MAMMALS 



451 



of tlie antero-posterior axis of the globe. In the hump-back whale, 

 Megajitera, for example, the antero-posterior diameter of the eye is 

 40 mm., the tliickness of the sclera at the posterior pole is 30 mm., 

 wliile its thickness at the limbus is only 3 mm. (Rochon-Duvigneaud, 

 1943) (Fig. 564). The cornea of this species is correspondingly thin 

 (1-5 mm. at the jjeriphery ; 0-5 mm. at the apex). In addition, the 

 whale has an immensely thickened accessory optic nerve sheath 

 composed of comiective-fatty tissue lying outside the dural sheath 



Fig. 564. — The Eye of the Whale. 



A heniisection to sliow the enormous thickening of the .sclera and the 

 accessory optic nerve sheath. Note that the relatively small lens is kept in place 

 artificially by a glass strut (specimen from A. Arruga ; Museum, Institute 

 of Ophthalmology). 



encased in a thick aponeurotic-like capsule. Set on this massive stalk, 

 the globe, of course, is immobile. A similarly tliick accessory sheath 

 surrounds the optic nerve of the elephant and the liippopotamus 

 (Rochon-Duvigneaud. 1943) : in both of these the sclera is very thick 

 and the eyes are capable of little movement. 



The phenomenal tliickness of the sclera in the whale is often said to be 

 necessary to resist the enormous pressures involved when the animal sounds to 

 great depths. It is to be remembered, however, that the cornea is thin and that 

 abyssal fish do not share this characteristic ; the sclera of the deeply 

 diving shark, Etmopterus, is niicroscopicalh' thin and that of the Chimteras 

 discontinuous.^ It is probable, indeed, that reinforcement in this sense is un- 



1 p. 290. 



