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 (P1. III, 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 myelonal or oblongata artery 
(Pl. 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 metaccele (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 
auditory artery (Pl. III, figs. 23, 23a, 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 ¢hzrd cranial cavity artery (P\. II, figs. 23 
