THE HEART AND ARTERIES OF POLYODON 443 



given off on the outside of the sclerotic, some of them possibly 

 anastomosing with the a. retinalis. 



Returning to the internal carotid proper, we find it proceed- 

 ing from its anastomosis with the efferent pseudobranchial 

 artery through the basal cartilage into the cranial cavity. This 

 region is carefully described by Allis (I.e., pp. 289-290). In 

 this part of its course it gives off a long slender vessel, apparently 

 confined entirely to the cartilage, which arches forward, ultimatel}^ 

 reaching the median line. Just inside the cranium the a. retin- 

 alis (a.ret.) separates and accompanies the optic nerve to the 

 eye. This is a very minute vessel which it is difficult to trace. 

 Lateral to the diencephalon the internal carotid divides pal- 

 mately into three branches, of which the posterior is largest, 

 the anterior smallest. Allis apparently found only two of these, 

 but he did not attempt to trace the encephalic arteries. The 

 anterior division, which is sometimes greatly reduced, runs 

 along the ventro-lateral side of the corpus striatum, dividing 

 in no very constant manner into the twigs that supply this 

 region. Its terminal rami run in among the coarse bundles 

 of the olfactory nerve and one of them gains the dorsal side of 

 the telencephalon. If the anterior division be reduced in size, 

 as is often the case, its place is taken by a branch from one of 

 the other two, either of which may send out a branch to cover 

 practically the same region. The middle division of the en- 

 cephalic artery, which, as just stated, often shows an anterior 

 branch, ascends in the angle between the telencephalon and 

 diencephalon. As it approaches the epiphyseal region it turns 

 abruptly backward over the dome-shaped midbrain where it 

 is joined by its fellow of the opposite side. ' There may be merely 

 cross anastomoses between the two or they may fuse completely, 

 in which case a single median vessel supplies the posterior aspect 

 of the midbrain and gives off on either side, a large branch which 

 turns laterally and ventrally between the optic lobes and cere- 

 bellum. These arteries are subject to considerable variations 

 and these variations are correlated for the most part with inverse 

 variations in the lateral rami of the third encephalic artery 

 with which they anastomose. The posterior and largest of the 



