DISTURBANCES OF CARBOHYDRATE METABOLISM 307 



glycemia is not evident under ordinary conditions, but becomes so after 

 the mgestion of glucose ; the physiological rise in content of blood sugar 

 being exceeded and prolonged. Exactly the contrary is the case in 

 myxedema, in which condition the tolerance for glucose is abnormally 

 high (Geyelin). The relation of the state of pituitary function to 

 carbohydrate metabolism is not quite so clear, but there is no doubt 

 that, in some cases of dyspituitarism, there is a low carbohydrate tolerance 

 with a tendency to spontaneous glycosuria, while in others the tolerance 

 is very high and the blood sugar is low (Cushing(c)). A similar low 

 level of blood sugar is observed in muscular .dystrophy ( Janney, Good- 

 heart and Isaacson, McCrudden and Sargent) and in Addison's disease 

 (Bernstein, Porges(&)). In the former there is also a high tolerance 

 for glucose. This does not seem to have been tested in Addison's disease. 



The significance of these facts for diagnosis, and for our understanding 

 of the function of the endocrine glands, will be more fully discussed in 

 other chapters. It is here intended to offer a few observations on the 

 possible paths of glucose metabolism, and to indicate the probable nature 

 of the disturbances. 



It would seem that there are involved three important chemical reac- 

 tions, or groups of consecutive reactions, one of which is reversible while 

 the other two are not. The reversible reaction is that of glucose <=* gly- 

 cogen. The apparently irreversible actions are concerned in the formation 

 of fat from glucose and in the oxidation of glucose. The various disturb- 

 ances of glucose metabolism are then due to conditions which raise, or 

 lower, the concentrations of blood sugar at which it is possible for these 

 reactions to occur. 



To use a mechanical analogy, tne blood sugar may be compared to a 

 mill pond, with four sluices, one to another pond, or reservoir, one to a 

 spillway, and the other two to mills, one of which (A) uses water power 

 directly, the other of which (B) first converts it into electric power, which 

 may be used at once or be accumulated in storage batteries. This mill is, 

 perhaps, located, not on the same pond as the other mill and the spillway, 

 but on the reservoir. According as the sluice gates are raised or lowered, 

 the water will flow in different directions. Let us assume there is water 

 in both ponds, with none going through B, which is delivering energy from 

 its storage batteries, while mill A is not receiving water to its full capacity. 

 As more water is supplied, mill A uses more, and more flows into the 

 reservoir. It may be that more water will flow through A than is required 

 and some of the energy may be wasted. The storage batteries in B are no 

 longer required and cease to supply energy. As the level of water rises, 

 the reservoir fills, and mill B begins to operate and to charge the batteries. 

 As the water reaches a still higher level it goes over the spillway. 



The reservoir is the tissue glycogen, the spillway is represented by the 

 kidneys, the two mills are, respectively, the direct oxidation of glucose 



