THE COMMON CAROTID ARTERIES. 579 



branches are not constant, and are most frequently met with in the Ass. 

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

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

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



POSTERIOR CEREBRAL ARTERIE'S. 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 and upward, to turn round the cerebral peduncles and reach Bichat's 

 fissure. 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 

 flexuous, and directed either to\vards 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 ARTERY. A very long vessel, lodged in the inferior 

 fissure 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 

 ascends at first to beneath the base of the cranium, outside the anterior 

 rectns muscles of the head, and bends forward to reach the lacerated 

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

 ticular 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 

 plexus. On arriving at the middle of the occipito-spheno-temporal hiatus, 

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

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

 the first looking forwards, occupies the carotid fossa of the sphenoid bone : 

 the second, with its convexity posterior, at which the internal carotid 

 receives an anastomosing branch from the basilar trunk : which branch is 

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

 very slender in the Horse. After the last inflexion, the two internal carotids 

 communicate by a very large transverse branch, which is always flexuous, 

 often reticulated, and leave the cavernous sinus in crossing the dura mater, 

 to gain the cranial cavity. 



These arteries are then placed at the sides of the pituitary gland, within 

 the superior maxillary nerve, proceed from behind forward, and terminate 

 in two branches before reaching the optic nerve. One of these branches 



