SENSES AND THE CENTRAL NERVOUS SYSTEM 



251 



Figure 130. Top view of the brain of a 

 Gangetic dolphin, showing small number 

 of convolutions of the cerebral cortex. 

 Note the marked grooving of the cere- 

 bellum which is not affected by the lack of 

 visual power. [Anderson, i8y8.) 



the greater the rate of excretion, glandular secretion and the need for food. 

 As a result of an increased need for food, the animal has to move about 

 faster and in much more differentiated ways, and will therefore need a 

 more highly developed sensory apparatus. All these factors may contribute 

 to the development of a larger brain. This is borne out by the fact that 

 warm-blooded animals which have to produce their own heat have a 

 higher metabolic rate and a much larger brain than fish and reptiles of 

 comparable size, which obtain their body heat from their environment. 

 Portmann and his colleagues have, moreover, shown recently that the 

 more active a particular species of fish the larger its brain. Here, too, 

 metabolism plays an important part in cerebral development. 



In Chapter 1 1, we shall see that Cetaceans have a very high metabolic 

 rate indeed, with a consequent high food intake and a need for great 

 activity. Hence the metabolic hypothesis would clearly explain their 

 highly developed brain, particularly since Odontocetes, which pursue 

 fast-moving fish, have a very much larger brain than Mysticetes whose 

 prey is so much more slow-moving. Still, as we have seen, the problem is 

 by no means solved, and a great deal of research will still have to be done, 

 before we can explain why, for instance, a porpoise has a brain comparable 

 in mass to that of man. 



We shall conclude this chapter by saying a few words about our know- 

 ledge of the brains of Archaeocetes, or rather about the lack of such 

 knowledge. The reader might wonder how we can hope to know anything 

 about so perishable an object as the brain of animals which have been 

 extinct for 30 million years. In fact, the brain is surrounded by bone, and 

 in most mammals fills the brain-case so completely that we can take 

 plaster-casts of the brain itself from the case. In that way, we can obtain 

 a fairly accurate idea of the shape and location of the brain, the size of its 

 different parts and of the cranial nerves, and even of its convolutions. 



