CHAPTEE L 



COMPARATIVE PHYSIOLOGY OF THE BRAIN 



IT will have been noticed in the preceding chapters how much of our 

 knowledge of cerebral functions is derived from observations and 

 experiments performed upon the lower animals. I propose in this 

 chapter to expand this part of the subject. It is important not only 

 because of its intrinsic interest, but also because a wider survey of 

 the conditions in various animals throws considerable light on what 

 is found in man.* 



The brain in the lower vertebrata is composed of a smaller 

 number of cells than is found in the human brain ; one notices also 

 that the massing of the nerve units towards the cerebral cortex and 

 in relation to the principal sense organs has gone on to a less extent. 



The doctrine of cerebral localisation is not accurately expressed 

 by the statement that a cortical centre is one, the stimulation of 

 which produces a definite response, and the extirpation of which 

 abolishes the response. We have, for instance, seen that the stimu- 

 lation of certain areas in the dog's brain produces certain movements, 

 but Goltz showed that in his dogs, the removal of an entire hemi- 

 sphere did not cause paralysis of the opposite side of the body. 



In the central nervous system there are few or no places, where 

 only one set of nerve units are situated, with fibres passing to and 

 from them. Almost every locality has several connections with 

 other parts, and also fibres passing through it which connect together 

 the parts on all sides of it. Hence in extirpating even a limited 

 area, numerous pathways are interrupted, and the damage is con- 

 sequently widespread. Much of the disturbance produced at first 

 gradually passes away, and the temporary effects must be distinguished 

 from those which are permanent ; the permanent effects have the 

 greater significance of the two. Moreover, it is clear that the relative 



* This subject is treated at some length in Dr Donaldson's article on the Central 

 Nervous System in the American Text-book of Physiology edited by Howell. I am 

 indebted to this article for much contained in the present chapter. 



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