34 TOPOGRAPHIC AND APPLIED ANATOMY. 



FIG. 10. The Gasserian ganglion (ganglion semilunare) which has been exposed in the middle cerebral fossa by 

 the removal of the dura mater. 



FIG. ii. Frontal section in the region of the cavernous sinus. 



which form the circle of Willis, situated in the subarachnoidean space at the base of the brain in 

 the region of the interpeduncular space (see Plate 3). 



The vertebral artery, soon after its origin from the subclavian (see Fig. 27), enters the foramen 

 in the transverse process of the sixth cervical vertebra, passes through the transverse processes of 

 all of the overlying cervical vertebras, runs toward the median line in the groove upon the upper 

 surface of the posterior arch of the atlas, perforates the posterior occipito-atlantal ligament and 

 the dura, and reaches the anterior surface of the medulla oblongata, at the superior boundary of 

 which it unites with the vessel of the opposite side to form the basilar artery. This artery lies in 

 the basilar groove of the pons, and at its superior margin divides into two terminal branches, the 

 posterior cerebral arteries. 



The internal carotid artery, commencing at the bifurcation of the common carotid at the 

 level of the upper margin of the thyroid cartilage, gives off no cervical branches, but ascends to the 

 base of the skull along the lateral wall of the pharynx. It is separated from the external carotid 

 artery by the styloglossus and stylopharyngeus muscles. It pursues a curved course through 

 the carotid canal of the petrous portion of the temporal bone (see Fig. 23), the concavity of the 

 curve being directed anteriorly. This leads us to remark that fatal hemorrhage has been ob- 

 served from the internal carotid artery in cases of caries of the petrous portion of the temporal 

 bone. After leaving the carotid canal the vessel passes across the middle lacerated foramen in 

 the carotid sulcus at the side of the body of the sphenoid bone and is enclosed in the cavernous 

 sinus. It then ascends to the optic foramen, where it describes a short curve, convex forwards, 

 from which is given off the only branch not supplying the brain, the ophthalmic artery, which, 

 together with the optic nerve, enters the orbital cavity through the optic foramen. After pene- 

 trating the dura, the internal carotid artery reaches the base of the brain at the angle between the 

 optic nerve and the optic tract, where it divides into two main branches, the anterior and 

 middle cerebral arteries. 



The chief branches of the vertebral and basilar arteries which supply the brain are: 



1. The posterior inferior cerebellar artery, from the vertebral to the lower surface of the 

 cerebellum. 



2. The anterior inferior cerebellar artery, from the basilar to the lower surface of the cere- 

 bellum. 



3. The superior cerebellar, which arises from the basilar at the anterior margin of the pons 

 and ramifies upon the superior surface of the cerebellum. 



4. The posterior cerebral arteries, the two terminal branches of the basilar. This vessel is 

 separated from the superior cerebellar at its origin by the oculomotor nerve and supplies the occip- 

 ital lobe and the greater portion of the temporal lobe. It is connected with the internal carotid 

 by the posterior communicating artery. 



5. The internal auditory artery, which reaches the internal ear through the internal auditory 

 meatus. 



The vertebral artery also gives off lateral spinal branches in the neck which pass into the 



