134 THE HUMAN 



shorten to one third of its resting length, but in the Body 

 the strongest effort of the will never produces a contrac- 

 tion of that extent. Apart from the rate of stimulation, 

 the strength of the stimulus has some influence, a greater 

 stimulus causing a greater contraction; but very soon a 

 I point is reached beyond which increase of stimulus pro- 

 / duces no increased contraction; the muscle has reached 

 / its limit. The amount of load carried by the muscle (or 

 the resistance opposed to its shortening) has also an influ- 

 ence, and that in a very remarkable way. Suppose we 

 have a frog's calf-muscle, carrying no weight, and find that 

 with a stimulus of a certain strength it shortens two milli- 

 meters (^ inch). Then if we hang one gram (15.5 grains) 

 on it and give it the same stimulus, it will be found to con- 

 tract more, say four or five millimeters, and so on, up to 

 the point when it carries eight or ten grams. After that 

 an increased weight will, with the same stimulus, cause a 

 less contraction. So that up to a certain limit, resistance 

 to the shortening of the muscle makes it more able to 

 shorten: the mere greater extension of the muscle due to 

 the greater resistance opposed to its shortening, puts it into 

 a state in which it is able to contract more powerfully. 

 Fatigue diminishes the working power of a muscle and rest 

 restores it, especially if the circulation of the blood be going 

 on in it at the same time. A frog ? s muscle cut out of the 

 body will, however, be considerably restored by a period of 

 rest, even although no blood flows through it. 



The Measure of Muscular Work. The work done by 

 a muscle in a given contraction, when it lifts a weight verti- 

 cally against gravity, is measured by the weight moved, mul- 

 tiplied by the distance through which it is moved. In the 

 above case when the muscle contracted carrying no load it 

 did very little work, lifting only its own weight;' when 

 loaded with one gram and lifting it five millimeters it did 

 five gram-millimeters of work, just as an engineer would 

 say an engine had done so many kilogrammeters or foot- 

 pounds. If loaded with ten grams and lifting it six 

 millimeters it would do sixty gram-millimeters of work. 

 Even after the weight becomes so great that it is lifted 



