GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 59 



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, amm 

 nium vapor, distilled water, glycerin, and sugar. 



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



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 

 current may be used. 1 . , 



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

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

 on opposite ends of a freshly prepared sartorius muscle of a frog wh 

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

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

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

 change in the muscle. If the current is strong, the muscle may, on the 

 contrary, remain in , state of continuous contraction. With the opening or 

 breaking of the current the muscle at once relaxes, or perhaps again co. 

 and then relaxes. The extent of the contraction depends mainly on ^t 

 strength of the current, being greater with strong, less with weak c 



i Since the study of the physiologic properties of both ^cle-tissue and nerve t.sue 



