448 THE METABOLIC PROCESSES OF THE BODY. 



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 carbo- 

 hydrate bank. It is destined to become part of the contractile substance, 

 and as such will contribute to the energy set free in a muscular contrac- 

 tion ; 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, or even as dextrose, 

 in the interstices of the muscular fibre. 



384. Glycogen may also be found in considerable quantity in the pla- 

 centa. Here, as we shall see in the latter 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 situa- 

 tions. In cases of diabetes, where the body is overloaded with carbohy- 

 drate 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 truth of the 

 view which we have expounded above, that the main purpose of the deposi- 

 tion 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 re- 

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

 lating 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 identi- 

 cal with that which occurs in the liver. Indeed, there seem to be interme- 

 diate stages between glycogen and starch, or dextrin. The physiological 

 value of these differences has not yet, however, been clearly determined, 

 and, with this caution, we may continue to speak of glycogen as a single 

 substance. 



Diabetes. 



385. Natural diabetes is a disease characterized by the appearance 

 of a large quantity of sugar in the urine, due, as we have already said, 

 to the presence of an abnormal quantity of sugar in the blood. Into 

 the pathology of the various forms of this disease it is impossible to enter 

 here ; but a temporary diabetes, the appearance for a while of a large 

 quantity of sugar in the urine, may be artificially produced in animals in 

 several ways. 



If the medulla oblongata of a well-fed rabbit be punctured in the region 

 which we have previously described ( 162) as that of the vasomotor centre 

 (the area marked out as the " diabetic area," agreeing very closely with that 

 defined as the vasomotor area), though the animal need not necessarily be 

 in any other way obviously affected by the operation, its urine will be found, 

 in an hour or two, or even less, to be increased in amount and to contain a 

 considerable quantity of sugar. A little later the quantity of sugar will 

 have reached a maximum, after which it declines, and in a day or two, or 

 even less, the urine will again be perfectly normal. The better fed the 

 animal, or, more exactly, the richer in glycogen the liver, at the time of 

 the operation, the greater the amount of sugar. If the animal be pre- 



