456 THE EYE IN EVOLUTION 



(rabbit), Zander and Weddell (1951) (rat, guinea-pig, rabbit, monkey and man, 

 as well as the dogfish, Scylliuni, and the frog, Rana), Rexed and Rexed (1951) 

 (rabbit), Itahashi (1952) (rabbit), BeW etal. (1952) (cat), Palumbi (1953) (rabbit, 

 rat, cow, horse, and man), and Wolter (1955-56) (rabbit). 



Branches of the ciliary nerves derived from the ophthalmic division of the 

 trigeminal enter the cornea at the limbus. After supplying a perilimbal plexus, 

 they lose their myelin sheaths and run radially into the corneal stroma in some 

 70 to 80 nerve-trunks (Fig. 574). As these branch they form a plexiform arrange- 

 ment at all levels in the stroma, more dense, however, in the siiperficial layers. 

 Some of the branches terminate in the stroma in bead-like thickenings (Fig. 575) ; 

 many of them terminate in the corneal epithelium penetrating Bowman's 

 membrane when this structure is present. In this layer the nerve fibres shed 

 their sheaths of Schwann and the naked axons subdivide to form a delicate 





Fig. 578. — The Posterior Segment of the Eye of the Kitten. 



ch, pigmented vascular choroid ; r, retina ; s, fibrous sclera ; t, tapetura 



( X 150) (Norman Ashton). 



plexus terminating in beaded formations in all layers of the eiDithelium. Although 

 there appear to be histological differences between the fibres, the evidence 

 suggests that these nerves are all of a sensory nature (Fig. 576). 



A most interesting finding has been reported by Wolter (1957) — the presence 

 of nerve fibres in the endothelium of the cornea in the rabbit (Fig. 577); their 

 function is unknown, nor have they been observed in other Vertebrates. 



An interesting peculiarity is the keratinization of the corneal epitheliufn seen 

 in two tyiDes of Placentals. In some aquatic forms (seals, dolphins and particu- 

 larly in whales) the epithelium is thick and keratinized as a protection against 

 sea-wateri ; while in the ant-eating Placentals (Xenarthra, as the armadillo ; 

 Tubulidentata such as the aard-vark) a similar keratinization occurs, correspond- 

 ing to that seen in the ant-eating Monotremes (the echidna), presumably a 

 protection against the formic acid emitted in defence by the termites. The 

 armadillo, Dasypus, is peculiar in that the cornea is vascvilarized to its apex, 

 probabi a necessary source of nutriment since the heavily cornified epithelium 



1 Co; nire the thick corneal epithelium of the platypus, Ornithorhynchiis (p. 433). 



