

VERTEBRAL ARTERY. 907 



(2) Spinal branches pass from the medial side of the second part of the vertebral 

 artery, through the intervertebral foramina, into the vertebral canal, where they give off 

 twigs which pass along the roots of the spinal nerves to reinforce the anterior and posterior 

 spinal arteries; they supply the bodies of the vertebrae and the intervertebral fibro- 

 cartilages, 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 descending branch (O.T. princeps 

 cervicis) of the occipital and with the prof unda cervicis artery. 



From the fourth part. (1) Meningeal. One or two small branches given off before 

 the vertebral artery pierces the dura mater. They ascend into the posterior 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. The posterior spinal branch springs most commonly from the 

 posterior inferior cerebellar branch of the vertebral (Stopford, 1916), but occasionally it 

 arises from the vertebral directly. It runs downwards upon the side of the medulla 

 oblongata and the spinal medulla, either in front of or behind the posterior nerve-roots. It is 

 a slender artery, which is continued to the lower part of the spinal medulla by means of 

 reinforcements 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 medial and 

 lateral sides of the posterior nerve-roots, and it ends by joining the anterior spinal artery. 



(3) The anterior spinal branch arises near the termination of the vertebral. It runs 

 obliquely downwards and medially, in front of the medulla oblongata, and unites with its 

 fellow of the opposite side to form a single anterior spinal artery, which descends along 

 the anterior median fissure -of the spinal medulla, and is continued as a fine vessel along 

 the filum terminale. The anterior spinal artery is reinforced as it descends by anasto- 

 mosing twigs from the spinal branches of the vertebral, intercostal, and lumbar arteries. 

 It gives off branches which pierce the pia mater and supply the spinal medulla, and it 

 unites below with the posterior spinal arteries. 



(4) The posterior inferior cerebellar is the largest branch of the vertebral artery. 

 It arises a short distance below the pons and passes obliquely backwards round the 

 medulla oblongata, at first between the fila of the hypoglossal nerve, and then between the 

 fila of the accessory and vagus nerves, into the vallecula of the cerebellum, where it divides 

 into lateral and medial terminal branches. 



The trunk of the artery gives branches to the medulla oblongata and to the chorioid 

 plexus of the fourth ventricle. Some of these branches supply the nuclei of the glosso- 

 pharyngeal, the vagus, and the accessory nerves, the spino-thalamic, spino-cerebellar, rubro- 

 spinal, olivo-cerebellar tracts, and possibly also the vestibular root of the acoustic and the 

 spinal root of the fifth nerve (Bury and Stopford). The medial terminal runs backwards 

 between the inferior vermis and the hemisphere of the cerebellum ; it supplies the former 

 structure, and anastomoses with its fellow of the opposite side. The lateral branch passes 

 to the lower surface of the hemisphere and anastomoses with the superior cerebellar artery. 



Arteria Basilaris. The basilar artery is formed by the junction of the two 

 vertebral arteries; it commences at the lower border and terminates at the upper 

 border of the pons (Varolii), bifurcating at its termination into the two posterior 

 cerebral arteries. 



Course and Relations. It runs upwards, in the median part of the cisterna 

 pontis, in a shallow groove on the front of the pons, behind the sphenoidal section 

 of the basi-cranial axis and between the two abducent nerves. 



Branches. (1) Pontine, a series of small arteries which pass across the front and 

 round the sides of the pons, supplying the pons, the brachia pontis (O.T. middle peduncles 

 <>f the cerebellum), and the roots of the trigeminal nerve. 



(2) The internal auditory, a pair of long slender branches. Each internal auditory 

 branch may spring either from the basilar or from the 'anterior inferior cerebellar artery 

 of the same side (Stopford, 1916). It enters the corresponding internal acoustic meatus 

 with the facial and acoustic nerves, and, after it has passed through the lamina cribrosa, 

 it is distributed to the internal ear. 



(3) The anterior inferior cerebellar, two branches which arise, one on each side, from 

 the middle of the basilar artery. They pass backwards, on the anterior parts of the 

 lower surfaces of the lateral lobes of the cerebellum, and anastomose with the posterior 

 inferior cerebellar branches of the vertebral arteries. 



(4) The superior cerebellar branches, two in number, arise near the termination of the 

 basilar. Each passes laterally, at the upper border of the pons, directly below the 



