CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 765 



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 paeked 

 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. 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 spinal bulb 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 con- 

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

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

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

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

 becomes loaded with sugar, much if not all of which passes away 

 by the urine. In' the absence of any proof to the contrary, we may 

 assume that in this form of artificial diabetes the glycogen 

 previously present in the liver becomes converted into sugar, just 

 as we know that it does become so converted by post-mortem 

 changes. The glycogenic function of the liver is therefore subject 



