240 



WHALES 



Figure 124. Cross-sections through the spinal cord of a porpoise and a horse. Top: cervical 

 region; Bottom: lumbar region. {After: Hepburn and Watersion, 1904, and Pressy and 



Cobb, i92g.) 



extent, compared with the sensory nerves of other mammals. There 

 is nothing very surprising in this discovery, for we saw in Chapter 3 that 

 a very large proportion of the Cetacean body consists of muscle. From the 

 fact that the sensory apparatus of Cetaceans is relatively small, various 

 biologists have inferred that they have reduced sensitivity, but we have 

 just seen that this inference does not tally with the knowai facts. True, 

 because of the animal's general shape and the almost complete absence 

 of limbs, the total skin surface is much smaller than it is in terrestrial 

 mammals. Moreover, aquatic mammals need fewer temperature recep- 

 tors in the skin, since water is not subject to such sudden fluctuations of 

 temperature as the air. This, and also the fact that the ventral horns are 

 exceptionally large, probably explains why the dorsal horns have been 

 taken to be much smaller than they really are. 



It may be objected that a strongly developed muscular apparatus must 

 go hand in hand with a well-developed system of proprioceptors and a 

 consequent increase in the size of the dorsal horns. In fact, this is not so, 

 since most proprioceptive reflexes by-pass the cells of the dorsal horns, the 

 nerve fibres from the spinal ganglion which transmit these stimuli going 

 directly to the ventral horns. The size of the dorsal horn is thus not very 

 much affected by the presence of a well-developed system of propriocep- 

 tors for registering tensions in muscles and tendons. The Cetacean spinal 



