FATIGUE AND RECOVERY OF MUSCLES AND NERVES 443 



animal, the poison will be gradually thrown off from the body, and the end 

 plates will again recover their function. But several hours intervene, and a 

 stimulus applied during the interval is of course without effect on the muscle. 

 But when the poison wears off, the effect of stimulation returns with all its orig- 

 inal force, which means that stimulation continued for hours has not fatigued the 

 nerve. Brodie and Halliburton observed likewise that nonmedullated nerves, such 

 as the splanchnic, were not fatigued by artificial stimulation lasting six hours. 



Besides this any number of natural phenomena show that nerves have a 

 much greater endurance. We know, indeed, that several efferent nerves, espe- 

 cially the vagus branches to the heart, are all the time under a tonic excitation 

 of greater or less intensity, also that the same is true of the afferent nerves, 

 examples of which we have in the constant pains of certain nervous maladies. 



From these facts the conclusion has been drawn that nerves in general are 

 not fatigued, and it cannot be denied that this conception is, to a certain extent, 

 well founded. Nevertheless, one must not imagine that no metabolic processes 

 are taking place in an active nerve or that it mediates the transmission of 

 stimuli, as for example a wire does an electric current ; such a supposition has 

 little probability in its favor on purely antecedent grounds, for a nerve is a 

 living tissue. Moreover, there are a number of direct observations at hand which 

 show the presence of chemical processes in nerve with perfect definiteness. 



For example, a nerve deprived entirely of oxygen becomes completely inex- 

 citable within three to five hours, but recovers its excitability again within three 

 to ten minutes when oxygen is supplied. This phenomenon as well as the pro- 

 duction of carbon dioxide in active nerves (cf. page 434) substantiates the view 

 that a nerve, so far as processes taking place within it are concerned, presents 

 no essential difference from the other organs of the body. On the other hand 

 its extraordinary resistance to fatigue presupposes a very low state of metabolism 

 and a very great power of recuperation. This ability to recover is probably 

 different also in different nerves; for in the olfactory nerves of the pike unmis- 

 takable signs of fatigue make their appearance after only a short period of 

 excitation (Garten). 



Contractions can still be induced by direct stimulation long after the mus- 

 cle fails to respond to a tetanizing stimulus applied to its nerve. Since the 

 nerve itself does not fatigue we must suppose that the nerve endings fatigue 

 much earlier than the muscle substance itself (Waller). 



B. FATIGUE OF HUMAN MUSCLES AND NERVES 



The phenomena of fatigue in man have recently been studied by several 

 authors by means of the ergograph, an apparatus first constructed by Mosso. 



This ergograph is especially constructed for the flexion of the middle finger, 

 and consists of two parts, one to which the hand is fastened and another which 

 records the contractions of the muscle. The whole apparatus is shown in Fig. 

 179. The forearm is fixed in position by means of the clamps and the hand by 

 means of the two tubes into which the index and ring fingers are thrust. A 

 string fastened to the middle phalanx of the middle finger, carries the load and 

 moves the writing lever. The latter records the contraction of the muscle, 

 enlarged about twice, on a slowly rotating drum. The work of the muscle is 

 of course the product of the actual height of contraction by the load. 



If now the load be not too light and the interval between contractions not 

 too great, the height continually declines until finally the subject is no longer 



