SENSES AND THE CENTRAL NERVOUS SYSTEM 233 



terrestrial mammal's. True, this disadvantage is less serious than might 

 be thought because the eyes are small and built into a streamlined body, 

 but frictional forces are still great enough to necessitate a special corneal 

 structure. Thus the outer cornea of Cetaceans is cornified, the cornified 

 substance uniting by means of papillae with the living tissue beneath. 

 Because of this cornification, the eye is protected not only against friction 

 but also against the stinging effect of brine. The conjunctiva is also 

 protected by a cornified layer. Nor are these the only means of protecting 

 the Cetacean eye against irritation by salt water, for, though Cetaceans 

 have no tear glands, other glands, e.g. Harderian glands in the outer 

 corners of the eyes and also glands in the conjunctiva of the eyelids, excrete 

 an oily substance which regularly bathes the cornea, thus protecting it 

 and the eyelids against the harmful effects of sea-water. Any surplus oil 

 is washed away by the sea, so that there is no need for special drains such 

 as our own lachrymal ducts. Such ducts have, however, been found in 

 the skulls of Archaeocetes. 



Naturally, the eyelids themselves, which can shut off the vulnerable eye 

 from the outside world, are its best protection against superficial injury. 

 That the eyelids of Cetaceans can shut and open is best seen in a newly 

 killed whale, where they can be moved up and down with ease. Though 

 they look somewhat rigid in Odontocetes, the Bottlenoses of Marineland 

 and other dolphins are known to be capable of shutting their eyes, and the 

 eyelids of Odontocetes and Mysticetes alike are, moreover, provided with 

 well-developed muscles ; a set of muscles connected to each of the four 

 recti muscles of the eye keeps them open, and an annular muscle shuts 

 them. In theory, therefore, whales and dolphins would not only be able 

 to sleep with their eyes shut, but also to wink, if they chose. 



Though the Cetacean pupil can be greatly dilated, the relative smallness 

 of the cornea is a disadvantage in the poor light in which Cetaceans 

 normally move. In animals which live in perpetual twilight, one might 

 have expected to find the exceptionally large eyes of nocturnal animals, 

 e.g. of lemurs. But we have seen why a large eye would be a drawback to 

 Cetaceans. Now, nature has in fact made some amends for these deficien- 

 cies by providing Cetaceans with an exceptionally well-developed 

 tapetum liicidiim, a special layer at the surface of a part of the choroid layer 

 adjacent to the retina. The tapetum lucidum contains large quantities of 

 guanine crystals which give it a metallic appearance and enable it to 

 reflect light like a mirror. The eyes of a cat or a horse glow in the dark 

 because they possess a tapetum whose function it is to send light which 

 has passed to the retina back to the retina a second time. In this way 

 vision in the dark is greatly increased, and it is therefore not surprising 

 that in terrestrial mammals a tapetum is found, particularly in carnivoi'es 



