218 PHYSIOLOGY CHAP. 



factors, e.g. the tissue fluids by which they are irrigated, and 

 which regulate their metabolism. 



Although under physiological conditions excitation never 

 occurs along the course of a nerve, it is, as we have seen, excitable 

 at any point, when acted on by an artificial stimulus of sufficient 

 strength. Its excitability is indicated by the minimal intensity 

 of the effective stimulus, when the latter can be measured with 

 sufficient accuracy. Speaking generally, we may say that the 

 minimal intensity of effective stimulus is less for nerve than for 

 muscle, which shows that nervous excitability is greater than 

 muscular excitability, and that the two forms of excitability have 

 a different organic substrate. 



Of the many external agents which throw a nerve into activity 

 when applied experimentally, electrical and mechanical stimuli 

 are usually adopted : the former because they are easily graduated 

 and do little harm to the integrity of the nerve; the latter 

 because their action can be localised to the point of application. 

 Thermal and chemical stimuli are less used, because they are 

 not easy to graduate and are more or less harmful. 



(a} Little need be said in regard to thermal stimuli. The 

 intrinsic temperature of an animal (homoiothermic or poikilo- 

 thermic) does not act as a stimulus on the nerve, but regulates 

 the normal degree of its excitability. 



Nor does abnormal rise or fall of general or local temperature 

 as a rule act as a stimulus when it occurs gradually ; it merely 

 modifies the excitability of the nerve. The rapid heating of a 

 frog's motor nerve, by dipping it into water at 38 C., or bathing 

 any given point with the same, may, according to Valentin, excite 

 a muscular twitch without causing local death of the nerve. But 

 this observation was not confirmed by Eckhard, who found that 

 contractions were only produced by a temperature of 66-68 C., 

 i.e. when the rise of temperature was so great as to destroy the 

 structure of the nerve or permanently alter it. According to 

 Valentin, a rapid fall of temperature to - 5 C. also excites a nerve, 

 though gradual freezing produces neither excitation nor final loss 

 of excitability. 



The later work of Eosenthal, Afanasieff, Grlttzner, and others 

 was directed more to the influence of temperature upon the ex- 

 citability and conductivity of nerve than to its stimulating action. 

 It is true that when the temperature rises above 35 C. or sinks 

 to - 4 C., signs of excitation often ensue, but this fact can be 

 interpreted either as meaning that stimuli that are normally inert 

 become effective in consequence of the rise of excitability, or that 

 the too acute rise or fall of temperature develops specific stimuli 

 of a mechanical or chemical nature. 



In regard to the stimulating action of abnormal temperatures 

 along the course of a sensory nerve, E. H. Weber observed on 



