CHAP. IL] THE BRAIN. 1061 



efferent impulses along the pyramidal fibres is markedly affected 

 by sensory impulses and especially by sensory impulses started in 

 the skin overlying and corresponding to the muscles put into 

 movement. How those sensory impulses reach the cortex we do 

 not exactly know ; but we have no evidence to shew that afferent, 

 centripetal impulses can travel backwards so to speak along the 

 pyramidal fibres; and it is more reasonable to suppose that the 

 sensory impulses in question reach the cortex by the ordinary paths 

 of sensory impulses, which we shall presently discuss. We may 

 therefore take the results of the experiments as shewing how close 

 is the connection of the motor area with the sensory mechanisms of 

 the spinal cord and lower parts of the brain, and as illustrating the 

 complexity of the chain of events by which the motor area brings 

 about voluntary movements. 



662. We have above used the general phrase 'movements of 

 the limb,' since in the dog it is not easy to pick out certain 

 movements as being particularly skilled movements. In the 

 monkey such a distinction is easier. In this animal, as we have 

 said, recovery of voluntary movement also takes place after removal 

 of a cortical area, or at least has done so in many cases ; and while 

 the phenomena immediately following removal on the whole 

 resemble those witnessed in the dog, a certain order of recovery 

 may be observed; the more skilled movements are the last to 

 return. When for instance the arm area is removed, the delicate 

 movements of the hand, of the thumb and finger, are the last to 

 be re-established ; and a condition of things may be met with in 

 which the animal after removal, say of the arm area in the left 

 hemisphere, uses by preference the left hand at a time when, if 

 prevented from using that hand, he is able to use the right ; that 

 is to say, the recovery in the right limb after the removal of the 

 area on the left side is nearly but not quite complete ; the ' will ' 

 can gain access to the right hand, but not so easily as to the left 

 hand, and this latter is used, though under ordinary circumstances 

 it would not be used. 



When we turn to man, in whom the great development of the 

 pyramidal system and differentiation of the cortical area is paral- 

 leled by the prominence of skilled and trained movements, the 

 analogy of the phenomena of speech, if it be true as clinical 

 histories seem to shew that destruction by disease of the speech 

 area of both sides causes permanent aphasia, would lead us to 

 conclude that at least highly skilled voluntary movements are 

 carried out by the pyramidal system and by that alone. But in 

 reference to this it must be remembered that such a permanent 

 aphasia may be due, not to mere loss of the pyramidal channel, 

 not to the will being merely unable to gain access to lower 

 coordinating mechanisms, but to the absence of the differentiated 

 cortical grey matter, by reason of which absence the will cannot 

 initiate the first processes of the act of speech ; it may be that 



