130 THE HUMAN BODY. 



nervous impulse is the natural physiological muscular 

 stimulus it is not the only one known. If a muscle be 

 exposed in a living animal and a slight but suddenjtap be 

 given to it, or a jiot bar be suddenly brought near it, or an 

 electric shock be senT through it, or a drop of glycerine or 

 oTsolutioirof ammonia be placed on it, it will cbnlract; so 

 that in addition to tlTe natural nervous stimulus, muscles 

 are irritable under the influence of mechanical, thermal, 

 electrical, and chemical stimuli. One condition of the ef- 

 ficacy of all of them is that they shall act with some sud- 

 denness; a very slowly increased pressure, even if ultimately 

 very great, or a very slowly raised temperature, or a slowly 

 increased electrical current passed through it, will not ex- 

 cite the muscle; although far less pressure, warmth, or 

 electricity, more rapidly applied would stimulate it power- 

 fully. It may perhaps still be objected that it is not proved 

 that any of these stimuli excite the muscular fibres, and 

 that in all these cases it is possible that the muscle is only 

 excited through its nerves. For the various stimuli named 

 above also excite nerves (see Chap. XIII), and when we ap- 

 ply them to the muscle we may really be acting first upon 

 the fine nerve-endings there, and only indirectly and 

 through the mediation of these upon the muscular fibres. 

 That the muscular fibres have a proper irritability of 

 their own, independently of their nerves, is, however, shown 

 by the action of certain drugs for example cur^ud, a South 

 American Indian arrow poison. When this substance is 

 introduced into a wound, all the striped muscles are 

 apparently poisoned, and the animal dies of suffocation 

 because of the cessation of the breathing movements. But 

 the poison does not really act on the muscles themselves: 

 it kills the muscle nerves, but leaves the muscle intact; and 

 it kills the very endings of the muscle-nerves right down 

 in the muscle fibres themselves. Now after its administra- 

 tion we still find that the various non-physiological stimuli 

 /eferred to above make the muscles contract just as 

 powerfully as before the poisoning, so we must conclude that 

 the muscles themselves are irritable in the absence of all 

 nerve stimuli or, what amounts to the same thing, when 



