SENSES AND THE CENTRAL NERVOUS SYSTEM 239 



From the structure of the central nervous system, i.e. the brain and the 

 spinal cord, we can, in fact, draw a great many interesting inferences not 

 only about the senses, but also about propulsion, metabolism and all 

 sorts of other vital processes. The central nervous system is like a switch- 

 board controlling all the functions of an animal, and its structure clearly 

 reflects the animal's behaviour. For this reason, we shall devote the rest 

 of this chapter to a discussion of the Cetacean brain and spinal cord. 



The Cetacean central nervous system, like that of all vertebrates, lies 

 inside the braincase and the vertebral column. The spinal cord is, as we 

 have seen in Chapter 5, surrounded by a thick vascular network, and so 

 is part of the brain of Mysticetes. Odontocetes have less highly developed 

 retia in the braincase, though vascular networks are certainly found in the 

 base of their skull and round the large nerves emanating from this region, 

 e.g. the optic nerve. In addition to being cushioned by this mass of blood 

 vessels, the delicate tissue of the central nervous system is protected by 

 meninges, connective-tissue membranes identical with those found in 

 terrestrial mammals. According to Gersh (1938), the epiphysis, a small 

 organ protruding from^ the roof of the brain, is also very similar to that of 

 terrestrial mammals. This organ is thought to regulate the pressure of 

 the cerebro-spinal fluid. 



If we look at the spinal cord of a terrestrial mammal in cross-section, 

 it appears to consist of white matter surrounding an H-shaped mass of 

 grey matter. While the grey matter is made up of nerve cell bodies, the 

 white matter consists of nerve fibres most of which are medullated, i.e. 

 surrounded by a sheath. Thus the grey matter may be said to be the 

 switchboard proper, while the white matter constitutes the plugs and 

 wires carrying messages to and from the rest of the body. Whereas the 

 dorsal horns of the grey matter (in man, the posterior horns of the 

 H-shaped mass) are composed almost entirely of cells which receive 

 sensory stimuli, the ventral roots consist almost entirely of motor cells 

 whose fibres run to the muscles, causing them to contract when necessary. 



If we look at a Cetacean spinal cord in cross-section and compare it 

 with that of a typical terrestrial mammal, we are immediately struck by 

 the fact that, particularly in the thoracic and lumbar regions, the ventral 

 (in man, anterior) horns are extraordinarily large while the dorsal (in 

 man, posterior) horns are comparatively small (Fig. 124). The same is 

 true also of the white matter, which is much thicker at the bottom than 

 it is on top, and of the nerve roots emanating from the cord. In other 

 words the motor nerves, i.e. the nerves causing muscles to contract, are 

 much more highly developed in Cetaceans than they are in terrestrial 

 mammals, while the sensory nerves are much less developed, at least in 

 comparison with the motor nerves and possibly, though to a lesser 



