CH. L.] COMPARATIVE PHYSIOLOGY OF THE BRAIN 711 



and absolute value of any locality in the central nervous system 

 depends largely on the degree to which centralisation has progressed, 

 and on the amount of connection between the various areas. The 

 closer the connection, the more numerous and intricate the path- 

 ways, the greater will be the permanent effects of an extirpation, 

 and the recovery of function the more remote. The lower the 

 animal in the zoological series, or the less the age of the animal, the 

 more imperfectly developed will be the connecting strands, and so 

 the possibility of other parts taking up to some extent the functions 

 of those that are removed will be increased. 



If the cerebral hemispheres are removed in a teleostean or bony 

 fish (and in such animals there is practically no cortex), the animal 

 is to all intents and purposes unaffected ; it can distinguish between 

 a worm and a piece of string, and will rise to red wafers in preference 

 to those of another colour. The operation does not damage the 

 primary centres of vision, and in these fishes the eye is the most 

 important sense organ. 



A shark, however, subjected to the same operation, is reduced to 

 a condition of complete quiescence ; this is due to the circumstance 

 that in this fish the principal sense organ is that of smell, and sever- 

 ance of both olfactory tracts produces the same result as removal 

 of the entire hemispheres. In either case the path between the 

 olfactory bulbs and the centres that control the cord are interrupted. 



Going a little higher in the animal scale to the frog, we find 

 that removal of the hemispheres only does not entirely abolish its 

 apparent spontaneity ; it still continues to feed itself, for instance, 

 by catching passing insects. It is not until the optic thalami are 

 removed also that it becomes the purely reflex animal described 

 on p. 678. 



If the brain and the anterior end of the bulb are removed the 

 frog becomes incessantly active, creeping and clambering about the 

 room ; but if the whole bulb is removed strong stimulation is required 

 to produce movements ; these, however, remain co-ordinated. 



If the frog's cerebellum is removed there is some tremor of the 

 leg muscles, and a loss of co-ordination in jumping. If the removal 

 is confined to one side of the twixt-brain, mid-brain, or bulb, there is 

 a tendency to forced positions and movements, action being most 

 vigorous on the side of the body associated with the uninjured 

 portions. 



We thus see that a progressive removal of portions of the brain 

 is followed by a progressive loss of responsiveness, until we reach the 

 anterior end of the bulb, the removal of which sets free the lower 

 centres of the cord, and the result is incessant movement provoked 

 by slight stimuli. Further removal, however, lessens responsiveness, 

 and this is not easy to explain. 



