THE BRAIN, AND COMPARATIVE BEHAVIOUR 375 



ability for variation or modification by experience. When 

 any particular sensory system is very highly developed, the 

 corresponding centre in the brain is enlarged. So in the carps, 

 which are well supplied with taste-organs, the medulla oblon- 

 gata is enlarged owing to the expansion of the visceral lobe. 

 This expansion is due to the increase in number of neurons 

 in the centre, parallel with the increased number of afferent 

 fibres coming from the numerous receptors. In the catfish, 

 the lateral-line system and the " ear-brain " are well developed. 



A certain amount of correlation exists between the primary 

 centres. For example, in the catfish, the " skin-brain " is 

 connected with the " taste-brain," so that food may be recog- 

 nised by touch and by taste, and these two types of sensation 

 co-operate in producing those movements which lead to 

 feeding. In other words, the reflex arc can pass from one 

 functional component system to the other. 



But this interrelation and team work between primary 

 centres is best brought about by special correlation- centres, 

 which are not related to any single primary centre but to 

 several. The history of the development and evolution of 

 these correlation-centres really makes up the evolution of the 

 brain in vertebrates. 



In the fish, the correlation-centres are not well developed, 

 with the exception of the cerebellum. The cerebellum lies 

 on the dorsal side of the medulla, and from its position its 

 connexions are mostly with the neighbouring centres : " ear- 

 brain " and " eye-brain." The ear-brain is concerned with 

 the balance of the animal as reported from the semicircular 

 canals, and the eyes report its position relatively to external 

 objects. At the same time, fibres of the general somatic 

 system run to the cerebellum and convey impulses of tactile 

 sensations, and of the state of the muscles and joints of the 

 body (proprioceptive). As a result of the commingling of 

 these impulses, the cerebellum comes to be an organ for the 

 regulation of the posture of the body and of bodily movements. 

 It keeps the muscles in " tone," and as a whole regulates the 

 execution of reflexes. It may, in a sense, be compared with 

 the steam steering gear of a ship, which smoothly carries out 



