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. cerebelli 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 joi 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 remforced 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, 56f, 562) 
This artery (A. earotis 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 $-shaped 
curve. It is connected with 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 Also termed the retrograde or anastomotic branch. 
° Also termed the musculo-occipital. 
° Not rarely it arises with the occipital artery from a common trunk of variable length. 
| 
| 
| 
