its anterior slope is marked by a median elevation, termed the tuberculum self, and behind that the surface is hollowed out to, form the hypophyseal fossa (fig. 331), which lodges the hypophysis- an important ductless gland. The floor of the hypophyseal fossa forms the roof of the sphenoidal sinus. Posterior to the fossa the bone projects upwards and forwards to form the dorsum sellae. On each side the superolateral angle of the dorsum sellae is expanded to form the posterior clinoid process. Lateral to the sella turcica the body of the sphenoid presents a shallow groove for the internal carotid artery, which runs forwards from the foramen lacerum in contact with the bone. A small elevation marks the anterior part of the medial edge of the carotid groove and is termed the middle clinoid process ; it may be joined to the anterior clinoid process by a thin bar of bone. Posteriorly the lateral edge of the carotid groove may be deepened by a small projection, termed the lingula.

The lateral part of the middle cranial fossa is deep and supports the temporal lobe of the brain. Its floor is formed in front by the cerebral surface of the greater wing of the sphenoid and behind by the anterior surface of the petrous part of the temporal bone, while the cerebral surface of the temporal squama occupies the interval between these two bones in the lateral portion of the floor. It is related in front to the posterior part of the orbit: laterally, to the temporal fossa ; and below, to the infratemporal fossa.

Anteriorly it communicates with the orbit through the superior orbital fissure, which is placed obliquely and is bounded above by the lesser wing, below by the greater wing, and medially by the side of the body of the sphenoid. The fissure is wider at its medial than at its lateral end and its long axis is directed upwards, laterally and forwards. It transmits the ophthalmic nerve and veins, the oculomotor, trochlear and abducent nerves, and some smaller vessels.

The foramen rotundum pierces the greater wing of the sphenoid immediately below the medial end of the superior orbital fissure, It leads forwards into the pterygopalatine fossa, to which it conducts the maxillary nerve.

The foramen ovale pierces the greater wing of the sphenoid posterior to the foramen rotundum and lateral to the lingula and the posterior end of the carotid groove. It leads downwards into the infratemporal fossa and transmits the mandibular nerve to that region.

The foramen spinosum lies close to the posterolateral margin of the foramen ovale, and transmits the middle meningeal artery from the infratemporal to the middle cranial fossa. The artery, with its accompanying veins, runs laterally to gain the temporal squama on which it runs upwards, forwards and laterally. Crossing the temporosphenoidal suture for a second time it ascends on the greater wing and divides into anterior and posterior branches about 2.5 cm. below the pterion. The anterior branch proceeds upwards across the cerebral surface of the pterion and gains the anterior part of the parietal bone. In the region of the pterion the artery is often enclosed in a bony canal. The posterior branch runs backwards and upwards on to the upper part of the temporal squama and crosses the parietosquamosal suture to gain the posterior part of the parietal bone. These arteries and their branches produce conspicuous grooves in the floor and lateral wall of the middle cranial fossa.

At the posterior end of the carotid groove and posteromedial to the foramen ovale the foramen lacerum is situated. It is bounded behind by the apex of the petrous temporal and in front by the body of the sphenoid and the posterior border of its greater wing. This end of the foramen lacerum transmits the internal carotid artery and its accompanying nervous and venous plexuses, together with some smaller structures.

Behind the foramen lacerum the anterior surface of the petrous temporal presents a shallow depression adjoining the apex of the bone. It is occupied by the trigeminal (semilunar) ganglion, and is named the trigeminal impression. The ganglion lies in a special recess of the dura mater and extends forwards above the lateral part of the foramen lacerum. Behind the trigeminal impression the surface presents a shallow hollow, limited posteriorly by a transversely rounded elevation, termed the arcuate eminence. This elevation is produced by the superior semicircular canal, which is closely related to the floor of the middle cranial fossa in this situation. Lateral to the trigeminal impression the anterior surface of the petrous temporal presents a narrow groove which is directed backwards and laterally and soon disappears into the hiatus for the greater superficial petrosal nerve. Posterolateral to the trigeminal impression and anterolateral to the arcuate eminence the anterior surface of the petrous temporal is formed by the tegmen tympani, a thin lamella of bone which forms the roof of the tympanic cavity, and extends forwards and medially above the pharyngotympanic (auditory) tube. Lateral to the arcuate eminence the posterior part of the tegmen tympani forms the roof of the tympanic antrum, an air-space in the bone which communicates in front with the tympanic cavity,

The superior border of the petrous temporal separates the middle from the posterior cranial fossa. Behind the trigeminal impression it is grooved by the superior petrosal sinus, which connects the posterior end of the cavernous sinus to the upper end of the sigmoid sinus.

