500 THE EYE IN EVOLUTION 



rete of Hiirlimann, 1912) situated intracranially in the cavernous 

 sinus. 



In the cat the circulation is unique in that the internal carotid in 

 the adult is vestigial, being reduced to imperforate connective tissue 

 strands (Fig. 659). The external carotid, on the other band, is well 

 developed and its large internal maxillary branch provides the basis 

 of an elaborate anastomotic network (the carotid rete) which is situated 

 extracranially near the apex of the orbit. From this rete large anasto- 

 motic vessels supply the circle of Wilhs by way of the orbital fissure. 

 Also from this rete seven independent trunks (corresponding to the 

 ophthalmic circulation of human anatomy) supply the orbital tissues 

 and the globe. The largest branch of the internal maxillary ^ — the 

 ciliary artery — reaches the optic nerve where it breaks up into its 

 numerous terminal ciliary branches which enter the eyeball ; there is 

 no central artery of the retina (Tandler, 1899-1906 ; Hiirlimann, 

 1912 ; Daniel et al., 1953 ; etc.). Davis and Story (1943) found that 

 from the circle of Willis a tenuous ophthalmic artery sometimes entered 

 the orbit to anastomose with the ciliary artery ; but even when it 

 occurs it is small and incidental. The whole of the orbit and eye is 

 therefore supplied from the external carotid as well as the greater part 

 of the circulation of the brain. 



Among Ungulates, in the pig the circulation resembles that of the 

 dog, but a well-formed rete is present supplied proximally by the 

 ascending pharyngeal artery; it empties into a large trunk which 

 is the only persistent portion of the internal carotid artery and con- 

 tributes to the circle of Willis (Fig. 660). Arising from this last vessel 

 there is a tenuous internal ophthalmic artery which anastomoses with 

 the ciliary (Versari, 1900 ; Daniel et al., 1953). In the sheep, goat, ox 

 and horse, the external ophthalmic artery may arise directly from the 

 internal maxillary, as it does in the dog, or from one of the group of 

 vessels which form anastomotic channels tlu"Ough the carotid rete with 

 the circle of Willis. As in the dog, a tenuous internal ophthalmic 

 artery is present in the sheep and the goat but not in the ox. In the 

 sheep and goat the rete is supplied wholly from the external carotid 

 and, as occurs in the pig, the internal carotid only exists as an afferent 

 vessel from this arterial network to the circle of Willis. In the ox and 

 horse, however, an internal carotid vessel is present (Figs. 661-2) 

 (Zietzschmann, 1913 ; Daniel et al., 1953). 



The orbital veins have not been fully worked out but in a general 

 way they correspond with the arterial supply. In man, the greater 

 part of the venous system returns into the intracranial system ; in the 

 lower Mammals the return is more and more to the extracranial system. 

 In the rabbit the veins from the globe and orbit empty into an extensive 

 ORBIT SINUS which ramifies throughout the apex of the orbit, 



