SENSES AND THE CENTRAL NERVOUS SYSTEM 235 



placed eyes which stick out of the head would be exposed to exceptional 

 frictional and saline effects, and so Cetaceans must content themselves 

 with small, laterally placed eyes, recessed in the head with consequent 

 loss of field vision. Animals with small snouts, e.g. dolphins, are still best 

 off in this respect, and in Bottlenose Dolphins the visual fields of left and 

 right eyes overlap to some extent so that part of their vision is stereo- 

 scopic. This, together with the flexibility of their heads, undoubtedly 

 accounts for the agility with which they catch fish in and out of the water. 

 An animal like the Pilot Whale with its blunt and bulbous head, has its 

 frontal view greatly obstructed ; this applies even more to the Sperm Whale 

 and to Mysticetes. However, all these animals feed on slow-moving 

 cuttlefish or on small shrimps, and good vision is less important to them. 

 A Pilot Whale was seen to refuse food placed directly in front of it, but to 

 respond when the food was moved towards the side. 



While the axis of the porpoise's eye at rest appears to be directed 

 towards the side, that of most other Cetaceans is, according to Putter, 

 directed obliquely downwards and towards the front. Since the Cetacean 

 eye is elliptically flattened, the horizontal visual field is undoubtedly 

 greater than the vertical field. Detailed investigations of the size of the 

 visual field have not yet been made, though Fischer found in 1946 that 

 the Sperm Whale had a horizontal visual field of about 125° on either side 

 of the head (Fig. 121). While the animals are in the water, they are 

 unlikely to see much above the surface, because of refraction and reflec- 

 tion by the water, though some of the tricks of captive Bottlenose 

 Dolphins suggest that they must have some idea of what goes on above 

 the surface while their eyes are still submerged. 



Naturally, the total visual field of all mammals can be increased by 

 moving head and eyeball. We have discussed the Cetacean neck and 

 possible head movements in Chapter 3, and need say no more about 

 them. As for the eyeball, we know that the Cetacean eye muscles are fairly 

 well developed. Hunter described all seven as early as 1 787, and Hosokawa 



Figure 121. Top view of head of Sperm Whale, showing 

 limits of its visual fields. {After Mann Fischer, ig46.) 



