POSTERIOR CRANIAL FOSSA (Figs. 327, 328)
The posterior fossa is the largest and deepest of the three cranial fossae. It is bounded in front by the dorsum sellae and clivus of the sphenoid and the basilar part of the occipital bone ; behind, by the lower portion of the squamous part of the occipital bone; on each side, by the petrous and mastoid parts of the temporal bone, the condylar (lateral) part of the occipital bone and, above and behind, by a small part of the posterior inferior angle of the parietal bone. It contains the cerebellum behind and the pons and medulla oblongata in front.
The foramen magnum lies in the floor of the fossa within the occipital bone. Its circumference is formed by the basilar part in front, by the condylar (lateral) part on each side and by a small portion of the squamous part behind. Just in front of its transverse diameter it is encroached on by the irregular medial aspects of the occipital condyles, so that it is somewhat ovoid in shape and is wider behind than in front. Its narrower, anterior part lies above the odontoid process (dens) of the axis vertebra; its wider posterior part communicates below with the vertebral canal, and through it the medulla oblongata passes down to become continuous with the spinal cord.
In front of the foramen magnum the basilar part of the occipital bone and the clivus of the sphenoid bone form a sloping surface, gently concave from side to side, to support the medulla oblongata below and the pons above. On each side this area is separated from the petrous part of the temporal bone by the petro-occipital fissure, which is occupied in the recent state by a thin plate of cartilage. The fissure is limited behind by the jugular foramen, and its margins are grooved by the inferior petrosaI sinus.
The jugular foramen separates the petrous part of the temporal bone from the condylar (lateral) part of the occipital bone, and leads forwards, downwards and laterally to the external surface of the base. Its upper border is sharp and irregular and presents a notch for the glossopharyngeal nerve. Its lower border is smooth and regular. The posterior part of the foramen transmits the sigmoid sinus, which is continuous below with the internal jugular vein. In front of the vein the accessory, vagus and glossopharyngeal nerves, in that order from behind forwards, traverse the foramen to gain the upper part of the neck. The most anterior part of the foramen transmits the inferior petrosal sinus.
Medial to the lower border of the jugular foramen a rounded elevation, termed the jugular tubercle, marks the condylar part of the occipital bone. It lies above and somewhat in front of the inner opening of the anterior condylar (hypoglossal) canal, which pierces the bone at the junction of the basilar with the condylar part and transmits the hypoglossal nerve.
The posterior surface of the petrous part of the temporal bone forms a large portion of the lateral (or anterolateral) wall of the posterior fossa, Above the anterior part of the jugular foramen it is pierced by the internal auditory meatus (fig. 328), which runs transversely in a lateral direction. It is a short passage, about 1 cm. long, closed laterally by a perforated plate of bone which separates it from the internal ear. It transmits the facial and auditory nerves.
Behind the petrous temporal the lateral wall of the posterior cranial fossa is formed by the mastoid part of the temporal bone. Anteriorly it is marked by a wide groove, which runs forwards and downwards, then downwards and medially and finally forwards to the posterior limit of the jugular foramen. This groove contains the sigmoid sinus and i5 termed the sigmoid sulcus (fig. 328). At its upper end, where it touches the posterio-inferior angle of the parietal bone, the groove is continuous with the groove for the transverse sinus and crosses the parietomastoid suture. As it descends, it lies behind the tympanic antrum and forms a very important relation of that structure. In this part of its course the large mastoid foramen opens near its posterior margin and transmits an emissary vein from the sinus. In its lowest part the sigmoid sulcus crosses the occipitomastoid suture and grooves the jugular process of the occipital bone. It is usually deeper on the right than on the left side.
Behind the foramen magnum the squamous part of the occipital bone is marked in or near the median plane by the internal occipital crest, which ends above and behind in an irregular elevation, named the internal occipital protuberance. On each side of the protuberance a wide shallow groove curves laterally with a slight upward convexity to the postern-inferior angle of the parietal bone. It is produced by the transverse sinus, is usually deeper on the right side and at its lateral extremity is continuous with the groove for the sigmoid sinus. Below the groove for the transverse sinus the internal occipital crest divides the bone into two gently hollowed fosssc, which lodge the cerebellar hemispheres.
When the posterior condylar canal is present (fig. 346), its inner orifice usually lies behind and lateral to the orifice of the anterior condylar (hypoglossal) canal, but it may lie nearer to the jugular foramen. It transmits an emissary vein from the lower end of the sigmoid sinus.
Particular features.-The clivus of the sphenoid and the basilar part of the occipital bone are related to the basilar plexus of veins, which connects the two inferior petrosal sinuses and communicates below with the internal vertebral venous plexus. A little in front of the foramen magnum the membrana tectoria is attached to the basilar part of the occipital bone (fig. 617), covering the attachment of the apical ligament of the odontoid process. The jugular tubercle is often grooved by the glossopharyngeal, vagus and accessory nerves, as they pass to the jugular foramen. In addition to the hypoglossal nerve the anterior condylar (hypoglossal) canal transmits a meningeal branch of the ascending pharyngeal artery. The canal is often subdivided into two parts by a small bar of bone, and this may be related to the composite origin of the hypoglossal nerve (p. 1014). The roughened medial aspect of the occipital condyle (fig. 345) gives attachment to the alar ligament.
The lower and posterior borders of the jugular foramen are smooth and regular, but its upper border is sharp and interrupted by a notch, the ends of which may succeed in dividing the foramen into two or sometimes three compartments. The notch-lodges the inferior (petrom) ganglion of the glossopharyngeal nerve and at its deepest part is pierced by the cochlear canaliculus, which contains the aqueduct of the cochlea.
The internal auditory meatus transmits the auditory nerve, both the motor and the sensory roots of the facial nerve and the internal auditory vessels. It is about 1 cm in length and its fundus is separated from the internal ear by a vertical plate, which is divided into two unequal portions by a transverse crest (fig. 330). Above the crest interiorly the bone is pierced by the facial canal, which conducts the facial nerve through the petrous temporal to the stylo-mastoid foramen. Behind the opening of the facial canal there is a small depression, termed the superior vestibular area., which is perforated by a number of small openings for the passage of the nerves to the utricle and the superior and lateral semi circular ducts. Below the transverse crest anteriorly lies the cochlear area, in which a number of small, spirally arranged openings encircle the central canal of the cochlea and constitute the tractus spiralis foraminosus. Behind the cochlear area the inferior vestibular area presents several openings for the nerves to the saccule. Below and behind the inferior vestibular area the foramen singular gives passage to the nerve to the posterior semicircular duct.
In addition to an emissary vein the mastoid foramen transmits meningeal branch of the occipital artery, which is sometimes large enough to produce a groove on the squamous part of the occipital bone.
The internal occipital crest gives attachment to the falx cerebelli and may be grooved by the occipital sinus, which is sometimes duplicated and occasionally forms a large vessel. Its lower end is related to the inferior vermis of the cerebellum. The internal occipital protuerance is related to the confluence of sinuses and is grooved on each side by the commencement of, the transverse sinus. The margins of the groove for the transverse sinus give attachment to the two layers of the tentorium cerebelli. Traced laterally the groove reaches the lowest part of the posterior inferior angle of the parietal bone, where it becomes continuous with the sigmoid groove. On each side of the internal occipital crest the bone is thin, and translucent, in marked contrast to the regions of the crest and of the internal occipital protuberance.
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