THE METABOLISM OF THE CARBOHYDRATES 671 



with a sustentacular material. By disrupting this combination and thus 

 exposing the glycogen to the action of glycogenase, glycogenolysis will 

 occur. We may call this the mechanical hypothesis and it deserves 

 serious consideration, for it has been shown that very little postmortem 

 glycogenolysis occurs in the intact liver of frogs in winter, even though 

 at this time the organ contains an excess of glycogen, but becomes 

 marked when the liver is broken down by mechanical means. 



The third view depends on the well-known fact that enzyme activities 

 become most markedly altered by slight changes in the chemical nature 

 of the environment in which they act. Diastatic enzymes are partic- 

 ularly susceptible to the reaction (C H ) of their environment, a very 

 slight degree of acidity favoring and a trace of alkalinity markedly 

 depressing their activities. That a tendency to increasing acidity in 

 the liver cells may accelerate the breakdown of glycogen is suggested by 

 the depressing effect produced on the assimilation limit of sugars by 

 administering acids, and by the observation that postmortem glycogen- 

 olysis becomes marked in proportion as the dying liver becomes acid in 

 reaction. It might be thought then that glycogenolysis in the liver cell 

 could be set up by the local production of a certain amount of acid. 

 Such a liberation of free acid could be brought about by a curtailment 

 in the arterial blood supply of the hepatic cell, producing a local accu- 

 mulation either of carbonic or of other less completely oxidized acids 

 (e.g., lactic). It may be that asphyxia causes hyperglycemia by such 

 a mechanism. Vasoconstriction and consequent curtailment of arterial 

 blood supply occurs in the liver when the hepatic nerves are stimulated, 

 and it is possible that the glycogenolysis which is also set up by such 

 stimulation is due to the appearance of acids. The accelerating effect 

 of epinephrine on glycogenolysis might also be explained as due to 

 limitation of blood supply on account of vasoconstriction and local 

 asphyxia. 



THE REGULATION OF THE BLOOD SUGAR LEVEL 



The level at which the concentration of sugar in the systemic blood 

 is maintained represents the balance between two opposing factors: (1) 

 the consumption of glucose bv the tissues, and (2) the production of 

 glucose by the liver. Since this is the most readily oxidizable of all 

 the proximate principles of food (page 652), muscular activity causes 

 large quantities of it to be consumed, so that its concentration in the 

 blood tends to fall below the physiological level, a tendency which is 

 immediately met by an increased discharge of glucose from the liver. 

 The question therefore arises as to lnow the muscles or other tissues 

 transmit their requirements for glucose to the liver. There are two 