Particular features.-The optic nerve carries with it through the optic foramen a sheath derived from the membranes of the brain. The ophthalmic artery lies below the nerve in contact with the posterior root of the lesser wing.

On each side of the body of the sphenoid the cavernous sinus extends from the medial end of the superior orbital fissure to the apex of the petrous part of the temporal bone. In addition to the internal carotid artery and its plexus of sympathetic nerves the sinus contains the oculomotor, the trochlear, abducent and ophthalmic nerves, but these structures do not come into contact with the bone, An anterior intercavernous sinus, which crosses the tuberculum sellae;, and a posterior intercavernous sinus, which crosses the front of the dorsum sellae, connect the two cavernous sinuses to each other. An additional connection is established by an irregularly loculated sinus which intervenes between the hypophysis cerebri and the floor of its fossa. The diaphragma sellae, which surrounds the infundibulum, is connected to the tuberculum in front and to the dorsum sellse behind. The posterior clinoid process gives attachment to the anterior extremity of the attached margin of the tentorium cerebelli and to the petrosphenoid ligament.

The superior orbital fissure (fig. 315) opens into the orbit between its roof and its lateral wall. Its lower border is marked by a small projection which gives attachment to the lateral part of the annulus tendinis communal. Lateral to this projection the fissure is narrow, and transmits the lacrimal, frontal and trochlear nerves, and, in addition, a small orbital branch from the middle meningeal artery and a meningeal branch from the lacrimal artery. The wider, medial end of the fissure transmits the upper and lower divisions of the oculomotor, the nasociliary-which usually lies between them, and the abducent nerve-which usually lies below them, and the ophthalmic veins. The latter pass directly into the anterior end of the cavernous sinus. At the lateral extremity of the fissure the greater wing and the orbital plate of the frontal bone articulate with each other.

The foramen rotundum, like the medial end of the superior orbital fissure, is intimately related to the lateral wall of the sphenoidal sinus. Originally a part of the fissure, it be comes separated secondarily. The root of the greater wing may be pierced medial to the foramen ovale by a small foramen which transmits an emissary vein from the cavernous sinus and is termed the emissary/ sphenoidal foramen. In addition to the mandibular nerve the foramen ovale transmits the accessory meningeal artery, and, sometimes; the lesser superficial petrosal nerve. In addition to the middle meningeal artery the foramen spinosum. transmits the nerves spinosus. Both these foramina are represented at first by notches on the margin of the greater wing, which subsequently become converted into foramina. The foramen lacerum, as described above, is a short bony canal, traversed in its whole extent only by minute meningeal branches from the ascending pharyngeal artery and a few small veins and meningeal lymphatic vessels. The internal carotid artery pierces its posterior wall and ascends through its upper opening. The greater superficial petrosal nerve emerges from its hiatus and runs forwards in the groove which marks the anterior surface of the petrous part of the temporal bone. It turns downwards through the foramen lacerum on the lateral side of the internal carotid artery and is joined by the deep petrosal nerve to form the nerve of the pterygoid canal. This nerve leaves the foramen lacerum above its lower opening by traversing the pterygoid canal, which opens on its anterior wall. The lesser superficial petrosal nerve lies to the lateral side of the greater as it emerges on the anterior surface of the petrous part of the temporal bone and may occupy a small groove.

In a young skull the suture between the petrous and the squamous parts of the temporal hone may be visible at the lateral margin of the tegmen tympani, but it is usually obliterated in the adult skull. In this situation anteriorly the lateral margin of the tegmen tympani turns downwards, forming the lateral wall of the bony part of the pharyngotympanic (auditory) tube, and its lower border may be visible in the floor of the squamotympanic fissure.

Lateral to the anterior part of the tegmen tympani the part of the temporal squama which helps to form the floor of the middle cranial fossa is thin and translucent over a small area. This corresponds to the deepest part of the articular fossa on the external surface of the base of the skull.

In front of the commencement of the groove for the superior petrosal sinus the upper border of the petrous temporal shows a shallow smooth notch, termed the trigeminal notch opposite the trigeminal impression. In this situation the roots of the trigeminal nerve intervene between the superior petrosal sinus and the bone. A tiny spicule marks the anterior extremity of the notch and gives attachment to the lower end of the petrosphenoidal ligament. The abducent nerve extends forwards sharply across the upper border of the temporal bone immediately in front of this bony spicule, and so lies between the petrosphenoidal ligament and the side of the dorsum sellae.

Figure 329
Horizontal section through the nasal and orbital cavities superior view - Figure 329

 


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