730 DIABETES. [BOOK n. 



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 

 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 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. 



465. 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 ( 176) as that of 

 the vaso-motor centre (the area marked out as the "diabetic area" 

 agreeing very closely with that defined as the vaso-motor 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 con- 

 siderable 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 be again 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 previously starved so that the liver con- 

 tains little or no glycogen, the urine will after the operation 

 contain little or no sugar. It is clear that the urinary sugar of this 

 form of artificial diabetes comes from the glycogen of the liver. 

 The puncture of the medulla causes such a change in the liver 

 that the previously stored-up glycogen disappears, and the blood 



