\iu Mrsri.K-MOVF.MKNT-> 69 



fixed in the position in which they are when dying, so that the 

 body of the animal becomes stiff and rigid. This condition is 

 called rigor mortis. After a few hours this stiffness passes 

 off, the myosin being changed some little time before putre- 

 faction commences. 



The living muscle contains a proteid substance called 

 myosinogcn which forms the substance myosin, much in the 

 same way that the blood plasma contains fibrinogen which 

 forms the substance fibrin. Besides the myosinogen there 

 is also some albumin in muscle, and a peculiar substance, 

 like starch, called glycogen ; and besides the salts, which 

 are chiefly phosphates and chlorides of potassium, and in 

 smaller amount of sodium and other metals, there are also 

 organic bodies of which we may have something to say later 

 on. The muscle also contains a large amount of water ; 

 about 7 5 per cent of the muscle is water. 



During life the muscle, like the blood and the other 

 tissues of the body, is alkaline, due to the presence of alkaline 

 salts, but when the formation of myosin occurs and death sets 

 in, the muscle becomes acid. This acidity is due to the forma- 

 tion of an acid called sarcolactic acid, not very different from 

 the acid which is formed in milk when it turns sour. 



A living muscle, like all living tissues, is constantly taking 

 oxygen from the blood, and its complex substances are con- 

 stantly breaking down into simple substances, of which carbonic 

 acid is one of the chief. This breaking down of the substance 

 of the muscle sets free energy in the form of heat, so that the 

 muscle is constantly giving out heat. When a muscle contracts 

 there is a large increase in the amount of carbonic acid which 

 it gives off, that is to say, the breaking down of the complex 

 substances of the muscle is much more active, and consequently 

 a larger amount of energy is set free. Some of this energy 

 is used to perform the work which the muscle does, the rest 

 of it is given out as heat. The heat given out in this way when 

 a muscle contracts is much greater than the heat given out 

 when it is at rest. 



Relation of Muscles to Nerves. Passing to the muscles 

 are nerves, which enter between the bundles of fibres and 

 divide into branches. The nerves, as we shall see, are them- 

 selves bundles of delicate long " nerve fibres," and they divide. 



