430 ANATOMY AND PHYSIOLOGY OF THE BEAIN. 



Injury to one side causes, as already noticed, a rotary 

 movement of the body, probably from the induced blindness 

 and vertigo. 



The optic thalami, notwithstanding their name, do not 

 preside over vision ; and they may be removed without, to 

 any extent, interfering with this sense. The removal of 

 one optic thalamus, however, leads to continued rotary 

 movements ; the animal standing and turning perpetually 

 to the injured side. If both are removed, the animal can 

 still stand, and even use his limbs in walking, but no 

 longer turns to one side. The turning thus seems depen- 

 dent on a want of balance between the action of the two 

 organs. The true physiological importance of these bodies 

 is by no means well understood. 



The corpora striata are placed between the crura cerebri 

 and the hemispheres, and contain many fibres passing from 

 the one to the other. They may consequently be supposed 

 to exercise some influence on sensation and volition, but 

 what their real functions are has not been satisfactorily 

 made out. A more or less complete paralysis of the poste- 

 rior extremities followed their mechanical injury in the 

 experiments of Colin and others; the effects, when one 

 only was injured, showing themselves on the opposite side of 

 the body. This is, however, not invariable ; and the state- 

 ment of Magendie, that animals, after removal of these 

 bodies, show an irresistible tendency to move rapidly for- 

 ward, has been contradicted by more recent investigators. 



The hemispheres of the brain are acknowledged, on all 

 hands as implicated in the performance of the higher 

 mental acts in the human subject, and in those acts of 

 memory and intellect by which the animals immediately 

 below man are characterised. The size of the hemispheres, 

 accordingly, and the amount of the superficial grey matter, 



