CH. XII.] FATIGUE IN NERVE 135 



We have made similar experiments with vaso-rnotor nerves, such as the cervical 

 sympathetic nerve in the rabbit, the splanchnic nerve of the dog, and the sciatic 

 nerve in a curarised dog, and have obtained corresponding results. This confirms 

 the work previously published by Eve. Eve excited the cervical sympathetic for 

 twelve hours, and found no loss of excitability at the end of that time. Eve 

 stimulated the nerve below the upper cervical ganglion, and the main object of his 

 work was to ascertain whether any histological evidence of fatigue could be found 

 in the cells of the ganglion. The only change he could find there was a somewhat 

 diffuse staining of the cells by methylene blue, which he attributes to the formation 

 of acid substances in the cells. A blue stain of similar appearance may be induced 

 in the motor cells of the spinal cord, after exhaustion is produced in them by giving 

 strychnine. In such experiments the spinal cord becomes, as a rule, distinctly acid 

 to litmus paper. Max Verworn has more recently employed strychnine as a means 

 of producing fatigue. He considers that the only specific effect of this alkaloid is 

 increase of reflex activity, and he attributes the subsequent paralysis to vascular 

 conditions and the accumulation of fatigue products, among which he places carbon 

 dioxide in the first rank. Eve, on the contrary, did not find that carbonic acid 

 alone produces the effects. 



We must conclude from such experiments that Dr Waller's theory is unproved, 

 and that while fatigue is demonstrable in nerve-cells, it cannot be shown to occur 

 in nerve-fibres of either the medullated or non-medullated variety by these methods. 



In carrying out these experiments we noticed that though no functional fatigue 

 can be demonstrated, there is noticeable, especially in vaso-motor nerves, a 

 phenomenon which Howell terms stimulation fatigue ; this means that the actual 

 spot of nerve stimulated becomes after a time less excitable, and finally, inexcitable, 

 though it will still transmit impulses, if the excitation is applied above the spot 

 originally stimulated. We think that the use of the term "fatigue" in this con- 

 nection is a mistake ; the prolonged electrical excitation causes injurious polarisa- 

 tion (due to electrolytic changes) of the nerve, which renders it less excitable. This 

 view has been confirmed by Prof. Gotch by means of experiments with the capillary 

 electrometer. This so-called *' stimulation fatigue " was not excluded in Miss 

 Sowton's experiments, and will possibly explain her results. The splenic nerves, 

 curiously enough, do not exhibit this phenomenon in any marked degree, and so 

 were peculiarly well adapted to test the question of functional fatigue. On a priori 

 grounds we should hardly expect non-medullated nerves to be peculiarly susceptible 

 of real fatigue, when one considers that many of them, such as the vaso-con- 

 strictors, are in constant action throughout life. 



It should be clearly understood that all these experiments prove only that 

 nerve-fibres are not fatiguable under ordinary conditions of stimulation. If we 

 assume that nerve is entirely "unfatiguable," we must assume also that its 

 activity is not associated with the consumption of material and the production of 

 waste products. This would render nerve unique among all the other tissues of the 

 body, and is, moreover, contradicted by recent discoveries of evidence of metabolic 

 changes in a nerve during its activity. We are therefore driven to the conclusion 

 that repair is exceedingly rapid and perfect, although it is impossible to agree with 

 Waller that the repairing process is definitely associated with the presence of a 

 medullary sheath. The interval between successive induction shocks is certainly 

 short, but it is apparently long enough to allow the nerve to recover completely 

 before the next stimulus arrives. If, however, the interval between two successive 

 stimuli is made very brief indeed (O'OOG sec.), the second stimulus is ineffective 

 because of the fatigue due to the first. If the irritability of the nerve is depressed 

 by cold, by asphyxia, or by an anaesthetic (such as yohimbine), the irresponsive 

 period may be lengthened to as much as one to two tenths of a second. 



In one of the foregoing paragraphs the following sentence occurs : "fatigue is 

 demonstrable in nerve-cells." M'Dougall has recently adduced evidence that 

 fatigue in the central nervous system has its seat not so much in the bodies of the 

 nerve-cells as in their synaptic junctions, which are the points of highest resistance 

 (that is, where impulses pass with greatest difficulty) in all mental and other 

 operations in which the brain and spinal cord share, even when no fatigue exists. 



