THE COMMON CAROTID ARTERIES. 665 



course, numerous additional vessels. Two series of ramuscules, in fact, emanate 

 either from the vertebral, intercostal, lumbar, or sacral arteries, enter the spinal 

 canal by the intervertebral foramina, and 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 vessels placed beside the venous sinuses, and united by transverse anasto- 

 moses ; this disposition is most evident in the cervical region of the Ox 

 (Fig. 382). 



Internal Carotid Artery (Figs. 380, 7 ; 381, 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 straight 

 muscles of the head, and bends forward to reach the foramen lacerum basis 

 cranii. In this primary portion of its course, it is suspended in a particular 

 fold of the guttural pouch, margined by the superior cervical ganglion, ac- 

 companied by the cavernous branch of the sympathetic nerve, and crossed in 

 various directions by the nerves that 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 in the sphenoid bone ; the second, with its convexity posterior, at 

 which the internal carotid receives an anastomosing branch from the basilar 

 artery — which branch is voluminous and nearly constant in the Ass; but is 

 rare and, when present, very slender in the Horse. After the last inflection, 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 constitutes 

 the posfcrior communicating artery ; the other soon bifurcates to form the middle 

 and anterior cerebral arteries. 



Posterior Cerebral Artery. — This vessel is inflected from before to 

 behind, to one side of the pituitary gland (Fig. 380, 14), and anastomoses behind 

 it, forming on the crura an arch which is often reticulated, and which receives 

 the basilar artery in the middle of its convexity. 



A multitude of hair-like ramuscules escape from this artery and enter the 

 substance of the crura ; but the principal branches it emits proceed in a flexuous 

 manner inwards and backwards, towards the great cerebral fissure, and terminate 

 either on the posterior extremity of the cerebral hemisphere or in its interior — 

 in the plexus choroides, or even in the cerebellum. The disposition and number 

 of these branches are very variable, but there is one which may be regarded as 

 constant ; this is the largest of all, and certainly merits the designation of 

 posterior cerebral artery (Fig. 380, 11). 



Middle Cerebral Artery. — This vessel separates itself from the anterior 

 cerebral artery, external to the optic commissure, is lodged in the fissure of 

 Sylvius, passing through it in a flexuous manner, and at its extremity separates 

 into several branches which ramify on the lateral and superior faces of the brain, 

 and anastomose by their terminal divisions with the posterior and anterior 

 cerebral arteries. 



