SENSES AND THE CENTRAL NERVOUS SYSTEM 243 



subject in 1954 and i960, that these organs may have some other unsus- 

 pected function. In Mysticetes, the central olfactory system is, of course, 

 more fully developed since they have an olfactory nerve, even though, as 

 we have seen, it does not play a very important part in their lives. 



Little can be said about the visual brain centres, as these have not been 

 sufficiently investigated, and as what evidence has been published is 

 largely contradictory. According to Breathnach (i960) they are not as 

 poorly developed as has been previously thought. 



The auditory centres, on the other hand, have been investigated more 

 thoroughly, and seem to be particularly well developed - not surprisingly, 

 if we recall how important hearing is in the lives of Cetaceans. All the 

 nuclei and sensory paths associated with the auditory centres, e.g. the 

 nucleus ventralis, the trapezoid body, the superior nucleus of the oliva, the 

 lateral lemniscus, the corpora geniculata and the temporal lobes of 

 the cerebrum, are very large, especially in Odontocetes. Those parts of the 

 brain assumed to assimilate high notes, e.g. the nucleus ventralis, 

 appear particularly well developed, the more so since the nucleus dorsalis 

 which is associated with the assimilation of low notes is so small that even 

 experienced neurologists usually miss it. This, too, is only to be expected 

 from our discussion of the Cetacean ear, and so is the fact that the auditory 

 nerve (the eighth cranial nerve) of Odontocetes is the biggest of all cranial 

 nerves. In Mysticetes, however, it takes second place to the fifth cranial 

 nerve (the trigeminal), of which more will be said later. The central 

 auditory apparatus of Mysticetes, though very much more highly 

 developed than that of terrestrial mammals, is not as effective as that of 

 dolphins, in which certain centres that may well be associated with 

 directional hearing and the reception of high-pitched tones are particu- 

 larly prominent. Since Mysticetes feed on slow-swimming prey, a minor 

 development of these centres is only to be expected. 



The eighth or acoustic nerve also contains vestibular fibres which 

 convey impulses from the semi-circular canals, i.e. from the equilibriating 

 oi'gan. The ratio of vestibular to cochlear fibres is very small, but absolutely 

 the vestibular fibres are no smaller than they are in terrestrial mammals. 

 Some central nervous regions, such as Deiter's nucleus, and probably 

 certain parts of the cerebellum, are well developed in accordance with the 

 important part that balance plays in the lives of Cetaceans. 



The trigeminal nerve is, as we have seen, the largest of all cranial 

 nerves in Mysticetes, and the second largest in Odontocetes. This is 

 certainly not due to the presence of a large number of motor fibres, for in 

 neither does chewing of food play an important part. However, the 

 trigeminal not only supplies the muscles of the jaws but also all the tactile 

 bodies in the entire head which, as we know, is supposed to be particularly 



