546 BLOOD-VASCULAR SYSTEM OF THE HORSE 



artery, and the spinal branch similarly forms by union with its fellow the middle 

 spinal artery. 



The basilar artery (A. basilaris cerebri) passes forward in the median groove 

 on the ventral surface of the medulla and pons and divides into the two posterior 

 cerebral arteries. The collateral branches of the basilar are: 



1. Medullary branches (Rami medullares), ten or twelve in number, dis- 

 tributed to the medulla oblongata. 



2. Posterior cerebellar arteries (Aa. cerebelli caudales) which pass outward 

 around the medulla behind the pons to the cerebellum, to which they are distributed 

 after giving twigs to the medulla and pons. 



3. The small auditory artery (A. auditiva interna) accompanies the eighth 

 nerve to the internal ear. It often arises from the posterior cerebellar. 



4. Anterior cerebellar arteries (Aa. cerebelli nasales). These are very variable 

 in number and origin. There are often two or three on either side and they fre- 

 quently arise from the posterior cerebrals. They pass outward in front of the 

 pons and supply the anterior part of the cerebellum. 



The posterior cerebral arteries (Aa. communicantes caudales) diverge at an 

 acute angle and join the posterior communicating branches of the internal carotid 

 arteries on the inferior surface of the cerebral peduncles. They are connected by a 

 transverse branch and by a network of fine twigs which form often a rete mirabile. 



THE INTERNAL CAROTID ARTERY (Figs. 434, 436, 437) 



This artery (A. carotis interna) is somewhat smaller than the occipital. It 

 usually arises just behind that artery, crosses its deep face, and runs upward and 

 forward on the guttural pouch to the foramen lacerum. It is closely related to 

 the vagus nerve and the superior cervical ganglion of the sympathetic nerve, fibers 

 from which accompany it. It is crossed externally by the ninth and twelfth 

 cranial nerves and the pharyngeal branch of the vagus. It passes through the 

 inferior petrosal sinus and enters the cavernous sinus, within which it forms an 

 S-shaped curve. It is connected with the opposite artery by a transverse branch 

 (A. intercarotica), which lies behind the pituitary body in the intercavernous 

 sinus. A branch (A. caroticobasilaris) sometimes connects it with the basilar artery. 

 It then perforates the dura mater, gives off the posterior commimicating branch, 

 and passes forward and divides at the side of the optic chiasma into anterior 

 and middle cerebral arteries. 



The posterior communicating artery (A. communicans posterior) turns back- 

 ward and joins the posterior cerebral. It gives off the deep cerebral artery (A. 

 cerebri profunda) which winds around the cerebral peduncle and is distributed 

 chiefly to the mid-brain. A smaller collateral branch is the anterior choroid 

 artery (A. chorioidea nasalis) which passes along the optic tract and is distributed 

 in the choroid plexus of the lateral ventricle. 



The anterior cerebral artery (A. cerebri anterior) unites with the corresponding 

 branch of the opposite artery above the optic chiasma. From this junction pro- 

 ceeds the artery of the corpus callosum (A. corporis callosi), which turns around the 

 genu of the corpus callosum, enters the great longitudinal fissure, divides into two 

 branches, and is distributed to the inner aspect of the cerebral hemispheres. A 

 small anterior meningeal branch (A. meningea nasalis) of the anterior cerebral is 

 distributed to the anterior part of th(^ dura, and assists in forming a network 

 in the ethmoidal fossa (Rete ethmoidale), anastomosing with the ethmoidal branch 

 of the ophthalmic artery. 



The middle cerebral artery (A. cerebri media) passes outward in the lateral 

 fissure (of Sylvius) and divides into branches on the outer surface of the hemisphere. 



The arterial circle of Willis (Circulus arteriosus) (Fig. 434) is formed at the 



