498 THE EYE IN EVOLUTION 



to allow ample scope for a wide gape of the jaws. On the other hand, 

 among many Ungulates, particularly the horse and all horned animals, 

 the orbit is enclosed and heavily reinforced, as if for protection 

 against the severe injuries caused by horns, and also for strengthening 

 the skull for combat. Among the Prosimians the orbit is incompletely 

 closed, maintaining continuity with the temporal fossa ; among the 

 Anthropoidea it is completely enclosed. A lining periorbita is invariably 

 present, associated with muscular elements (Burkard, 1902 ; Ashley- 

 Montague, 1931). The orbits vary much in position depending on 

 whether the eyes look frontally or laterally (Koschel, 1883) ^ ; their 

 capacity compared with the size of the globe also varies within wide 

 limits (pig, 2-2 : 1 ; sheep, 1-6:1; horse, 3:1; ox, 6:1; man, 4-5 : 1, 

 Dexler, 1893). Even among the Primates themselves the size of the 

 orbit varies only very loosely with that of the globe, large Primates 

 having a relatively small orbital capacity (Imai, 1934-36 ; Schultz, 

 1940 ; Chamberlain, 1954). 



Tke vascular systein is extremely variable throughout the verte- 

 brate phylum. In man, the entire intra-ocular blood supply and most 

 of the orbital blood supply is derived from the internal carotid artery ; 

 in the lower Mammals, the external carotid takes the larger share and 

 sometimes is the sole source of supply. In Rodents such as the rat 

 and the rabbit the arrangements are relatively simple (Fig. 657). The 

 main blood supply to the globe and the orbit is derived from the internal 

 maxillary branch of the external carotid. The external ophthalmic 

 divides into several branches which supply the muscles and tissues of 

 the orbit, as well as the long and short ciliaries wliich enter the globe. 

 A second artery of supply, the internal ophthalmic artery, is small. It 

 is derived from the circle of Willis and ultimately from the internal 

 carotid ; it runs tlu-ough the optic foramen into the orbit, sends an 

 anastomotic branch to the nasal long ciliary artery and enters the 

 optic nerve near the globe to supply the retina as a central retinal 

 artery (Krause, 1868 ; Henderson, 1903 ; Davis, 1929 ; Daniel et al., 

 1953 ; Janes and Bounds, 1955). 



Among the Carnivores, the dog and cat may be taken as typical. 

 In the dog the arrangement is not very different from that in the 

 rabbit (Fig. 658). Again, the main blood supply to the orbit and globe 

 is by way of the external oj3hthalmic branch of the internal maxillary 

 artery which is ultimately derived from the external carotid. In the 

 same way an internal ophthalmic artery derived from the circle of Willis 

 (that is, ultimately from the internal carotid) also enters the orbit 

 to anastomose with the ciliary branch of the external ophthalmic. 

 There is, however, a large anastomotic branch (the arteria anastomotica) 

 betv, L the internal carotid and the external ophthalmic arteries, so 



1 p. 672. 



