GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 65 



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. 



(a) The Influence of Irritants 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 falling 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 short intervals, without interfering with the activity 

 of the nerve. A nerve can be irritated thirty to forty times, at intervals of 

 three to four minutes, by blows from a weight of 0.485 gram, falling 1 to 20 

 millimeters, the contractions of the muscle, weighted with 30 to 50 grams, 

 varying from minimal to from 3 to 4 millimeters in height. Rapidly following 

 light blows or twitches applied to a motor nerve, by the tetanomotor of Heiden- 

 hain or Tigerstedt, excite a series of contractions in the corresponding muscles 

 which fuse more or less into a form of continuous contraction, known as 

 tetanus. 



Mechanical applications to nerve and muscle first increase and later lessen 

 and destroy the irritability. Thus pressure gradually applied first increases 

 and later reduces the power to respond to irritants. Stretching a nerve acts in 

 a similar way, for this also is a form of pressure ; as Valentin said, the stretch- 

 ing causes the outer sheath of the nerve to compress the myelin, and this in 

 turn to compress the axis-cylinder. Tigerstedt states : 2 " From a tension of 

 up to 20 grams the irritability of the nerve is continually increased, but 

 it lessens as soon as the weight is further increased." 



Surgically the stretching of nerves is sometimes employed to destroy their 



1 " Studien uber mechanische Nervenreizung," Ada Soeietatis Scientiantm Fennicce, 1880, 

 torn. xi. p. 32. 2 Op. cit., p. 43. 



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