GLYCOGEN. 353 



stroma, which remains after the extraction of the soluble albumins 

 with a 5 per cent, solution of ammonium chloride, we know only 

 that the material in question consists of an albuminous substance. 

 It is apparently a native albumin, and, like the soluble muscle- 

 albumins, characterized by the ease with which it is transformed into 

 alkaline albuminate on treating with dilute solutions of alkalies. 



According to Danilewski and Holmgren, the structure of the 

 muscle-fibre is in no ways altered by dissolving out the soluble 

 albumins, and it would thus appear that the stroma represents the 

 actual contractile substance of the tissue. Whether or not this is 

 actually the case, however, is as yet unknown. 



The sarcolemma apparently consists of a substance which belongs 

 to the albuminoids, and resembles elastin in its general properties. 



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 body of 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 radicles 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. 



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