446 PHYSIOLOGY 



With the development of the cerebral hemispheres in the higher 

 mammals there is a considerable shifting of motor reactions from 

 those which are immediate and ' fatal ' or inevitable to those which 

 are educatable. The cerebral hemispheres in man take a large part 

 in the determining of even the common reactions of everyday life. 

 Ablation of the hemispheres therefore, or even part of the hemispheres, 

 in the ape and man gives rise to much more lasting symptoms than is 

 the case in the animals we have just studied. These defects we shall 

 have to consider more fully later. The results, however, obtained on the 

 lower animals, from the dog downwards, show that the brain stem, from 

 the head ganglion of the optic thalamus back to the medulla, with the 

 spinal cord, represents a complex mechanism which can be played 

 upon by impulses received through all the sensory apparatus of the 

 body, and is able to adjust the motor and visceral reactions to the 

 immediate environment of the animal. 



Certain of these immediate reactions are susceptible of further 

 physiological analysis. We have seen that the spinal cord contains 

 the co-ordinated mechanism for the movement of the limbs. We may 

 now discuss how the movements of the limbs are co-ordinated with 

 those of the trunk and head in the maintenance of the unstable position 

 of the animal in standing and in locomotion. For this purpose there 

 has been developed the great mass of nerve matter in the roof of the 

 metencephalon, viz. the cerebellum. 



