444 C. H. DANFORTH 



encephalic arteries like the second may or may not give rise 

 to a vessel to the corpus striatum. In one specimen, where 

 such a branch was well developed, no anastomosis could be 

 detected between it and the anterior artery by the gross methods 

 employed. The main trunk turns medially between the in- 

 ferior lobe and the anterior end of the medulla. As it arches 

 backward it gives off one or two lateral branches which run 

 up between the optic lobe and the cerebellum to anastomose 

 as above described with the dorsal artery 'of the brain. The 

 posterior arteries of the two sides join each other in the angle 

 between the inferior lobes and the medulla, but from this point 

 of union two vessels instead of one run caudally as far back as 

 the level of the vagus nerve and may even extend further back 

 before finall}^ uniting. From these two parallel basilar arteries 

 in front and their single posterior prolongation behind, a number 

 of small branches are given off to the medulla, to the roots of 

 the nerves, (nearly all of which are accompanied laterally by 

 small arteries) and to the floor of the cranial cavity. Several 

 small vessels of the latter group go to the optic capsule. In 

 addition to these smaller branches there are on either side two 

 large branches which seem to be constant in occurrence, but 

 not in position. The more anterior of these runs laterally, either 

 in front of the auditory nerve and between the roots of the V- 

 VII complex, or posterior to all of these, to gain the lateral side 

 of the medulla along which it runs caudally. The other and 

 more posterior gives off a small branch and then follows the 

 ninth nerve laterally as far as the auditory region where it turns 

 aside to be distributed about the sacculus. No end to end 

 connection between the basilar artery and the vertebral branch 

 of the subclavian, which enters the spinal canal further caudad, 

 could be detected, although it is not improbable that small anas- 

 tomoses between the two arteries do occur. It is evident, at 

 any rate, that the type of circulation, common to many of the 

 vertebrates, including some of the fishes, in which the vertebral 

 arteries are an important factor in the cerebral circulation, has 

 not been attained by Polyodon. 



