THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 757 



B 



support of the physical nature of the conduction process. Neverthe- 

 less, it is possible to show by special methods that nerve can be tempor- 

 arily 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 0-002 second for the frog's sciatic. But it can 

 be greatly prolonged 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. 

 And while the nerves of warm-blooded animals at body temperature 

 and those of cold-blooded animals at about 32 C. can hardly be shown 

 to undergo fatigue when tetanized in atmospheric air, fatigue phe- 

 nomena are easily elicited when the temperature is lowered even 

 although air is supplied (Thorner). 



Stimulation of Nerve. With some differences, the same stimuli 

 are effective for nerve as for muscle (p. 711); but chemical stimula- 

 tion is not in general so easily obtained. The so-called thermal 



Fig. 265. Electrical - Resistance g g 



Thermometer (Natural Size) as i ^ * 1 



used for investigating heat -pro- 

 duction in mammalian nerves in 

 situ. A, a piece of hard rubber in 

 the hook-shaped part of which the 

 fine platinum wire P is fixed, and 

 covered with insulating varnish; 

 c, c, thick copper wires connected A 

 with P, fastened in grooves, and A 

 covered with paraffin. Above they 

 end in contact with the small 

 binding posts, By B 2 . B is a hard 

 rubber sliding piece, with a slot *. 

 When B is in position the screw, a, 

 projects through the slot. By a nut 

 on this screw B is fixed on A when 

 the nerve has been arranged in the / 



groove. P 



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 desicca- 

 tion of the nerve. 



Chemical Stimulation. When hyper- or hypotonic solutions are em- 

 ployed, 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 iso tonic 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. 422). 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 by 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 



