908 NUTRITION AND HEAT REGULATION. 



only during the conditions of ordinary living, but under such an 

 abnormal condition as prolonged starvation. It is assumed that 

 this constancy of composition is effected mainly by an enzyme 

 formed in the liver cells, which converts the glycogen to dextrose 

 in proportion as the sugar of the blood is used up by the tissues, 

 and, as was stated in the chapter on Internal Secretions, this 

 process of glycogenolysis may be accelerated or retarded by the 

 action of hormones arising in the ductless glands. 



Intermediary Metabolism of Carbohydrates. — The ordinary 

 carbohydrates of our food after the completion of the processes 

 of digestion and absorption, glycogenesis, and glycogenolysis are 

 found in the blood in the form of dextrose, and in this form they 

 are presented to the tissues for utiHzation. Eventually the dextrose 

 undergoes oxidation to carbon dioxid and water, but it can hardly 

 be supposed that this oxidation takes place directly upon the 

 sugar molecule. The general assumption at present is that the 

 dextrose undergoes cleavage and other intermediary reactions 

 before the oxidation takes place. Numerous suggestions have been 

 made in regard to the nature of these intermediary changes, but 

 our positive knowledge at present on this point is scanty and 

 unsatisfactory. In the first place there is considerable evidence 

 that this normal destructive metabolism does not take place with 

 the dextrose itself. This substance undergoes certain preparatory 

 changes in which the internal secretion of the pancreas is concerned. 

 We know that if the pancreas is extirpated the whole process of 

 the metabolism of the dextrose is suspended — the dextrose formed 

 in the body is excreted unchanged in the urine. Observations 

 upon isolated organs, such as the heart, perfused with solutions 

 containing dextrose indicate (Clark) that the muscular tissue can- 

 not readily use the dextrose unless it is first perfused through the 

 pancreas. What this preparatory change is and how the pancreas 

 is concerned in it are matters for speculation. Since dextrose solu- 

 tions when submitted to perfusion through the pancreas and the 

 heart undergo a diminution in optical rotation and in reducing 

 power (p. 887) it maybe assumed provisionally that the simple sugar 

 is condensed into some form of polysaccharide, and it is possible, of 

 course, that other substances may enter into this synthesis. The 

 several chemical theories that have been suggested to explain the 

 processes involved in the oxidation of sugar in the body do not take 

 account of this preliminary synthesis, but consider the metabolism 

 as beginning with the dextrose molecule. There is much evidence 

 of a varied character which indicates that in tissue activity lactic 

 acid is formed from the dextrose. The dextrose molecule may yield 

 two molecules of lactic acid. 



CeHiaOe = 2(C3H603) 



