MAMMALS 453 



necessary for the pressure on the surface is equally transmitted to all the fluid 

 contents of the body including the inner eye. It is more likely that the rein- 

 forcement of the posterior region of the sclera is necessary to maintain the non- 

 spherical shape in the huge cetacean globe rendered mechanically weak by ita 

 great size, thus taking over the supportive function of the scleral cartilage in 

 fishes with similarly shaped eyes. 



The cornea of Placentals is usually circular or almost so, but in 

 Cetaceans and in a great number of the Ungulates (Equiclse, Ruminants 

 and the liippopotamus) it is horizontally oval corresponding to the 

 configuration of the pupil. In many, a pigmented ring encircles the 

 limbus sjDreading a considerable distance into the corneal tissue ; 

 sometimes this is confined to epidermal pigment (Rodents such as 

 rabbit, hare, guinea-pig, rat, marmot, etc. ; the horse and the gorilla) 

 (Fig. 607) ; sometimes to this is added pigment in the deep interstitial 

 tissues (Carnivores such as the cat, dog, fox, lion ; Ruminants such as 



Fig. 57.3. — The Endothelium of the Cornea of the Rabbit. 



Showing a sheet of corneal endothelium lining the anterior chamber 

 wliich has been stripjjed away from Descemet's membrane. No nerve tibres 

 are seen but there are a few circular blobs of .stain lying between the cells 

 ( X 400) (Zander and Weddell). 



the ox and deer ; the jjorj^oise, the dolphin, the whale and the 

 chimpanzee). In the rhinoceros the pigmented region of the cornea 

 is vascularized. The ]iigmentation may be an anti-glare device for it 

 is absent in crepuscular or nocturnal animals. 



The histological structure of the cornea is biult on the typical 

 vertebrate plan seen in man excej^t that most species have no Bowman's 

 membrane ; Descemet's membrane with its endotheliiun. however, is 

 always present and is often very substantial. Although Bowman's 

 membrane is a relative rarity, the basal membrane of the epithelium 

 seems always to be present (Calmettes et a!., 1956 ; Sheldon, 1956). 

 The thickness of the epithelium varies considerably (Figs. 565 to 572)^; 

 that of the endothelium is constant (Fig. 573). Blood vessels some- 

 times invade the cornea proper from the limbus, whereas in Primates 



1 20 layers of cells in the horse ; 10-12, pig ; 9-11, ox; 8-10, dog; 6-8, rabbit ; 

 5-6, guinea-pig, rat (Virchow, 1910 ; Calmettes et al., 1956). 



