574 GLYCOGEN IX THE MUSCLES. [Boon n. 



glycogen present is enormous ; it frequently amounts to 40 

 p.c. of the dry material. At this period the hepatic cells are 

 immature and very little glycogen is present in them. Later 

 on, as the muscles become more wholly striated, the glycogen 

 largely disappears from the muscle, and very soon afterwards 

 begins to be stored up in the liver. The meaning of this can 

 hardly be mistaken. The glycogen in the immature muscle is 

 a store of carbohydrate material, laid down on the spot, and 

 ready at once to be used in what we may probably call the 

 fierce metabolic struggle by which the simple protoplasmic cell- 

 substance of the rudiment of the muscular fibre is transformed 

 into the highly differentiated striated contractile substance. 

 And we shall probably not err in considering the glycogen of 

 the mature muscle to hold a similar position ; it is carbohydrate 

 material stored up on the spot, a local branch so to speak of 

 the great carbohydrate bank. It is destined to become part of 

 the contractile substance, and as such will contribute to the 

 energy set free in a muscular contraction ; but its energy is 

 only available in this way after it has undergone the necessary 

 metabolism and become part of muscular substance ; it cannot 

 be fired off in a contraction while it lies as raw glycogen in the 

 interstices of the muscular fibre. 



368. Glycogen may also be found in considerable quan- 

 tity in the placenta. Here, as we shall see in a later part of 

 this work, it is laid down in epithelial cells which lie on the 

 boundary between the maternal and the foetal tissues. And 

 here too there can be little doubt that it serves as a store of 

 carbohydrate material for the nourishment of the foetus. 



It has also been found in leucocytes, and in cartilage cor- 

 puscles, especially in those large rapidly growing and rapidly 

 multiplying cartilage corpuscles which lie in the outer zone of 

 endochondral ossification, and in other situations. In cases of 

 diabetes, where the body is overloaded with carbohydrate mate- 

 rial, it has been found in considerable quantity in the testis, in 

 the brain and elsewhere. Its occurrence in these situations, and 

 under these circumstances, may be regarded as additional evi- 

 dence of the truth of the view which we have expounded above 

 that the main purpose of the deposition of glycogen is to afford 

 a store, either general or local, of carbohydrate material, which 

 can be packed away without much trouble so long as it remains 

 glycogen, but which can be drawn upon as a source of soluble 

 circulating sugar whenever the needs of this or that tissue demand 

 it. It thus forms a very complete analogue to the vegetable 

 starch, and fitly earns the name of animal starch. 



We have some reasons for thinking that there are several 

 varieties of glycogen, and that the glycogen which exists in 

 muscle is not quite identical with that which occurs in the liver. 

 Indeed there seem to be intermediate stages between glycogen 



