680 A MANUAL OF PHYSIOLOGY 



process which need not be either entirely chemical or entirely 

 physical, and very likely is a complex one, the suggestion, so far as 

 it goes, is undoubtedly in favour of the chemical hypothesis. 



That chemical changes go on in living nerve we need not hesi- 

 tate to assume ; and, indeed, if the circulation through a limb of a 

 warm-blooded animal be stopped for a short time, the nerves lose 

 their excitability. But the metabolism is very slight compared 

 with that in muscle or gland. Even in active nerve no measurable 

 production of carbon dioxide has ever been observed, nor, in fact, 

 has any chemical difference between the excited and the resting 

 state ever been unequivocally made out. Neither in cold-blooded 

 nor in mammalian nerves is there any sensible rise of temperature 

 during stimulation. It has already been stated that, under ordinary 

 conditions, nerve-fibres are practically insusceptible of fatigue, and 

 this has been considered a strong support of the physical nature of 

 the conduction process. Nevertheless, it is possible to show by 

 special methods that nerve can be temporarily fatigued, although 

 it recovers very rapidly. When a medullated nerve is stimulated, 

 a brief period ensues during which it refuses to respond to a second 

 stimulus. This refractory period is normally very short not more 

 than o'oo2 second for the frog's sciatic. But it can be greatly pro- 

 longed by cold, asphyxia, or anaesthesia, especially by the alkaloid 

 yohimbine (Tait and Gunn), and when the refractory period is thus 

 prolonged, fatigue phenomena are readily induced by stimulation. 



Stimulation of Nerve. With some differences, the same 

 stimuli are effective for nerve as for muscle (p. 632) ; but chemical 

 stimulation is not in general so easily obtained. The so-called 

 thermal stimulation is not a real stimulation due to the sudden 

 change of temperature. The irregular contractions of the 

 muscle caused by the local application of heat to the nerve are 

 dependent on desiccation of the nerve. 



Chemical Stimulation. When hyper- or hypotonic solutions are 

 employed, the withdrawal or entrance of water may be an important 

 factor. 



For salts which penetrate the fibres with equal difficulty this 

 factor can be eliminated by applying them as isotonic solutions. 

 There is evidence that chemical stimulation proper, as distinguished 

 from the stimulation produced by changes in the water content of 

 the fibres by osmosis, is connected with the electrical charges on 

 the dissociated ions of the salts (p. 400). Electrical stimulation, 

 indeed, may only be a variety of chemical stimulation (Loeb, 

 Mathews, etc.). 



Mechanical stimulation may be applied to a nerve by allowing a 

 small weight to fall on it from a definite height or permitting mercury 

 to drop upon it from a vessel with a fine outflow tube. A regular 

 tetanus may thus be obtained. Tigerstedt found that the smallest 

 amount of work spent on a frog's nerve which would suffice to excite 

 it was a little less than a gramme-millimetre that is, the work done 

 by a gramme falling through a distance of a millimetre, or (taking 

 an erg as equivalent to TTT \y^ gramme-centimetre) about 100 ergs. 

 No doubt a great part of this is wasted, as a much smaller quantity 

 of work done by a beam of light on the retina or by an electrical 

 current on an isolated nerve, both of which may be supposed to act 

 more directly on the excitable constituents, suffices to cause stimu- 



