136 THE HUMAN BODY. 



raised. For example, if the above muscle were loaded with 

 fifty grams it would maybe lift that weight only 1.5 millime- 

 ters, but it would then do seventy-five gram-millimeters of 

 work, which is more than when it lifted ten grams six milli- 

 meters. A load is, however, at last reached with which the 

 muscle does less work, the lift becoming very little indeed, 

 until at last the weight becomes so great that the muscle can- 

 not lift it at all and so does no work when stimulated. Starting 

 then from the time when the muscle carried no load and did 

 no work, we pass with increasing weights, through phases in 

 which it does more and more work, until with one particular 

 load it does the greatest amount possible to it with that stim- 

 ulus: after that, with increasing loads less work is done, until 

 finally a load is reached with which the muscle again does no 

 work. What is true of one muscle is of course true of all, 

 and what is true of work done against gravity is true of all 

 muscular work, so that there is one precise load with which 

 a beast of burden or a man can do the greatest possible 

 amount of work in a day. With a lighter or heavier load the 

 distance through which it can be moved will be more or less, 

 but the actual work done always less. In the living Body, 

 however, the working of the muscles depends so much on 

 other things, as the due action of the circulatory and respira- 

 tory systems and the nervous energy or "grit" (upon which 

 the stimulation of the muscles depends) of the individual 

 man or beast, that the greatest amount of work obtainable is 

 not a simple mechanical problem as it is with the excised 

 muscle. 



From what precedes it is % clear that the molecular changes 

 which take place in a contracting muscle fibre are eminently 

 susceptible of modification by slight changes in its environ- 

 ment. The evidence indicates that the contractility of a 

 muscle depends, not upon a vital force entirely distinct from 

 ordinary inanimate forces, but upon an arrangement of its 

 material elements which is only maintained under certain 

 conditions and is eminently modifiable by changes in the 

 surroundings. 



Influence of the Form of the Muscle on its Working 

 Power. The amount of work that any muscle can do de- 

 pends of course largely upon its physiological state; a healthy 

 well -nourished muscle can do more than a diseased or starved 

 one; but allowing for such variations the work which can be 



