GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 55 



such circumstances the nerve might respond well to the direct battery current 

 and yet fail to respond to the induced current. This would be still more 

 markedly the case with the muscle, which at the same time that it gave no 

 response to induction shocks would react better than normally to battery 

 currents. At such times galvanotonus is easily excited. Thus during 

 degeneration the irritability of the nerve and muscle approaches that of 

 slowly reacting forms of protoplasm (see p. 70). 



Conditions -which Determine the Irritability of Nerves and Muscles. 

 — We have thus far considered the conditions which determine the efficiency 

 of such an irritant as the electric current. Other irritants are subject to like 

 conditions, their activity being controlled to a considerable extent by the sud- 

 denness, strength, density, duration, and, possibly, direction of application. It 

 is not necessary for us to consider how each special form of irritant is affected 

 by these conditions; it will be more instructive for us to study how different 

 irritants alter the irritability of nerve and muscle, and the relation of irri- 

 tability to the state of excitation. 



The power to irritate is intimately connected with the power to heighten 

 irritability — for a condition of heightened irritability is difficult to distin- 

 guish from a state of excitation. The irritability of cell-protoplasm is very 

 dependent upon its physical and chemical constitution, and even slight altera- 

 tions of this constitution, such as may be induced by various irritants, 

 will modify the finely adjusted molecular structure upon which the normal 

 response to irritants depends. If this change be in the direction of increased 

 irritability, the result may be irritation. But we must defer the discussion of 

 the relation of irritability to irritation until we have considered the conditions 

 upon which the irritability of nerve and muscle depends. These conditions 

 can be best studied in connection with the influences which modify them — 

 namely : 



(a) Irritants. 



(6) Influences which favor the maintenance of the normal physiological 

 condition. 



(c) The effects of functional activity. 



(«) The Influence of Irritant* upon the Irritability of Nerve and Muscle. — 

 Effect of Mechanical Agencies. — A sudden blow, pinch, twitch, or cut excites 

 a nerve or muscle. All have experienced the effect of a mechanical stimulation 

 of a sensory nerve, through accidental blows on the ulnar nerve where it passes 

 over the elbow, " the crazy bone." The amount of mechanical energy required 

 to cause a maximal excitation of an exposed motor nerve of a frog is estimated 

 by Tigerstedt 1 to be 7000 to 8000 milligrammillimeters, which would corre- 

 spond roughly to a weight of 0.500 gram filling fifteen millimeters — at least 

 a hundred times less energy than that given out by the muscles in response to 

 the nerve-impulse developed. Such stimuli can be repeated a great many 

 times, if not given at too shori intervals, without interfering with the activity 



1 Studien iiber niechanisclie Nervenreizung," Acta Societatis Scientiarum Fennica, 1880, 

 Bd. xi. S. 82. 



