56 



THE HUMAN MECHANISM 



retreat who feel that they must " drop in their tracks " 

 until the discharge of musketry close behind stimulates 

 them to move faster than ever. 



The feeling of fatigue has its seat in the nervous system, 

 and its study must be postponed until we have learned some- 

 thing of the physiology of the brain and spinal cord. In the 

 present chapter we are not immediately concerned with this 

 side of the question, but rather with the diminution of work- 

 ing power produced by 

 work. Such fatigue 

 must be measured not 

 by our sensations but 

 by the work accom- 

 plished, whether that 

 work be physical or 

 mental. And as we 

 studied the physiology 

 of work in its simplest 

 form in a single work- 

 ing organ, such as a 

 muscle or a gland, so we 



can best begin our study 

 FIG. 33. Diagram of apparatus for recording ? j 



successive muscular contractions 01 diminished working 



power or fatigue in one 

 of these same organs, namely, the skeletal muscle. 



1. Fatigue of an isolated muscle and of a muscle with in- 

 tact circulation. The course of fatigue in a muscle is best 

 studied by causing the muscle to contract to its utmost, at 

 regular intervals of time, against the resistance of a suitable 

 spring. If now we record the height of each contraction, we 

 obtain a series which shows at once the effect of the work 

 on the working power ; that is, the course of fatigue. Fig. 33 

 gives a diagram of the arrangement of such an experiment 

 with an isolated muscle ; that is, a living muscle detached 

 from the rest of the body. One tendon is made fast in a 



