THE CENTRAL NERVOUS SYSTEM. 89 



to ascribe any sensory function to the pre-central convolution, and it 

 must be considered as purely motor. 



The effect of removal of the motor area varies in different animals. 

 In all cases the immediate effect is paralysis of the muscles on the 

 opposite side of the body. In the dog recovery takes place, and the 

 power of movement becomes almost as complete as it was before the 

 operation. In the monkey recovery is less complete, and a certain degree 

 of weakness may remain as a permanent result. When the motor area or 

 any part of it is destroyed by disease in man, recovery is still less com- 

 plete. In the ascent of the animal scale the functions of the nervous 

 system become more and more transferred* to the higher centres ; hence 

 injury of these centres in the higher animals, and especially in man, 

 is productive of more serious interference with the neuro-muscular 

 mechanism than is the case in animals lower in the scale. 



The Area for Skin and Muscle Sense. It is obviously a matter of 

 considerable difficulty to locate the area for tactile and muscular sensi- 

 bility, inasmuch as stimulation of a sensory area in animals is followed 

 by no objective phenomena, and opportunities rarely arise for observa- 

 tion of the results of stimulation in conscious human beings. There 

 are, however, recorded cases of stimulation of the post-central convolu- 

 tion in conscious individuals, and in these sensations of numbness and 

 touch were evoked. Apart from stimulation, the localisation of the 

 sensory areas rests upon the effects of removal, the distribution of the 

 thalamo-cortical fibres which represent the upward continuation of the 

 fillet, and histological and clinical observations. 



Stimulation of the post-central convolution in animals is followed 

 . by muscular movements, but a stronger stimulus is required to elicit 

 these than is necessary if the pre-central convolution is stimulated, and 

 the latent period is longer in the case of the post-central, indicating that 

 the impulse has to traverse a larger number of neurons. This result 

 is what might be anticipated,- as it is probable that the post-central and 

 pre-central convolutions are connected by short association fibres. 



Removal of the post-central convolution in monkeys is said to result 

 in ataxia of the muscles of the opposite side of the body without 

 paralysis. On the other hand, Graham Brown has excised a part of 

 the post-central convolution opposite the arm area of the pre-central 

 in a young chimpanzee, and records that after a short period of weak- 

 ness of the opposite fore-limb there was no appreciable permanent motor 

 defect. 



The thalamo-cortical fibres are distributed not only to the post- 

 central convolution but also to the temporal, frontal, and occipital 

 lobes, and therefore sensory impulses are distributed tc- a much wider 



