THE COMMON CAROTID All I V /.'//> 579 



brunches ore not constant, and arc most frequently met with in the Ass. 

 They begin at the basilur trunk, in front of the posterior border of 

 tin- annular protuberance, traverse the dura mater to enter the cavernous 

 sinus, and join the carotid arteries at their second curvature. 



POSTERIOR CEREBRAL ARTERIES. These terminate the basilar trunk, and 

 separate into right and left of it, behind the pisiform tubercle (Fig. 285, 11). 

 They first proceed forward, receiving posterior communications, then pass 

 outward ana upward, to turn round the cerebral peduncles and reach Bichat's 

 tissnro. On their course, they furnish a multitude of hair-like twigs that enter 

 the substance of the peduncles ; but the principal branches they give off are 

 lli xuuus, ami directed either towards the great cerebral fissure, where they 

 terminate, to the posterior extremity of the hemisphere of the cerebrum, or its 

 interior, to the plexus chorides more particularly, or even to the cerebellum. 

 The disposition and number of these branches are very variable. 



MEDIAN SPINAL AUTERY. A very long vessel, lodged in the inferior 

 li-sure of the spinal cord, and measuring the whole extent of that organ, 

 which it follows from before to behind. It is from this artery that are 

 given off the branches which cover with their arborisations the medullary 

 tissue, or penetrate its substance. This emission, which ought soon to 

 exhaust the artery, does not sensibly diminish its diameter, as it receives on 

 both sides, during its course, numerous additional filaments. Two series of 

 ramuscules, in fact, emanate either from the vertebral, intercostal, lumbar, 

 or sacral arteries, and enter the spinal canal by the intervertebral foramina, 

 and go to join this artery. Generally, however, they do not pass to the 

 spinal cord until they have anastomosed with each other outside the dura 

 mater, so as to form on the floor of the vertebral canal two lateral conduits 

 placed beside the venous sinus, and united by transverse anastomoses ; this 

 disposition is most evident in the cervical region of the Ox (Fig. 288). 



INTERNAL CAROTID ARTERY. (FigS. 285, 8 ; 286, 5.) 



One of the terminal branches of the common carotid, the internal carotid 



.1- at first to beneath the base of the cranium, outside the ai ; 

 rectus musel.-s of the head, and bends forward to reach the lae.-rat.-il 

 foramen. In this primary portion of its course, it is suspended in a j^ar- 

 tieular fold of the guttural pouch, margined by the superior cervical 

 ganglion, accompanied by the cavernous branch of the sympathetic nerve, 

 and crossed in various directions by the nerves which form the guttural 

 1*1 \ns. On arriving at the middle of the occipito-spheno-temporal-hiatns, 

 it enters the cavernous sinus, and in the interior of that cavity, where it i 

 bathed in venous blood, describes two successive and opposite curvatures; 

 the lir.-t looking forwards, occupies the carotid fossa of the sphenoid bone: 

 :.!, with its convexity posterior, at which tho internal carotid 

 r- -rives an anastomosing branch from the basilur trunk: which branch is 

 voluminous and nearly constant in tho Ass; but is rare and, when present, 

 very slender in the Horse. After the last inflexion, the two internal < ;M 

 rommmiicato by a very large transverse branch, which is always flexuous. 



ulat.-.l. and leave tho cavernous sinus in crossing the dura m 

 in the ernuial cavity. 



.ire then placed at tho sides of the pituitary gland, within 

 tho superior maxillary nerve, proceed from In-hind forward, and ; 

 in two branehi s l.t-foro reaching the optic nerve. One of these branches 



2 p 2 



