32 TOPOGRAPHIC AND APPLIED ANATOMY. 



FIG. 9. The base of the skull with the cranial nerves, as seen after the removal of the brain. Upon the left side 

 the dura inater has been left in place and the tentorium cerebelli incised so that it is possible to look into the posterior 

 cerebral fossa. Upon the right side the dura has been removed and the places of exit of the nerves are visible at the 

 base of the skull. 



nections between the cavernous sinuses; the spheno parietal sinus (sinus alae parvae), running 

 along the lesser wing of the sphenoid, ascending behind the coronal suture with the anterior 

 branch of the middle meningeal artery, and occasionally imbedded in a very deep bony groove, 

 to reach the superior longitudinal sinus; and the occipital sinus, in the attached border of the 

 falx cerebelli. 



In addition to the cerebral veins, the sinuses also receive the diploic veins, the internal 

 auditory veins from the internal auditory meatus, the meningeal veins, the superior ophthalmic 

 veins, and the emissary veins (see page 22). 



Nerves. After the removal of the brain, which is necessarily preceded by a division of the 

 cranial nerves, the latter structures may be observed while they are still within the dura mater 

 at the base of the skull (see Fig. 9). A number of them, the optic, the facial, and the auditory 

 nerves, for example, disappear at the same place at which they leave the cranial cavity. Others, 

 however, pass into or through the dura and pursue an intradural or a subdural course for a vary- 

 ing distance before they leave the skull. This is particularly the case with the nerves in the 

 middle cranial fossa, which on account of their course are therefore most likely to be affected by 

 fractures of the base (see page 23). The oculomotor and the abducent nerves, for example, per- 

 forate the dura at the anterior extremity of the tentorium, but do not leave the cranial cavity until 

 they reach the sphenoidal fissure. The abducent nerve perforates the dura upon the clivus, but 

 does not leave the skull until it reaches the sphenoidal fissure, consequently having the longest 

 subdural course of any of the cranial nerves. The trunk of the trifacial nerve disappears beneath 

 the edge of the tentorium in the posterior cranial fossa and forms the semilunar ganglion, which 

 is lodged in a depression near the apex of the petrous portion of the temporal bone in the middle 

 cranial fossa. This ganglion is covered by the dura mater, and the three branches of the tri- 

 facial cannot be dissected until the dura has been removed (see Figs. 9 and 10). When very 

 obstinate trifacial neuralgia makes it necessary to attack the ganglion through the temporal 

 region, carefully elevating the temporal lobes of the cerebrum, the structures most endangered, 

 in addition to the middle meningeal artery (which may be ligated), are the cavernous sinus and 

 the carotid artery. The extirpation of the ganglion is followed not only by sensory changes, but 

 also by a unilateral paralysis of the muscles of mastication, since the small motor root in intimate 

 relation with the ganglion is also removed. Unpleasant symptoms of cerebral compression are 

 not infrequently observed after the operation. 



The relations of the nerves in the middle cranial fossa to the cavernous sinus and to the 

 internal carotid artery are best shown by a frontal section (see Fig. u). 



The abducent nerve lies upon the internal carotid within the sinus, while the illustration 

 distinctly shows the oculomotor, the trochlear, and the first and second divisions (ophthalmic and 

 superior maxillary) of the trifacial within the wall of the sinus. 



Fractures or caries of the base of the skull may involve one or more of the cranial nerves, and 

 the symptoms produced by the affected nerve may furnish conclusions as to the site of the lesion. 



