MUSCLE. 87 



plying it which excite it to action. It is more usual to stimulate 

 the nerve, as this is more convenient and more effectual. There 

 are four kinds of excitants that may be used : 1, mechanical, as by 

 a blow or prick or pinch ; 2, heat, as by a hot needle ; 3, chemical, 

 as dilute acids, etc. ; 4, electrical, as by the galvanic or faradic cur- 

 rent. This is the most usual as well as most convenient kind of 

 stimulus for experiment. 



How are the phenomena of a contraction observed ? 



They are commonly observed in a tracing upon an instrument 

 called the myograph, and usually a large muscle of a frog and its 

 nerve are taken for the demonstration. The time of the observa- 

 tion is recorded by the vibrations of a tuning-fork. Such a prepa- 

 ration is known as " a nerve-muscle preparation," and the tracing 

 obtained is called a " muscle-curve." 



At what period of an electrical current do the muscles contract ? 



Only at the making and breaking. If a nerve-muscle prepara- 

 tion be arranged for experiment and stimulus be applied by means 

 of a galvanic cell, it will be found that the muscle will contract 

 only at the instant the circuit is completed and at the instant it is 

 broken. During the time the current is passing no contractions of 

 the muscle occur, unless the character of the current changes or 

 the strength of the current is considerable. The mere passage of 

 an electrical current does not cause a muscle to contract, but the 

 entrance or exit of the current is the stimulus. It does not matter 

 if the positive pole be applied to the nerve nearer the muscle, or if 

 the negative pole is the nearer : the contraction is the same with 

 an ascending as with a descending current. 



Describe the phenomena observed in a single contraction fol- 

 lowing a stimulus. 



There is a brief period during which no action occurs, the latent 

 period, during which the stimulus applied to the nerve is being 

 transmitted to the muscle. Then follows a sharp, sudden contrac- 

 tion of the muscle (stage of contraction), which is intensified 

 steadily to a maximum, and then the muscle more slowly relaxes 

 (stage of elongation), and the muscle returns to its former condi- 

 tion of rest. There are sometimes less violent contractions which 

 follow, and are due to the elasticity of the muscle. The whole 

 time consumed is about one-tenth of one second, and of this time 



