CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 575 



and starch or dextrin. The physiological value of these differ- 

 ences has not yet however been clearly determined, and, with 

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

 substance. 



Diabetes. 



369. Natural diabetes is a disease characterized by the 

 appearance of a large quantity of sugar in the urine. This is 

 due, as we have already said, to the presence of an abnormal 

 quantity of sugar in the blood. The system can only dispose 

 (either by oxidation, or as seems more probable in other ways) 

 of a certain quantity of sugar in a certain time. Sugar injected 

 into the jugular vein reappears in the urine whenever the injec- 

 tion becomes so rapid that the percentage of sugar in the blood 

 reaches a certain (low) limit. Sugar in the urine means an 

 excess 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 spinal bulb of a well-fed rabbit be punctured 

 in the region which we have previously described ( 154) as 

 that of the vaso-motor centre (the area marked out as the " dia- 

 betic 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 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 be 

 again perfectly normal. The better fed the animal, or, more 

 exactly, the richer in glycogen the liver, at the time of the oper- 

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

 viously starved so that the liver contains 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 

 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 glycogeiiic function of the liver is therefore subject to the 

 influence of the nervous system, and in particular to the influ- 

 ence of a region of the cerebro-spinal centre which we already 



