THE STUDY OF MUSCLE PHYSIOLOGY. 195 



including' the muscles of the embryo, is found in small quantity 

 in the muscles of the adult; and in the heart-muscle a peculiar 

 sugar (inosit) is present. 



It is, of course, very difiBcult to say to what extent the bodies 

 known as extractiyes exist in living muscle, though that glyco- 

 gen, fats, and certain salts are normally present admits of little 

 doubt. 



There is a coloring matter in muscle, more abundant in the 

 red muscles of certain animals than the pale, allied to hsemo- 

 globin, if not identical with that body. 



It may be stated as a fact, the exact significance of which 

 is unknown, that during contraction the extractives soluble in 

 water decrease, while those soluble in alcohol increase. 



It will, however, be very plain, from what has been stated 

 in this section, that life processes and chemical changes are 

 closely associated, and to realize this is worth much to the 

 student of Nature. 



THERMAL CHANGES IN THE CONTRACTING MUSCLE. 



Since very marked chemical changes accompany miiscular 

 contraction, it might be expected that there would be some 

 modification in temperature, and probably in the direction of 

 elevation. Experiment proves this to be the case. 



But why should a muscle when at rest, as may be shown, 

 maintain a certain temperature, unless chemical changes are 

 constantly taking place ? As already stated, such is the case, 

 and the rise on passing into tetanus is simply an expression of 

 increased chemical action. 



No machine kjiown to us resembles muscle except super- 

 ficially. The steam-engine changes fuel into heat and mechani- 

 cal motion, but there the resemblance ends. Muscle changes 

 its food, or fuel, not directly either into heat or motion, but into 

 itself ; yet as a machine it is more effective than the steam- 

 engine, for more work and less heat are the outcome of its 

 activity than is the case with the steam-engine. 



THE PHYSIOLOGY OF NERVE. 



Muscle and nerve are constantly associated functionally, 

 and have so much in common that it becomes desirable to study 

 them together. Much that has been established for muscle 



