56 ALLEN 
2 layers; an outer layer of large arteries and veins, and an 
inner layer of capillaries. The capillary layer is separated 
from the retina only by the thin pigment layer of the choroid. 
A little dorsad to the point of union of the hyoidean artery 
with the external carotid, the latter sends off, caudad, a smaller 
posterior hyoidean artery (P1. 1, fig. 1; P.Hyo.A.). Close to 
its point of origin this vessel gives off a dorsal branch, which 
runs in front of the preopercular and directly behind the ramus 
mandibularis VII, supplying the inner side of the deeper 
adductor mandibule muscle. Passing ventro-caudad through 
the same foramen as the hyoidean artery it runs parallel with 
it. In its course along the inner side of the preopercular it 
passes along the dorsal surface of the interhyal a little below 
the hyoidean vein; then curving around the ventral edge of the 
epihyal it comes to lie above the vein, finally terminating in 
several vessels to the hyohyoideus superior muscle in the region 
of the last branchiostegal ray. 
(6) Lnuternal Carotid or Carotis Anterior Artery (Pl. I, figs. 
1 and 5; I.Car.A.).— This vessel after leaving the common 
carotid bends inward, passes ventrad across the jugular vein to 
penetrate the internal carotid foramen (a foramen formed by the 
dorsal process of the parasphenoid, the parasphenoid, and the 
prootic bones) into the eye-muscle canal. Here it divides into 
a cephalic and a horizontal trunk. The former is the orbito- 
nasal artery, and the latter unites in the median line, above the 
parasphenoid, with the corresponding trunk from the opposite 
side, the combined trunk being the encephalic or brain artery. 
The encephalic or brain artery (Pls. I, II and III, figs. 1, 
5, 15, 23 and 25; Enc.A.) proceeds dorsad between the 
external recti muscles and penetrating the floor of the brain 
case directly cephalad of the hypophysis, and exhausts itself in 
4 branches, which are given off at right angles to one another. 
The cephalic one may be designated as the anterior cerebral 
artery, the lateral ones as the right and left posterior cerebral 
arteries, and the small posterior one as the infundibular artery. 
Soon after leaving the main stem the anterior cerebral artery 
(Pl. III, figs. 23 and 25; A.Cer.A.) divides; the 2 branches 
running parallel for a short distance in a sort of zig-zag course 
