640 BLOOD-VASCULAR SYSTEM OF THE HORSE 



branch (Ramus recurrens)^ of the occipital passes up through the foramen trans- 

 versarium of the atlas and joins the vertebral artery. It gives branches to the 

 obliquus capitis posterior, which covers it. 



The anterior or occipital branch (Ramus occipitalis)^ passes through* the alar 

 foramen of the atlas and supplies the muscles and skin of the poll, anastomosing 

 with the deep cervical artery and its fellow of the opposite side. , In the alar furrow 

 it gives off the cerebrospinal artery (A. cerebrospinalis) , which enters the spinal 

 canal through the intervertebral foramen of the atlas, perforates the dura mater, 

 and divides into cerebral and spinal branches. The cerebral branch (Ramus 

 cerebralis) unites with that of the opposite side to form the basilar artery, and the 

 spinal branch (Ramus spinalis) similarly forms by union with its fellow the ventral 

 spinal artery. 



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

 on the ventral surface of the medulla oblongata 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 aborales), 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. cereljelli orales). 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 cerebral. They pass outward in front of the pons 

 and supply the anterior part of the cerebellum. 



The posterior cerebral arteries (Aa. cerebri aborales) diverge at an acute angle 

 and join the posterior communicating branches of the internal carotid arteries on 

 the ventral 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 ventral spinal artery (A. spinalis ventralis) runs along the ventral median 

 fissure of the spinal cord, which it supplies. It is reinforced along its course by 

 branches from the vertebral, intercostal, lumbar, and lateral sacral arteries, which 

 enter the vertebral canal through the intervertebral foramina. 



THE INTERNAL CAROTID ARTERY (Figs. 559, 561, 562) 



This artery (A. carotis interna) is usually 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 anterior cervical ganglion of the sympathetic nerve, fibers 

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

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

 petrosal sinus and enters the cavernous sinus, within which it forms an S-shaped 

 curve. It is connected Avith the opposite artery by a transverse branch, the inter- 

 carotid artery (A. intercarotica), which lies in the intercavernous sinus behind the 

 pituitary body. A branch (A. caroticobasilaris) often connects it with the basilar 

 artery. It then perforates the dura mater, gives off the posterior communicating 

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

 and middle cerebral arteries. 



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



1 -Vlso termed the retrograde or anastomotic branch. 



' Also termed the musculo-occipital. 



' Xot rarely it arises with the occipital artery from a common trunk of variable length. 



