130 STUDIES IN ADVANCED PHYSIOLOGY. 



living muscles. Muscles which have begun to die, or have 

 been fatigued by excessive work, or have been cooled too 

 much, or have been heated to above one hundred and ten 

 degrees, react much more slowly, and finally refuse to act 

 at all. 



The Actual Lifting Power of Muscles. 



It is the function of muscles to pull and lift, and their 

 maximum capacity to lift is of course a matter of general in- 

 terest. The amount which a muscle can lift varies with its 

 cross-section in the same way that the capacity of a rope to 

 sustain a weight depends upon the thickness of the rope. 

 With frog's muscle, experiments indicate that a muscle a 

 square centimeter in cross-section can lift about six and one- 

 half pounds. A square centimeter of muscle in man may 

 lift as much as twenty-five pounds under the stimulus of the 

 will. When we remember that some muscles are many 

 square centimeters in cross-section the force with which they 

 may be made to lift and pull is easily explained. 



Amount of Shortening of Muscles. 



Another question of interest is the proportionate length 

 to which a muscle may contract itself. This, of course, 

 varies somewhat with different muscles. A short, thick 

 muscle would not shorten itself as much as a longer slender 

 muscle. Varied experiments indicate that the actual reduc- 

 tion in length may be from sixty-five to eighty-five per cent, 

 of the entire muscle. 



Changes in Volume. 



A muscle when it contracts occupies a little less volume 

 than it did in an expanded state. This is no doubt due to 

 the fact that in the contraction, some of the muscle sub- 

 stance is more compressed. This may be easily demon- 

 strated by hanging a muscle in a vessel of water and then 

 making it contract. On the contraction, the height of the 

 water in the vessel sinks a trifle. 



