. 



fjf 



CEREBRUM 199 



of impressions, so that they may be associated with present 

 sensations, is indicated by the following considerations : 

 It is this association of present stimuli with past sensations 

 which is the basis of intellectual life, and in man, where 

 apparently the intellectual functions are most highly de- 

 veloped, the frontal and parietal lobes of the brain are much 

 larger than in the lower animals. So far stimulation of these 

 lobes has failed to give indication of resulting sensations or 

 to produce muscular movements. They may be extensively 

 injured without loss of sensation and without paralysis, and 

 hence it has been concluded that the storing and associating 

 functions must be chiefly located in them. Further, in these 

 regions the nerve fibres acquire their medullary sheath at a very 

 late date. 



C. Discharging Mechanism. (a) The position of the discharg- 

 ing mechanism for cerebral action has been definitely localised, 

 by pathological and experimental observation, in the cerebral 

 convolutions round, or probably chiefly in front of, the fissure of 

 Rolando or sulcus centralis (fig. 99, p. 192). Destructive lesions 

 of this area on one side cause a loss of the so-called voluntary 

 action of groups of muscles on the opposite side of the body. 

 The cerebral arc is stimulated and acts along certain lines 

 possibly with the accompaniment of changes in consciousness 

 and a sensation of decision as to the line of action to be taken 

 and a desire to accomplish.it but this so-called volition is not 

 accompanied by the appropriate muscular action. From the 

 frequent involvement of the so-called volition in these actions, 

 and from the fact that it is the metaphysical changes which 

 figure in our consciousness rather than the physical changes 

 which are their basis, we are accustomed to assume that the 

 movements produced are the result of volition, and to speak of 

 them as voluntary movements, and of the brain mechanism 

 producing them as voluntary centres. There is no harm in 

 doing so, if we remember that these centres can and do act 

 without) the involvement of consciousness, and, therefore, without 

 volition ; and that their action generally implies the previous 

 action of parts of the receiving and associating mechanism of 

 the cerebrum. 



But certain lesions may directly stimulate these centres, 



