BLOOD-VASCULAR SYSTEM OF THE LORICATI 59 



and 23a; C."C."A."), which passes between the ventral later- 

 alis and the motor roots of the VII nerve to the roof of the 

 skull, supplying the adipose tissue surrounding the brain and 

 the semicircular canals. Emerging from beneath the motor 

 root of the VII nerve, the auditory artery divides into an ante- 

 rior and a posterior auditory artery. The antcrioi' atiditury 

 artery (PI. Ill, figs. 23^ and 25 ; A.Aud.A.) follows along 

 the anterior surface of the ramulus acusticus ampullae ante- 

 rioris ; passing beneath the anterior ampulla to which it gives 

 off a branch, it continues on to the external ampulla and its 

 semicircular canal. The -posterior auditory artery (PI. Ill, 

 figs. 23, 23^ and 25 ; P.Aud.A.), which at first passes caudad 

 under the ramulus acusticus ampulljE anterioris and the ramulus 

 acusticus sacculi, comes up through the center of the latter and 

 passes along in front of the ramulus acusticus ampullae poste- 

 rioris, to supply the posterior ampulla, its semicircular canal, 

 and the utriculus. The myelonal artery terminates in 2 forks 

 on the ventral surface of the my el, in the region of the first 

 spinal nerves. These branches usually anastomose with a 

 branch of the first neural or vertebral arteries, which having 

 their oricrin from the subclavians make them analogous to the 

 anastomosis of the basilar and vertebral arteries of mammals, 

 of which a more detailed description will be given under the 

 subclavian arteries. At the point where the posterior cerebral 

 artery bends to penetrate the mesencephalon it gives off, caudad, 

 the cerebellum artery (PI. Ill, figs. 23 and 24; Cer.A.). This 

 vessel continues parallel, but above the IV nerve, ventrad to 

 the optic lobes, and laterad to the crus. In its caudal course 

 it gradually rises higher on the crus, until in the region of the 

 posterior end of the optic lobes it gets to lie between the optic 

 lobes and the valvula cerebelli. A little behind the origin of 

 the IV nerve and the posterior end of the optic lobes this vessel 

 penetrates the dorso-lateral wall of the valvula cerebelli at the 

 point where the molecular layer of the valvula unites with the 

 corresponding layer of the cerebellum. Its course is then 

 caudad a little to one side of the median line, gradually ex- 

 hausting itself in the granular layer of the cerebellum. 



The fourth and smallest vessel to be given off from the en- 



