8 BIOTIC STRUCTURE AND BIOTIC ENERGY 



ordinary sense of the word; while skeletal muscle lacking, under 

 normal conditions, the more rapid rhythm can be fatigued. 



Between each beat the cardiac muscle passes rapidly through 

 its period of fatigue and recuperation. After each contraction its 

 structure is so changed that it is completely fatigued and so cannot 

 again be stimulated and kept at chemical work till this phase passes 

 over; then there follows another cycle of exactly similar form. But 

 each charge and discharge is complete in itself, and there can be no 

 accumulation of waste products. Skeletal muscle, on the other hand, 

 loses its waste products more slowly and does not at once become 

 saturated with them ; a slight accumulation does not cause it to be- 

 come inactive and relax, and it can have a second stimulus, causing 

 a second or continued contraction to be imposed upon the first. 

 This leads to continued chemical activity and a continued accumu- 

 lation of waste products similar to what is termed in therapeutics 

 the cumulative effect of a drug. The final result is stoppage of 

 activity, and that more prolonged period of inaction or diminished 

 activity ensues which we call fatigue of voluntary muscle. 



It is curious that it does not appear to have occurred to those 

 who have experimented upon the infatigability of nerve fibres, 

 that this physiologically essential property is due to the nerve fibre 

 possessing a rhythm which is very rapid, so that nerve fibre is never 

 continuously active, but merely transmits short waves of activity 

 with incomparably longer periods of repose between them in which 

 there is a phase of fatigue and recuperation, just as in the case 

 above described of cardiac muscle. 



It does not by any means follow that because what is commonly 

 called a tetanising current, from its effects upon muscle, is being 

 applied to a nerve, the nerve processes are thereby being teta- 

 nised or thrown into continuous activity. The muscle attached may 

 be in continuous activity, and yet the material of the nerve may be 

 enjoying its regular periods of rest and recuperation between 

 each stimulus just like a beating heart, and, for this reason, like 

 the heart, be incapable of ordinary fatigue. Consideration of the 

 known results of experimental work upon the nerve impulse due to a 

 single stimulus shows that this is the true state of affairs. The 

 electrical variation which we may take as a true index of the dura- 

 tion of the chemical activity of the nerve fibre has been shown to 

 pass over any given cross- section of the nerve in an almost infini- 

 tesimal part of a second, in something like TO^TO second. Now sup- 



