MUSCLE 255 



chief producers of heat. If they turned all the energy which 

 they receive into work, they would be inefficient as regards this 

 very important function. Yet even from the engineer's point 

 of view muscles are more efficient than the best of engines. 



It is almost impossible to determine with accuracy, in 

 regard to isolated muscles, the amount of food taken up from 

 the blood, and the return in work by the muscles of the 

 energy potential in the food. Calculations have to be based 

 upon observations of food consumed, gain or loss of body- 

 weight, work done by a man or an animal during a period 

 lasting for several days. We shall consider the evidence ob- 

 tained in this way in a subsequent section (p. 149). But 

 whether we study isolated muscles or the body as a whole, the 

 relation between work and heat varies within wide limits. So 

 wide, indeed, are the variations as to justify the conclusion that 

 there is no necessary relation between the two phenomena. 

 Muscles develop heat when they are quiescent. Activity is 

 accompanied with an increased evolution of heat ; but, if it 

 be desirable, the evolution of heat is reduced until it is, re- 

 latively to the output of work, much smaller than in the case 

 of any engine which has yet been made. It is sufficient in 

 this connection to state that, under certain conditions, the 

 return in work may amount to about one-half. The comparison 

 with an artificial motor, of whatever kind, breaks down. In an 

 engine combustion develops heat, heat causes steam or gas to 

 expand, the expanding gas pushes a piston. In muscle certain 

 of the carbon, hydrogen, and oxygen atoms contained in 

 protoplasm combine to form water and carbonic acid com- 

 pounds too stable to be reassociated with the remaining atoms 

 of the protoplasm-molecule. They are replaced by complex, 

 energy-yielding substances f oods and by oxygen, carried in 

 the blood. Their displacement brings about a change in 

 the form of the molecules which involves, owing to their 

 peculiar orientation, a change in shape of the muscle as a whole. 

 Such an explanation is, perhaps, more exact than our know- 

 ledge at present warrants ; or rather let us say, since we do 

 not know what the expression " the form of a molecule " 

 means, it has an appearance of an exactitude which does not 

 characterize it. It is merely intended to help the reader to 

 realize the hopelessness of attempting to compare muscle with 



