534 NUTRITION OF FARM ANIMALS 



general view considers that heat and work are simultaneously 

 produced, a portion of the energy taking one form and a portion 

 the other. The former view has been supported by no less 

 distinguished an authority than Englemann, but nevertheless 

 it has not been generally accepted by physiologists. In par- 

 ticular, it is difficult to conceive of the existence in a muscle 

 of sufficient temperature differences to account for its observed 

 efficiency. In other words, the muscle is not in general re- 

 garded as being a heat engine. The prevailing view, stated in 

 the broadest outline, is that in the chemical changes conse- 

 quent on a stimulus, energy is in part liberated as heat and in 

 part expended in producing or maintaining tension of the muscle 

 fibers. To use a simple illustration, it is as if by some process 

 the elasticity of a cord supporting a weight were to be sud- 

 dently increased. The cord would contract and the weight 

 would be lifted for a certain distance. In isotonic contraction, 

 that is, when the muscles are free to shorten, the increased ten- 

 sion set up does mechanical work. In isometric contraction 

 this increased tension is also finally converted into heat, as for 

 example in the case of muscular contraction applied to simply 

 sustaining a weight. In this case no work in the mechanical 

 sense is done, but energy is expended in what has sometimes 

 been called " static work." A familiar illustration of " static 

 work " is the muscular effort required in standing. 



632. Tonus. In the foregoing paragraphs it has been 

 tacitly assumed that before and after a contraction the muscle 

 is absolutely relaxed. Such is not normally the case. Even 

 in a state of rest, so-called, there is a greater or less degree of 

 tension of the muscles, especially during the wakening hours, 

 known* as tonus or tonic contraction. In other words, the 

 living muscle is slightly on the stretch, as is shown by the fact 

 that it gapes open when cut or shortens when its connections 

 with the bone are severed. This tension, like the much greater 

 one set up in active contraction, is maintained, in part at least, 

 by a continual katabolism in the muscle, which respires, taking 

 up oxygen and giving off carbon dioxid. In other words, the 

 " resting " muscle is in a state of slight isometric contraction 

 and is doing " static work." According to the principles just 

 enunciated, all the energy transformed in such a muscle finally 

 takes the form of heat, so that, as indicated in Chapter VII 



