636 HANDBOOK OF PHYSIOLOGY. 



actually represented in the cortical areas, and that the pyramidal tracts 

 are actually essential for voluntary movements. If the pyramidal 

 tracts be partially or wholly destroyed, anywhere in their course, a 

 paralysis corresponding with the amount destroyed invariably follows. 

 In the dog experiments have shown that this is not the case, and the 

 conduction of voluntary impulse to muscles may take place, for example, 

 in other parts of the cord besides the pyramidal tract, after hemisection. 



The pyramidal tracts in man, however, must be considered also as 

 the only path connecting the cortical centres with the co-ordinated 

 centres lower down in the brain, as, for example, in the bulb. The 

 impulses which pass down from the cortex, whatever they may be, are 

 not however of necessity connected with consciousness, and many volun- 

 tary movements of a complicated nature may take place really better with- 

 out consciousness than with it. This is shown in such co-ordinated 

 movements as writing, walking, marching, and the like, all of which are 

 acquired with time and much labor, but when once perfect in the 

 individual, can best be performed without voluntary effort. Such 

 movements must be represented by impulses passing in the pyramidal 

 tracts, for if they are interrupted, the movements are no longer per- 

 formed. 



What actually originates a voluntary action, or one performed by 

 an effort of the will, we are unable to say. No doubt impulses from the 

 periphery conducted to the cerebral cortex along all kinds of afferent 

 channels must have something to do with it; directly or indirectly, 

 sooner or later. In the human cortex it would seem that the apparatus 

 for performing all manner of possible co-ordinated movements which may 

 result in speech or action, are stored. This apparatus is capable of 

 being set in action either in the absence of consciousness by afferent 

 stimuli of some kind directly, or by what may be, indirectly or remotely, 

 in some way the result of afferent stimuli, viz., the will. It is also prob- 

 able that the will of another may take the place of the man's own will, 

 and may call for the movements, actions, and speech, all of which are, 

 as it were, ready to be called forth by a stimulus of some kind. It may 

 be supposed that the condition of development of the brain inherited by 

 the individual has something to do both with the potentialities of the 

 apparatus for co-ordinated acts, which he receives at birth, and with the 

 way in which the apparatus is set in motion. 



Unilateral Action. Respecting the mode in which the brain dis- 

 charges its functions, there is no evidence whatever. But it appears 

 that, for all but its highest intellectual acts, one of the cerebral hemi- 

 spheres is sufficient. For numerous cases are recorded in which no 

 mental defect was observed, although one cerebral hemisphere was so 

 disorganized or atrophied that it could not be supposed capable of dis- 



