

CHAPTER VII 

 MUSCULAR ACTIVITY 



The Study of Isolated Muscles. There are some simple facts of 

 muscle activity that one can learn by observing his own muscles; 

 for example, when the arm is bent at the elbow the muscle that 

 produces the movement, the biceps, can be seen under the skin 

 to shorten and thicken, and if felt will be found hard when con- 

 tracted, as compared with its soft flabbiness when relaxed. This 

 knowledge is the possession of every school boy. 



More detailed knowledge can be gained by the direct examina- 

 tion of living muscles, dissected away from their bodily attach- 

 ments. To isolate the muscles of the higher animals thus is ob- 

 viously impractical, but fortunately the " cold-blooded" animals, 

 notably frogs and turtles, are peculiarly suited for such studies as 

 this. If a frog is quickly killed, as by destroying the brain with a 

 sharp instrument, the large muscle of the calf of the leg, the gastroc- 

 nemius, can be dissected out, and if properly cared for will remain 

 alive for several hours, during which its activity can be studied. 

 The chief precaution to be observed is to prevent such evaporation 

 of water from the muscle as would disturb the physico-chemical 

 equilibrium and injure the tissue. This loss of water is prevented 

 by repeated moistening, but here again a precaution must be ob- 

 served since the application of pure water to the surface of the 

 muscle would be followed by a flow of water into the tissue under 

 the driving force of osmotic pressure (p. 19). This would bring 

 about a disturbance of equilibrium in the direction of too great 

 dilution as harmful to the tissue as the evaporation it is designed 

 to prevent. For moistening the muscle a liquid of the same os- 

 motic pressure as the tissue fluids must be employed. A solution 

 of common salt with a concentration of 0.7 per cent satisfies this 

 condition and is the fluid commonly used for keeping living tissues 

 moist. 



The Necessity of Stimulation. An important fact about skel- 

 etal muscle, one indeed which has much to do with the adaptive 



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