540 PHYSIOLOGY. 



electricity, no movements of the muscles of the limb follow. On the 

 other hand,, should the stimulus be applied directly to the muscles, 

 they immediately contract. Therefore the muscle^is irritable by itself. 



By this it would seem to be clearly demonstrated that irritability 

 belongs to the muscle, and does not depend upon the nerve-fibers 

 mingled with those of the muscle. 



In addition to this classical experiment there may be mentioned 

 several other facts which go to corroborate what has been mentioned 

 concerning irritability : 



1. The chemical excitants of the muscle are not the same as the 

 chemical excitants of the nerves. Thus, glycerine excites the nerve, 

 but has no effect upon the muscle. 



2. Isolated muscle-fibers have been seen which, according to 

 microscopical examination, contained no nervous elements and which, 

 notwithstanding, were contractile. 



3. If the decreasing progress of irritability be followed after 

 death, in the muscle as well as in the nerve, it will be found that 

 the nerve dies long before the muscle. When the nerves have lost 

 all irritability, the muscle is still alive, and can contract under the 

 influence of excitations directly applied to its tissue. It is at that 

 very moment when the nerves have lost all excitability that the mus- 

 cle is at its maximum of irritability. 



INFLUENCE OF .BLOOD UPON IRRITABILITY. It has been demon- 

 strated by experiment upon the frog that when the artery of a mem- 

 ber is ligated the muscle contraction is less high and less strong than 

 if the artery had been left intact. 



Sterion's experiment of ligating the abdominal aorta of a dog 

 is worthy of mention. In twenty to thirty minutes after the ligation 

 the dog seems paraplegic. He is unable to stand upon his hind 

 limbs. Reflex and voluntary movements are completely lost; muscle 

 irritability, however, persists for nearly three hours. 



When the ligature is removed movement does not return to the 

 limbs at once, but within a very short time the dog is able to stand 

 upon his four feet. 



Stimuli. Those extreme forces which bring into play the irrita- 

 bility of the muscle are simply various forms of energy. To them 

 the name stimuli has been applied. By their action the muscle is 

 thrown into a state of excitement whereby the chemical energy of 

 the muscle is transformed into heat and work. These muscle 

 excitants, or stimuli, are of five varieties: (a) nervous, (6) electrical, 

 (c) thermal, (d) mechanical, and (e) chemical. 



