578 TSE ABTEEIE8. 



atlas, places itself beneath the atlo-axoid muscle, and in a flexuous manner 

 advances to meet the vertebral artery, which it directly joins, after giving 

 off some branches to the great oblique and neighbouring muscles. This 

 anastomosis is the means of establishing a collateral communication between 

 the vertebral artery, and the divisions furnished by the common carotid ; so 

 that these two arteries can mutually assist or supplant each other. 



Terminal Bkanohes. — 1. Occipito-musoular Aeteet (Fig. 286, 10). — 

 Covered at its origin by the great oblique muscle, the occipito-mu- uu,r 

 artery is directed transversely inwards to the surface of the posterior 

 straight (recti) muscles, and soon separates into several branches — ascending 

 and descending — mixed with the nervous divisions of the first superior 

 cervical branch, all of vyhich are destined to the muscles and integuments of 

 the occipital region. The descending branches anastomose with the ter- 

 minal divisions of the superior cervical artery. 



2. Cekebbo-spinal Aeteet. — This vessel enters the spinal canal by the 

 anterior foramen of the atlas, traverses the dura mater, and divides into two 

 branches on the inferior face of the spinal cord. Of these two branches, the 

 anterior is united, by convergence, with the analogous branch of tl 

 opposite artery on its arrival at the middle of the length of the bulb (medulla 

 oblongata), and so forms the basilar trunk ; the other passes backwards, and 

 constitutes the origin of the median spinal artery, by anastomosing, after a 

 short course, with the corresponding branch of the other cerebro-spinal 

 artery. There results from this distribution a kind of vascular lozenge, 

 situated at the lower face of the medulla oblongata, which receives in its 

 middle the two cerebro-spinal arteries. This regular arrangement is not, 

 however, always observed ; these arteries may unite at the posterior extremity 

 of this lozenge, as is shown in figure 285. 



Basilar trunk. — This is a single vessel which creeps in a somewhat 

 flexuous manner on the inferior face of the medulla oblongata, beneath the 

 visceral arachnoid membrane, and passing over the annular protuberance 

 (pons Varolii), terminates at the anterior border of this portion of the 

 encephalic isthmus, by anastomosing with the two posterior cerebral arteries 

 (Fig. 285, 11, 11). 



On its course it gives off: 



1. A multitude of plexuous ramuscules, which enter the substance of the 

 medulla oblongata and the pons Varolii, or are distributed to the roots of the 

 nerves emanating from the medulla oblongata. 



2. ThQ posterior cerebellar arteries, vessels liable to numerous anomalies 

 in their origin ; they usually arise from the basilar trunk at a right angle, 

 behind the posterior border of the pons Varolii, and bend outwards, one to 

 the right, the other to the left, by gliding along the surface of the bulb 

 (meduUa oblongata), whose external border it thus reaches, and is then 

 inflected backwards beneath the cerebellar plexus choroides, whence they 

 spread their ramifications on the lateral and posterior parts of the cere- 

 bellum. 



3. The anterior cerebellar arteries, two or three on each side, only one of 



which IS constant. These vessels are very variable in their disposition, and 



arise from the terminal extremity of the basilar trunk, in front of the pons 



\arolii,and sometimes even from the posterior cerebral arteries. Usually 



united in fasciculi, they are directed outwards and a little backwards ra. 



turning round the cerebral pedunculi, and plunge into the anterior part of 

 the cerebellum. 



4. Two branches anastomosing with the internal carotid artery; these 



