764 GLYCOGEN IN THE MUSCLES. [BOOK n. 



But even granting that the glycogen in a muscle may be di- 

 minished during prolonged labour, it cannot be admitted that the 

 oxidation or other chemical change of glycogen is a necessary part 

 of the ordinary metabolism of a muscular contraction, since many 

 muscles wholly free from glycogen are perfectly well able to carry 

 on long-continued contractions. 



Another view of the use of glycogen in muscle is suggested by 

 the fact that undeveloped embryonic muscles are peculiarly rich 

 in glycogen. In a young embryo, at the time when the muscular 

 substance, though undergoing striation, is still largely ' proto- 

 plasmic ' in nature, the quantity of 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 meta- 

 bolism and become part of muscular substance ; it cannot be fired 

 off in a contraction while it lies as raw glycogen, or even as 

 dextrose, in the interstices of the muscular fibre. We have 

 already ( 87) discussed in part the metabolism of "contractile 

 substance," and shall probably again return to it later on. 



464. Glycogen may also be found in considerable quantity 

 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 is a store of carbohydrate material for 

 the nourishment of the foetus. 



It has also been found in leucocytes, in cartilage corpuscles, 

 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 material, 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 evidence of the 



