56 ALLEN 



2 layers ; an outer layer of large arteries and veins, and an 

 inner layer of capillaries. The capillar}?- 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 

 ■postci'io}' hyoidean artery (PI. I, fig. i ; 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, supphnng the inner side of the deeper 

 adductor mandibular muscle. Passing ventro-caudad through 

 the same foramen as the h3^oidean 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. 



{J)) Internal Carotid or Carotis Anterior Artery (PI. I, figs. 

 I 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 ence'piialic or brain artery (Pis. I, II and III, figs, i, 

 5, 15, 23 and 25 ; Enc.A.) proceeds dorsad between the 

 external recti muscles and penetrating the floor of the brain 

 case directl}'- 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 

 (PI. Ill, figs. 23 and 25; A.Cer.A.) divides ; the 2 branches 

 running parallel for a short distance in a sort of zig-zag course 



