CH. L.] CEREBRAL LOCALISATION 725 



Localisation of Cerebral Functions. 



The different parts of the brain and of its cortex are related to 

 different parts of the body. The right hemisphere, for instance, con- 

 trols the voluntary movements on the left side of the body, and 

 receives sensory impulses from the left side, and vice versd. 



Then in each hemisphere there are certain areas, termed motor 

 areas, which are the starting-points of those volitional impulses which 

 give rise to movements; and other areas primarily concerned in the 

 reception of sensory impulses ; these are termed sensory areas. These 

 various areas have been mapped out by means of experiments on 

 animals, and by the observation of disease in man. 



Before these facts were ascertained it was usual for physiologists 

 to say that " the brain acts as a whole," and although we do not now 

 attach the same meaning to that phrase as did the physiologists of 

 the past, it still has an underlying substratum of truth. Let us take 

 an example, and imagine the smell of an orange ; such an abstract 

 idea of an isolated sensation is impossible ; we cannot think of the 

 smell of the orange apart from the other characteristics of the fruit, 

 the smell recalls the taste, the shape, the colour, the act of peeling it, 

 fingering it, cutting it, eating it, and so forth. One sensation due to 

 the activity of one area, such as the olfactory area, calls into play the 

 activity of other sensory areas, and of the motor areas, and of the 

 links between the sensory and motor areas. The brain is acting as 

 a whole because its various parts are called into play simultaneously, 

 though the whole brain is not concerned in each of the component 

 sensations and volitions associated with any particular mental state. 



Moreover, the doctrine of cerebral localisation is not accurately 

 expressed by the statement that a cortical centre is one, the stimula- 

 tion of which produces a definite response, and the extirpation of 

 which abolishes the response. We shall, for instance, immediately 

 see that the stimulation of certain areas in the dog's brain produces 

 certain movements, but Goltz showed that in his dogs, the removal 

 of an entire hemisphere did not cause permanent 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 or from them. 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 



