120 THE HUMAN MOTOR 



The length of the path is diminished when the nervous centres 

 of the movement are those of the spinal cord ; for example, in 

 simple reflex actions and in " automatic " movements, there 

 being fewer relays. 



The duration of the reflex decreases also as the intensity o| 

 the excitation increases ; it is in fact influenced by the particular 

 state of organisation, acquired, or hereditary, which assists 

 the transmission called the " nervous influx." The time which 

 elapses from the instant of impression, until the instant when 

 movement begins, is the "personal equation/'; and it is, 

 obviously, a qualitative element. Numerically it is from J to T *& 

 of a second, for various circumstances and individuals. 



The following averages are quoted : 



Tactile reaction -^- 



Visual 1 ' 9 - 



Auditory r ' 5 



84. Laws of Muscular Contraction. Contraction does not 

 necessarily consist in the shortening of the muscle, for if there is 

 sufficient resistance to overcome, the muscle will keep its length 

 (" absolute " static contraction or absolute force) : but, as a 

 rule, it shortens, its width increasing and its length decreasing, 

 hence the volume is not appreciably altered. 



The shortening affects all the fibres; thus the force of contraction 

 is the sum of these elementary contractions. Chauveau ( l ) has 

 made a minute analysis of the elements which enter into the 

 effort of contraction, as follows : 



According to the law of elasticity, a muscle sustaining a weight 



PL 



P will be lengthened by a quantity L = = ^ (see 44). If by its 



contraction it resists this elongation, its internal foice will be 

 equal and opposite to P. If it also shortens by a quantity r while 

 sustaining the weight, the internal force will be greater than P 

 by the effort necessary to restore the muscle to its original length ; 

 this supplementary effort will therefore be proportional to the 

 shortening and will be P X r. The force of static contraction, to 

 balance a weight P with a shortening y, will have the total value : 



F = P + Pr = P (1 +r). 



It is therefore equal to an elastic force F, including the 

 "effective " elastic force P and a " disposable" elastic force Pr, 

 varying with the shoitening. It is evident that economy of foice 

 is attained by reducing the shortening of the muscle to a mini- 



(*) A. Chauveau (Comptes Rendus Sciences, vol. cxxvii p 983 1898 ' 

 Journal de Physiologic, 1899, p. 157). 



