MOTOR NERVES AND MUSCULAR CONTRACTION. 



257 



strain appear: the face becomes flushed, the superficial veins 

 enlarge, more or less sweating occurs, etc.: i.e., all the familiar 

 signs associated with continued effort assert themselves. Re- 

 sistance evidently underlies the whole process, and, as "resist- 

 ance" always implicates at least two contending forces, we are 

 led to divide the process itself into two parts: i.e., the weight 

 which tends to force the hand down and the muscular effort 

 exerted to hold it up. But if we analyze the muscular effort, 

 it soon becomes apparent that it is itself susceptible to a clearly 

 defined subdivision. Indeed, notwithstanding the weight, the 

 arm remains fixed in one position; and the entire organism 

 shows the effects of strain; muscles other than those of the 

 arm contribute work, the entire circulatory system (including 

 the heart, judging from its overaction) enter into a phase of 

 unusual activity, etc., all laboring to the one end, viz.: to 

 mechanically satisfy, regardless of the aggregate of energy 

 converted into work, the needs of the voluntary act physically 

 impressed upon tlie muscle and transmitted to it from the brain 

 through the motor nerve. We thus have, on the one side, a 

 form of volitional energy through which the muscle is fixed in 

 the one position; and, on the other, an oxidation process, 

 through which muscular work is carried out, sustained, and 

 intensified to the highest possible degree compatible with the 

 body's strength. 



That two distinct processes are present may be shown in 

 several ways. Professor Foster, referring to the "impulse- 

 wave" states, for example: "It is followed by an explosive de- 

 composition of material, leading to a discharge of carbonic 

 acid, etc." Not only does the active reaction occur after the 

 dispersion of the impulse, but Helmholtz ascertained that quite 

 a perceptible and computable period of time elapsed between 

 the two phenomena. By means of the Marey myograph this 

 "latent period" was found to occupy one-sixtieth of a second, 

 while the maximum contraction is only reached at the end 

 of about one-sixth of a second 6 in an average muscle. A 

 radical difference is also evident in the relative ability of the 

 two kinds of energy volitional and motor to undergo fluct- 



M. Duval: Loc. cit., p. 151. 



