661 THE ARTERIES. 



is directed transversely inwards to the surface of the rectus capitis posticus major, 

 and soon separates into several branches — ascending and descending ; these are 

 mixed with the divisions of the first superior cervical branch, all of which go to 

 the muscles and integuments of the occipital region. The descending branches 

 anastomose with the terminal divisions of the superior cervical artery. 



2. Cerebro-spinal Artery. — 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 the opposite 

 artery on its arrival at the middle of the inferior surface of the medulla oblongata, 

 and so forms the basilar artery ; the other passes backwards, and constitutes the 

 origin of the middle spinal arteri/, 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 Fig. 389. 



Basilar Artery. — This is a single vessel that creeps in a somewhat flexuous 

 manner on the inferior face of the medulla oblongata, beneath the visceral arach- 

 noid membrane, and, passing over the pons Varolii, terminates at the anterior 

 border of this portion of the isthmus, by anastomosing with the two posterior 

 cerebral arteries (Fig. 380, 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 Varohi, or are distributed to the roots of the 

 nerves emanating from the medulla oblongata. 



2. The posterior cereheUar arteries — vessels liable to numerous anomalies in 

 their origin. They usually arise from the basilar artery 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 passing along the surface of the medulla oblongata, the 

 external border of which it thus reaches ; they are then inflected backwards 

 beneath the cerebellar plexus choroides, whence they spread their ramifications on 

 the lateral and posterior parts of the cerebellum. 



3. The anterior cereheUar 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 artery, in front of the pons Varolii, 

 and sometimes even from the posterior cerebral arteries. Usually united in 

 fasciculi, they are directed outwards and a little backwards in turning round the 

 crura cerebri, and plunge into the anterior part of the cerebellum. 



4. Two branches anastomosing with the internal carotid artery. These 

 branches are not constant, and are most frequently met with in the Ass. They 

 begin at the basilar artery in front of the posterior border of the pons Varolii, 

 traverse the dura mater to enter the cavernous sinus, and join the carotid arteries 

 at their second curvature. 



Middle Spinal Artery. — A very long vessel, lodged in the inferior fissure 

 of the spinal cord, and measuring the whole extent of that organ, which it 

 follows from before to behind. It is from this artery that are given off the 

 branches that cover with their arborizations the medullary tissue, or penetrate its 

 substance. This emission, which ought soon to exhaust the artery, does not, 

 however, sensibly diminish its diameter ; as it receives on both sides, during its 



