1066 VOLUNTARY MOVEMENTS. [BOOK in. 



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 in- 

 considerable 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 infer that a hemi- 

 section 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. 



665. We may now briefly summarise 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 bear in mind 

 that-fin a voluntary movement fatigue is much more of nervous 

 than of muscular origin. 



Besides the active muscles, if we may so call them, which 

 directly carry out the movement, the metabolism of which supplies 

 the energy given out as work done, other muscles, some of which 

 are antagonistic to the active muscles and some of which may be 

 spoken of as adjuvant, enter into the whole act. In flexion for 

 instance of the forearm on the arm it is not the flexor muscles only 

 but the extensors also which are engaged. According to the 

 immediately preceding position and use of the arm, and according 

 to the kind and amount of flexion which is to be carried out, the 

 extensors will be either relaxed, that is to say inhibited, or thrown 

 into a certain amount of contraction. And in some of the more 

 complicated voluntary movements the part played by adjuvant 

 muscles is considerable. Hence in a voluntary movement the will 

 has to gain access not only to the active muscles, but also to the 

 antagonistic and adjuvant muscles; and every voluntary move- 

 ment, even one of the simplest kind, is a more or less complex act. 



The impulses which lead to the contraction of the active 

 muscles reach the muscles along the fibres of the anterior roots, 



