58 ALLEN 



the anterior cerebral artery ; they run ventro-laterad across the 

 optic nerves, the cerebral hemispheres, the III and IV nerves. 

 Shortly after crossing the IV nerve each vessel makes a sharp 

 curve at nearly right angles ; then passing caudad between the 

 IV nerve and the roots of the V and VII, parallel with, but 

 inside of the corresponding vein, they give off several super 

 ficial branches to the mesencephal (optic lobes) and hypoaria 

 (inferior lobes). The outer layer of the former contains a mass 

 of blood vessels. Close to the posterior end of the hypoaria 

 each posterior cerebral artery bends inward with the III nerve 

 and the corresponding vein, between the mesencephal, hypoaria 

 and crura cerebri, and when the saccus vasculosus is reached 

 this vessel divides into an anterior and a posterior branch. The 

 anterior branch unites with the corresponding vessel from the 

 opposite side in the median line above the anterior part of the 

 saccus vasculosus to form the mesencephalic artery (PI. Ill, fig. 

 25 ; Me. A.), which passes cephalad a short distance in the 

 crura; then turning dorsad, penetrates the floor of the mesen- 

 cephal directly in front of the valvula cerebelli (volvula of 

 other authors), and here sends out a lateral branch along the 

 dorsal surface of each torus semicircularis. In like manner 

 the posterior forks of the posterior cerebral arteries unite in the 

 median line above the posterior end of 'the saccus vasculosus, 

 and the vessel thus formed continues caudad along the ventral 

 surface of the oblongata as the myclonal or oblongata artery 

 (PI. Ill, figs. 23 and 25 ; My. A.). Along its short course 

 several branches are given off to the oblongata and one to the 

 auditory region. The first vessel for the oblongata is given off 

 near the source of the myelonal artery and passes up through 

 the crura to the metacoele (IV ventricle), where it branches out 

 caudad in the dorsal part of the crura. The second branch 

 comes up through the ventral fissure of the oblongata in the 

 neighborhood of the facialis lobe and breaks up similarly to the 

 first branch. The third branch, which is much larger, is the 

 anditory artery (PI. Ill, figs. 23, 23^^', 24 and 25 ; Aud.A.). 

 Its course is obliquely laterad across the oblongata, but before 

 coming out from under the roots of the VII nerve, sends up a 

 dorsal branch, the third craiiial cavity artery (PI. Ill, figs. 23 



