GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 71 



mal temperature. The muscles of the cold-blooded animals, and 

 especially the frog, retain their irritability for a much longer period 

 than the muscles of the warm-blooded animals. This is the case also 

 with the individual muscles after removal from the body of the animal. 

 The reason for this is found in all probability in the difference in the 

 rate of their nutritive activities and in the quantity of nutritive mate- 

 rial stored up in their cells. The duration of the irritability of isolated 

 muscles can be considerably prolonged by keeping them moist. 



Muscle Stimuli. Though consisting of a highly irritable tissue, 

 muscles do not possess spontaneity of action. They require for the 

 manifestation of their characteristic activity the application of a 

 stimulus. In the living body aH contractions, at least of the sj^ej^al 

 muscles, occurring under normal or physiologic conditions are caused 

 by_the action of "nerve impulses" transmitted by the nerves fromtfie 

 central nervous system to the muscles. The nerve impulse is the 

 normal or physiologic stimulus. After removal from the body and 

 freed from nerve connections muscles can be excited to action by 

 various agents e. g., mechanic, chejnic, thermic, electric. These 

 are artificial or non-physiologic stimuli. 



1 . Mechanic Stimuli. Cutting, pinching, sharply tapping the muscle 



will cause it to contract, providing the stimulus has sufficient 

 intensity. With each stimulation a short, fleeting contraction 

 ensues. If repeated with sufficient rapidity, a series of con- 

 tinuous but irregular pulsations are produced. 



2. Chemic Stimuli. Various chemic substances in solution will 



excite single or continuous pulsations if the strength of the solu- 

 tion is not such as to destroy at once the irritability. They owe 

 their efficiency as stimuli to the rapidity with which they alter 

 the composition of the muscle-substance. Among these may be 

 mentioned solutions of potassium and sodium, weak solutions of 

 the mineral and organic acids, ammonium vapor, distilled water, 

 glycerin, and sugar. 



3. Thermic Stimuli. The application of a heated object, such as a 



hot wire, causes the muscle to rapidly contract. 



4. Electric Stimuli. The most efficient stimulus and the one least 



injurious to the tissue is the electric current. Either the con- 

 stant or the induced current may be used.* 

 The Constant Current. If the ends of the wires in connection with 

 an electric cell be provided with non-polarizable electrodes and the 



* Since the study of the physiologic properties of both muscle-tissue and nerve- 

 tissue involves the employment of electricity as a stimulus, is becomes necessary for 

 the student to familiarize himself with certain forms of apparatus by which it is gen- 

 erated, controlled, and applied. For the purpose of not interrupting the continuity 

 of the text this information is embodied in an appendix. The facts therein contained 

 should be mastered by the student. 



