CONVOLUTIONS AND FISSURES OF SURFACE OF CEREBRUM. 61 



lie far hack between the corpora quadrigemina and the spinal 

 cord ; that they are, however, so connected with centres situated 

 in the cortex that irritation of the latter gives rise to movements. 

 There is a conflict of opinion as to the nature and importance 

 of the influence exercised by the higher centres over the lower. 

 For this reason we study with the utmost scrupulousness those 

 phenomena which follow the removal of portions of the cortex. 

 Doubtless the importance of the? role played by the cortex differs 

 in different animals. While in the lower animals the removal, 

 even of the whole cerebrum, does not prevent the vigorous exe- 

 cution of coarser movements, yet in mammals the destruction 

 of even circumscribed portions of the motor zone gives rise to 

 temporary paralysis, and in the human being disease of rela- 

 tively small parts of the cortex often leads to a permanent 

 paralysis. It is evident that all motor and many sensori-psychic 

 functions may start from deep-lying centres, but the higher we 

 ascend in the scale of animal life the more is the cortex con- 

 cerned in cerebral activity, and consciousness plays a more 

 prominent part. Man has reached a stage in this connection 

 in which many of the functions cannot be performed without 

 the participation of the cortex. In mammals all possible degrees 

 of variation are observed. Thus is explained the fact that irri- 

 tation of certain tracts of cortex will give rise to muscular 

 action, and yet movements may be executed after those particular 

 parts of cortex have been removed. In man the greater part 

 of the surface of the hemispheres has become indispensable to 

 the proper performance of such movements. 



