776 VOLUNTARY MOVEMENTS. [Book hi. 



is under normal circumstances more exclusively the channel of 

 volitional impulses in man than in such lower animals, but also, 

 bearing in mind the discussion in a previous chapter, § 464, 

 concerning the activities of the spinal cord of man, that the 

 potential alternatives presented by the spinal cord of the dog 

 are greatly reduced in that of man. And such clinical histories 

 of disease or accidental injury in man as we possess support this 

 conclusion. Lesions confined to one half of the cord, or even 

 lesions confined to the lateral column of one half, appear to lead 

 to loss of voluntary power on the same side, and the same side 

 only, in the parts below the level of the lesion ; and the same 

 symptoms have been observed to accompany disease limited 

 apparently to the pyramidal tract of one side. Moreover, 

 though cases of recovery of power have been recorded, we have 

 not such satisfactory evidence as in animals of the volitional 

 impulses ultimately making their way along an alternative 

 route ; but here the same doubts may be entertained as were 

 expressed in discussing the reflex acts of the cord in man. 



When we say that the loss of voluntary power is seen on 

 the side of the lesion only, we should add that this statement 

 appears to apply chiefly to the thoracic and lower parts of the 

 cord. We have seen that in man, in the upper regions of the 

 cord, the pyramidal tract is only partly crossed ; a variable but 

 not inconsiderable number of the pyramidal fibres do not cross 

 at the decussation of pyramids, but running straight down as 

 the direct pyramidal tract effect their crossing lower down in 

 the cervical and upper thoracic regions. Hence we should in- 

 fer that a hemisection of, or a lesion confined to one side of the 

 cervical cord, would affect the voluntary movements of the 

 crossed side as well as of the same side, though not to the same 

 extent. But we have no exact information as to this point. 

 And indeed the purpose of the direct tract is not clear ; there 

 is no adequate evidence for the view which has been held that 

 these direct fibres are destined for the upper limbs and upper 

 part of the body ; since they are the last to cross we should 

 a priori be inclined to suppose that they were distributed to 

 lower rather than higher parts. 



§ 493. We may now briefly summarize what we know con- 

 cerning voluntary movements. And it will be convenient to 

 trace the events in order backwards. 



Certain muscles are thrown into a contraction which even in 

 the briefest movements is probably of the nature of a tetanus. 

 In almost every movement more than one muscle as defined by 

 the anatomists is engaged, and in many movements a part of 

 several muscles is employed, and not the whole ^of each. It is 

 perhaps partly owing to the latter fact that a muscle which has 

 become tired in one kind of movement, may shew little or no 

 fatigue when employed for another movement, though we must 



