MEMBRANES AND BLOOD VESSELS 445 



part of the inferior surface of the corresponding cerebellar 

 hemisphere. 



Arteria Spinalis Anterior. The anterior spinal artery arises 

 near the lower border of the pons, and it is rare to find the 

 vessels of the two sides of equal size. They converge on 

 the anterior surface of the medulla oblongata and unite, 

 forming the commencement of the median vessel which 

 extends downwards on the ventral face of the spinal medulla. 



The bulbar arteries are minute vessels which enter the 

 substance of the medulla oblongata ; they spring both from the 

 vertebral artery itself and also from its branches. 



Arteria Basilaris. The basilar artery, which is formed by 

 the union of the two vertebral arteries, is a short trunk. It 

 extends, in the median plane, from the lower to the upper 

 border of the pons. At the latter point it ends by dividing 

 into the two posterior cerebral arteries. The basilar artery 

 lies in the middle part of the cisterna pontis and occupies 

 the median groove on the ventral or anterior surface of the 

 pons. Anteriorly it is supported by the basilar portion of 

 the occipital bone and the dorsum sellae of the sphenoid. 



The branches which spring from the basilar artery proceed 

 laterally for the most part from either side of the vessel. 

 They are : 



1. Pontine. 



2. Internal auditory. 



4. Superior cerebellar. 



5. Posterior cerebral. 



3. Anterior inferior cerebellar. 



Rami ad Pontem. The pontine branches are numerous 

 slender twigs which run laterally on the surface of the pons 

 and enter its substance. 



Arteria Anditiva Interna. The internal auditory artery 

 will be seen amongst the pontine branches. It accompanies 

 the acustic nerve into the internal acustic meatus, and is 

 distributed to the internal ear. 



Arteria Cerebelli Inferior Anterior. This artery inclines 

 postero- laterally to reach the anterior part of the inferior 

 surface of the cerebellum. 



Arteria Cerebelli Superior. The superior cerebellar artery 

 is a large vessel which springs from the basilar close to its 

 termination. It winds laterally and posteriorly, along the 

 upper border of the pons, to reach the upper surface of the 

 cerebellum, upon which it spreads out in a number of large 

 branches. 



