GLYCOGEN. 371 



THE MUSCLE-PIGMENTS. 



As I have already indicated, the color of the muscle-plasma is dif- 

 ferent in different animals, and practically coincides with the color 

 of the muscle-tissue itself. In some animals, and notably the mam- 

 mals, this is dark red, while the muscles of others are almost color- 

 less. But even in those vertebrate animals in which no color is 

 observed in the skeletal muscles as a whole the heart-muscle and 

 the diaphragm always appear dark red. This difference is thought 

 to depend upon the degree of activity of the different muscles, but 

 apparently has nothing to do with the velocity of contraction of 

 which a muscle is capable. 



The red-muscle pigment proper is now known to be identical 

 with the haemoglobin of the blood, and probably serves the same 

 purpose, as a carrier of oxygen, in the internal respiration of the 

 tissue. That it actually occurs within the cells is now undoubted. 

 Curiously enough, the same pigment is found in the red muscles of 

 certain insects, in which no haemoglobin otherwise occurs. 



In addition to haemoglobin various lipochromes may also be 

 encountered in muscle-tissue, and are especially abundant in certain 

 fishes, such as the salmon and the sea trout. Of their origin and 

 significance nothing is known. 



GLYCOGEN. 



The glycogen which is found in muscle-tissue does not occur in 

 the cells proper, but is distributed between the individual fibres in 

 the form of fine threads, which are apparently connected with the 

 connective-tissue corpuscles. 



The substance is formed synthetically in the muscle-tissue through 

 a polymerization of the anhydride radicle of glucose, which is 

 carried to the tissue either directly from the intestinal tract, or which 

 results from the hepatic glycogen through a process of depoly- 

 merization. That the muscle- tissue is in fact capable of effecting this 

 synthesis is now undoubted. It has thus been shown that in frogs 

 a deposition of glycogen occurs following the subcutaneous injection 

 of a solution of glucose, even after removal of the liver. The 

 amount of glycogen which is deposited in the muscle-tissue probably 

 represents about one-half of the total amount that is found in the 

 entire body, and in man corresponds to 150 grammes. It repre- 

 sents the most important source of energy which is at the disposal 

 of the tissue, and is constantly consumed, even when the muscle is 

 at rest. This is apparent from the fact that after section of the 

 nerves more glycogen is found in a given muscle than in the cor- 

 responding muscle of the other side, while ordinarily this is the 

 same. While at work the consumption of glycogen increases, and 

 after a comparatively short time already the substance has entirely 

 disappeared. If now a period of rest follows, glycogen is again 

 stored in the muscle, and so on. It is to be noted, however, that 



