774 THE VASCULAR SYSTEM. 
posterior spinal arteries ; they supply the bodies of the vertebree and the intervertebral 
discs, and they anastomose with corresponding arteries above and below. 
From the third part.—(1) Muscular branches to the sub-occipital muscles. 
(2) Anastomotic branches which unite with the princeps cervicis branch of the ocei- 
pital and with the profunda cervicis artery. 
From the fourth part.—(1) Meningeal (rami meningei).—One or two small branches 
given off before the vertebral artery pierces the dura mater. They ascend into the pos- 
terior fossa of the skull, where they anastomose with meningeal branches of the occipital 
and ascending pharyngeal arteries, and occasionally with branches of the middle meningeal 
artery. 
(2) Posterior Spinal (a. spinalis posterior).—The posterior spinal branch springs from 
the vertebral directly after it has pierced the dura mater. It runs downwards upon the 
Anterior communicating artery 
Anterior cerebral artery 
4 
3rd cranial nerve, 4 yy, q ‘\ p = ) \ Middle 
pF \ ) ‘ cerebral artery 
4th cranial Internal 
nerve —carotid artery 
Anterior 
chorioidal 
artery 
5th cranial Posterior com- 
nerve municating 
artery 
I Posterior 
6th eranial cerebral artery 
nerve A 
Superior cere- 
bellar artery 
Transverse 
pontine 
arteries 
7th eranial | 
nerve 
8th cranial 
nerve Basilar artery 
Anterior 
—_ inferior cere- 
9th eranial bellar artery 
nerve 
Posterior inferior 
10th cranial nerve 
cerebellar artery 
11th cranial nerve 
Vertebral artery 
. / \ . . 
12th cranial nerve Aniterior spinal artery 
Fic. 559.—THE ARTERIES OF THE BASE OF THE BRAIN. ‘THE CIRCLE OF WILLIS. 
side of the medulla and the spinal cord in front of the posterior nerve roots. It is a 
slender artery, which is continued to the lower part of the cord by means of reinforce- 
ments from the spinal branches of the vertebral and intercostal arteries. It gives off 
branches to the pia mater, which form more or less regular anastomoses on the inner 
sides of the posterior nerve roots, and it terminates below by joining the anterior spinal 
artery. 
(3) The anterior spinal branch (a. spinalis anterior) arises near the termination of the 
vertebral. It runs obliquely downwards and inwards, in front of the medulla, and unites 
with its fellow of the opposite side to form a single anterior spinal artery, which descends 
in front of the anterior fissure of the spinal cord, and is continued as a fine vessel along 
the filam terminale. The anterior spinal artery is reinforced as it descends by anastomos- 
