SENSES AND THE CENTRAL NERVOUS SYSTEM 229 



Figure iiy. The Gangetic Dolphin is practically blind and finds its food at the bottom of 



muddy rivers. 



from other rivers (e.g. the La Plata Dolphin and the Boutu) which find 

 their prey in less turbid waters have correspondingly better eyes. 



But to function at all, however poorly, Cetacean eyes had to undergo 

 certain adaptations to their aquatic environment. Firstly, they need no 

 device to keep their eyeballs moist, and all Cetaceans therefore lack 

 lachrymal glands and ducts. In other words, whales cannot weep. Nor 

 need whales worry about dust and sweat, and so they have no eyebrows or 

 eyelashes, or, for that matter, any hands or paws with which to brush 

 foreign bodies out of their eyes. Other adaptations are also dictated by 

 their aquatic environment, and we shall see that most of these adaptations 

 are more highly developed in Odontocetes than in Mysticetes, possibly 

 due to the greater mobility of the former's prey. 



Now a ray of light entering the eye lens on land is refracted differently 

 from one entering the eye under water. An image of an object which would 

 normally form on our retina would form behind it if we looked at the 

 object through water. We can correct this fault by accommodating our 

 lens, but Cetaceans have permanently compensated for their 'longsighted- 

 ness' by an exceptionally rounded lens, strongly resembling that in fish. 

 In many species the lens is very nearly perfectly spherical (Fig. 1 18), and 

 even in Rorquals, where the curvature is less pronounced, it is very much 

 more rounded than in terrestrial mammals. A more spherical lens has the 

 same effect as a pair of convex glasses worn under water by a man with 

 normal sight - the rays are bent forward and the image is made to fall on 

 the retina as it would do if he were out of the water without glasses 

 (Fig. 119). The Cetacean's lens also has a greater refractive index than 

 that of terrestrial mammals, and in most species the eyeball itself is oval 

 rather than spherical, thus enlarging the visual field (see below), but at 

 the same time decreasing the distance between lens and retina (thus 

 making them more longsighted). In Rorquals this is partly cancelled out 

 by their having a very much smaller lens than other Cetaceans. 



The first scientist to make a thorough study of the Cetacean eye with 

 relatively modern instruments was Dr Aiatthiessen, Professor of Ophthal- 

 mology at the University of Rostock, who spent his long vacation 



