8 June, 1907.] Elements of Animal Physiology. 345 



It is an every day experience to find that exercise increases muscular 

 power. This is due to two factors. First, the exercise itself actuallv 

 makes the muscle either bigger or more efficient; and secondly, when an 

 exercise is repeated and learnt, unnecessary muscular exertion is avoided. 

 When an animal performs a skilled action for the first time (a man 

 doing an athletic exercise, or a horse jumping or in harness) a number of 

 superfluous muscles are called into play and antagonistic muscles are too 

 forcibly contracted. When the exercise is learnt the requisite muscles 

 and no more, receive the nerve messages calling on them to contract, and 

 hence the work is done more economical I v and with less fatigue. When 

 -a muscle, acting under the influence of the will, and opposed bv a re- 

 sistance sufficiently great, continues contracting until fatigue has set in 

 and the resistance can no longer be overcome, we find that the contractile 

 substance of the muscle is still capable of doing vigorous work. Tf for 

 instance a weight be lifted by the flexion of a single finger until utter 

 fatigue has set in, and the muscle, or its nerve, be stimulated in the arm 

 by an electric shock, the weight will be lifted, and can continue to be 

 lifted, for a considerable time. Fatigue, in fact, is most strongly marked 

 in the motor nerve-cells of the central nervous system, much less strongly 

 marked in the receptive substance of the muscle, -and still less in the 

 contractile mechanism of the muscle ; for a muscle if stimulated directly 

 can contract long after it has ceased to contract on stimulation of its 

 nerve. 



That muscular power is different in different individuals ; is less in 

 the female than in the male ; and is greater in adult life than in youth 

 or old age, is well known. These differences are due, i)artly to the 

 differences in number and size of the muscle cells, and partly to the 

 \ariable efficiency of the contractile elements. 



Chemical composition of muscle. — The contractile cells of muscle 

 are arranged on a scaffolding of fibrous connective tissue. Moreover con- 

 nective tissue sheets pass through and through the muscle marking off the 

 muscle cells into bundles. With the connective tissue there is always some 

 fat. As the amounts of connective tissue and fat are very inconstant, the 

 chemical composition of muscle substance varies within wide limits. As 

 an average of the figures obtained for muscle which has been freed from 

 fat as far as the eye could determine we mav take the following as 

 useful : — 



Per Cent. 



Water ... ... ... ... 75 



Proteins ... ... ... ... 18 



Chondrogen and fat ... ... 2-^ 



Mineral matter ... ... ... 1-2 



Glycogen and various extractives* ... 0-5 



We have seen that muscle requires carbon compounds as sources of 

 energy, but it also requires true protein for repair. A muscle, like any 

 other machine, is constantly losing some of its substance from wear and 

 tear, but this (unlike the machine) is constantly being replaced from the 

 proteins of the blood. This amount of repair protein is very small as 

 compared with the amount of carbon compounds used as energy supply. 



By the term extractives is meant those simple chemical substances which make 

 up the main part of meat extract. 



