152 NUTRITION OF FARM ANIMALS 



212. Reversibility of metabolic reactions. It would appear, 

 then, that the action of the intracellular enzyms which are be- 

 lieved to play such an important part in metabolism may be 

 synthetic as well as analytic, and that the metabolic processes 

 may be conceived of as a complex of reversible chemical reac- 

 tions, now accelerated and now retarded by appropriate en- 

 zyms. 1 The idea that each cell of the body thus exists in a 

 state of constantly shifting chemical equilibrium, according as 

 the concentration of one or another substance in its domain 

 changes, is an attractive one in its breadth and comparative 

 simplicity, and there seems to be little doubt that it contains 

 elements of truth and will prove an important aid to research. 

 As yet, however, it is to be regarded as a probable hypothesis 

 rather than as a fully established fact. 



3. THE METABOLISM OF THE CARBOHYDRATES 

 The hexose carbohydrates 



213. Glycogenic function of the liver. The monosac- 

 charids (principally dextrose) produced in the digestive cleavage 

 of the carbohydrates are resorbed chiefly or wholly by the blood 

 capillaries of the intestines. These capillaries unite into the 

 portal vein leading to the liver, where it subdivides into a 

 capillary system in which the blood is brought into intimate 

 contact with the cells of that organ and from whence it passes 

 by way of the hepatic vein into the posterior vena cava, thus 

 entering the general circulation (182). 



The proportion of dextrose found in the blood of the general 

 circulation is remarkably constant, and if any considerable 

 excess be introduced it is promptly excreted through the kid- 

 neys (198). On the other hand, the supply of carbohydrates 

 from the digestive tract may be more" or less intermittent or 

 fluctuating, so that there is -evidently need for some regula- 

 tory mechanism to prevent a waste of sugar by excretion in 

 the urine. This regulation is effected chiefly in two localities, 

 viz., in the muscles and in the liver. The function of the liver 



1 That syntheses can be effected by the agency of enzyms seems established, but 

 that enzym reactions in general are reversible is questioned by good authorities. 

 For example, the ferment maltase, acting on dextrose, is stated to produce not mal- 

 tose but isomaltose, and it is claimed that a different enzym is required to reconvert 

 isomaltose into dextrose. 



