194 CIRCULATORY SYSTEM. 



the vertebral artery, as soon as the latter has emerged from the 

 foramen in that bone. From about the middle of the arch 

 comes off 



The occipital artkry, a vessel nearly equal in magnitude 

 to the trunk itself, which pursues a flexuous course to the occiput, 

 first ascending upon the coronoid process, and then climbing the 

 occipital ridge to reach the vertex. Its branches are — a. A long, 

 slender, deep-seated one, that mounts upward and enters the cra- 

 nium through the foramen lacerum to be dispersed upon the dura 

 mater, though this is sometimes a branch from the anastomotic 

 trunk itself, b. Twigs forwarded to the temporal muscle, c. 

 Terminating I'amifications to the straight and oblique muscles of 

 the occiput. 



The third division of the carotid is the 



Internal Carotid Artery. 

 This vessel, whose caliber is not more than half that of the 

 external carotid, is, originally, deeply lodged within the submax- 

 illary space, whence it ascends to the base of the skull, crossing 

 the upper extremity of the cornu of the os hyoides, inwardly, in 

 its course, which is rendered remarkable by several tortuous turns : 

 it first curves backward, next inward, then upward, and lastly 

 forward, to reach the anterior part of the foramen lacerum, through 

 which it enters the cavity of the cranium. It is accompanied in 

 its way by one of the principal branches of the jugular vein, by 

 the eighth pair of nerves, and by the sympathetic nerve. At its 

 entrance into the skull, we find the artery lodged within the 

 cavernous sinus, wherein it makes two more turns, one forward, 

 the other inward, from which last flexure comes off a vessel 

 named the 



1. Arteria coMMUNiCANS. This runs in a direction up- 

 ward and backward, passing under the crura cerebri, to join the 

 basilar, with which it makes one continued vessel, thus forming 

 one side of the circulus arteriosus — to be presently described. 

 After having given off this branch, the internal carotid pierces the 

 dura matter, and continues its course upward, alongside of the 

 optic nerve, and immediately over the optic decussation splits into 

 four principal divisions. At the point of division however, or prior 

 to it, are sent off — 2. Two or three long branches of small size, 

 which run forward and spread their ramifications upon the anterior 

 and inferior portions of the dura mater. 



Its four divisions or remaining branches are — 

 3. The anterior cerebral; which advances by the side 

 of the optic nerve, and in front of its decussation transmits a con- 

 siderable branch across, which unites with a similar one coming 



