56 TEXT-BOOK OF PHYSIOLOGY. 



for the purposes of ordinary muscle actions. Nevertheless if a long muscle, 

 such as the sartorius, from a curarized frog be stimulated at one end with 

 an induced electric current, the excitation and the contraction processes will 

 be conducted with extreme rapidity to the opposite end of the muscle. The 

 rapidity of conduction in human muscles has been estimated at from 10 to 13 

 meters per second, and in frog's muscle at from 3 to 4.5 meters per second. 

 The contraction process, the thickening of the muscle, is termed the con- 

 traction wave. As it is the result of the excitation process and immediately 

 succeeds it, its rate of conduction must be the same as that given above. 

 With appropriate apparatus the duration of the wave at any given point has 

 been shown to be, in the frog's muscle, about one- tenth of a second and its 

 length three-tenths of a meter 



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 all contractions, at least of the skeletal muscles, occurring under 

 normal or physiologic conditions are caused by the action of " nerve im- 

 pulses" transmitted by the nerves from the central nerve 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 of a mechanic, chemic, thermic, or electric 

 nature. 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 continuous but irregular pulsations 

 are produced. 



2. Chemic Stimuli. Various chemic substances in solution will excite single 



or continuous pulsations if the strength of the solution 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-sub- 

 stance. Among these may be mentioned solutions of potassium and 

 sodium salts, weak solutions of the mineral and organic acids, ammo- 

 nium vapor, distilled water, glycerin, and sugar. 



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



wire, causes the muscle to contract rapidly. 



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



to the tissue is the electric current. Either the constant or the induced 



current may be used. 1 



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

 electric cell be provided with non-polarizable electrodes and the latter placed 

 on opposite ends of a freshly prepared sartorius muscle of a frog which has 

 been previously curarized, it will be found on closing or making the circuit 

 that the muscle will exhibit a short, quick pulsation. During the actual 

 passage of the current, especially if it is weak, there may be no apparent 



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

 involves the employment of electricity as a stimulus, it is necessary for the student to 

 familiarize himself with certain forms of apparatus by which it is generated, controlled, and 

 applied. To avoid interrupting the continuity of the text this information is embodied 

 in an appendix. The facts therein contained should be mastered at this time by the student. 



