40 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



being supported by an upright (see Fig. 3). This preparation can then be 



subjected to a variety of tests. 



Mechanical Irritation. If the nerve be cut, pinched, suddenly stretched, or 



subjected to a blow, the muscles of the leg will 

 contract and the foot will be quickly moved. 



Chemical Irritation. If acid, alkalies, vari- 

 ous salts, glycerin, or some other chemical sub- 

 stances be placed upon the nerve, the muscles 

 of the leg begin to twitch irregularly, and as 

 the chemical enters more and more deeply into 

 the nerve the movements will become more 

 and more marked, until finally all the muscles 

 are actively contracted and the foot is held 

 straight up. 



FIG. 3.-Experiment for determining Thermal Irritation. If a hot iron, or the 



tl^e irritability of nerves. 



flame of a match, be applied to the nerve, a 



condition of activity will be developed in the rapidly heated nerve-fibres, and 

 be responded to by more or less vigorous muscular contractions. 



Electrical Irritation. If the wires connected with the two poles of a 

 galvanic cell, static machine, or induction apparatus be brought in contact 

 with the nerve, the muscles will twitch each time there is a sudden change in 

 potential. 



More exact statements with reference to these different forms of irritation 

 will be given later. By all these methods the nerve was excited by irritants 

 applied to it from without, and the muscle was excited to action by the physio- 

 logical stimulus coming to it from the excited nerve. The irritant produced 

 no visible change in the nerve, but the movement of the muscles was an evi- 

 dence that the nerve had undergone a change at the point of stimulation, and 

 that the active state thus induced had been transmitted through the length of 

 the nerve, and had been sufficiently marked to stimulate the muscle to contrac- 

 tion. This condition of activity which was transmitted along the nerve is called 

 the nerve-impulse. 



Independent Irritability of Muscle. In the above instances the irritants 

 were applied to the nerve, and the muscle was indirectly stimulated. Muscle 

 protoplasm, like nerve protoplasm, may be directly excited to action by various 

 forms of irritants. A nerve after entering a muscle branches freely, and the 

 nerve-fibres are distributed quite generally through the muscle. An irritant, 

 if directly applied to muscle, would probably excite the nerve-fibres present as 

 well as the muscle-fibres, and to obtain proof of independent irritability of 

 muscle-substance it would be necessary to prevent the nerves from stimulating 

 the muscle. This can be done by paralyzing the nerve-endings with curare. 



Curare, the South American arrow-poison, is used by the Indians in hunt- 

 ing. The bird shot by these poisoned arrows gradually becomes paralyzed, 

 and, losing power to move its muscles, is easily captured. The following 

 experiment reveals the method of the action of this drug, and at the same 



