96 THE HUMAN BODY 



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 stimulus: 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 respiratory 

 systems and the nervous energy or " grit " (upon which the stimu- 

 lation 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-fiber are eminently susceptible 

 of modification by slight changes in its environment. 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 main- 

 tained 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 depends 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 done by a muscle varies 

 with its form. The thicker the muscle, that is the greater the 

 number of fibers present in a section made across the long axes 

 of the fasciculi, the greater the load that can be lifted or the other 

 resistance that can be overcome. On the other hand, the extent 

 through which a muscle can move a weight increases with the 

 length of its fasciculi. A muscle a foot in length can contract 

 more than a muscle six inches long, and so would move a bone 

 through a greater distance, provided the resistance were not too 



