466 THE HUMAN BODY. 



energy needed to keep the heart and respiratory muscles at 

 work, and to maintain the temperature of the body, must 

 have been obtained from the oxidation of the fat reserve 

 with which the animal started. 



Glycogen. The size of the liver was long a stumbling- 

 block to physiologists: it was difficult to understand why so 

 large an organ should be developed for the mere secretion of 

 some bile, a not very important digestive liquid. But even 

 centuries ago some glimmering of the truth was guessed, 

 and the liver was believed to be concerned in the elaboration 

 of nutritive blood, which was distinguished from the blood, 

 charged with vital spirits, which came from the lungs and 

 the left side of the heart. Harvey's discovery of the real 

 course of the circulation, and Lavoisier's interpretation of 

 the meaning of respiration, upset these crude doctrines; and 

 for long the germ of truth which they contained was lost to 

 view in the glare of the new light. We have now learned, 

 on a new basis of actual experiment, that the liver is very 

 largely concerned in the nutritive processes of the Body : its 

 relation to proteid metabolism and urea formation has 

 already been considered, and we have now to study its 

 activity in regard to the formation, and storage, and trans- 

 mission of a carbohydrate substance, glycogen. 



If a liver be cut up two or three hours after removal from 

 the body of a healthy well-fed animal, and thoroughly ex- 

 tracted with water, it will yield much grape-sugar. If, on the 

 other hand, a perfectly fresh liver be heated rapidly to the tem- 

 perature of boiling water, and be then pounded up and ex- 

 tracted, it will yield a milky solution, containing little grape- 

 sugar, but much glycogen; a substance which chemically 

 has the same empirical formula as starch (C 6 H 10 B ), and in 

 other ways is closely allied to that body. The salivary and 

 pancreatic secretions rapidly convert it into the sugar maltose, 

 as they do starch. The transformation of glycogen into glucose 

 (grape-sugar) which occurs in the liver after death and prob- 

 ably also during life is then quite different from that brought 

 about by the digestive enzymes; and in fact no enzyme has 

 been extracted from fresh liver. The change is apparently 

 not a fermentative one, but one dependent on some vital 

 metabolic activity of the liver-cells, which activity is greatly 

 accelerated during their period of dying: hence the need of 

 killing them rapidly by boiling, if any considerable amount of 



