DIGESTION OF THE CARBOHYDRATES 23i 



normally lies between 0.5 and 0.15 per cent, and we may suppose that 

 when the consumption of carbohydrate in distant tissues results in a 

 certain degree of depletion of local stores, and of the circulating glucose, 

 the equilibrium between the glycogen stores in the liver and the 

 glucose in the fluids bathing the liver-cells is disturbed, and glycogen 

 breaks down in order to restore it. 



(C 6 HioO 5 )n + nH 2 O -> nC 2 Hi2O 6 ) 

 Glycogen Glucose 



We must suppose that Pentoses, in so far as they form constituents 

 of animal tissues, namely in the d-Ribose radical of guanylic and 

 inosinic acids, are derived from pentoses originally contained in the 

 diet. It will be recollected that guanylic and inosinic acids represent 

 fragments derivable by partial hydrolysis from vegetable nucleic acid, 

 and catalytic agents capable of bringing about this cleavage are found 

 widely distributed in animal tissues and tissue fluids The mono- 

 nucleotids are not improbably absorbed as such, the adenine mononucle- 

 otid being transformed by direct deaminization into the corresponding 

 hypoxanthine derivative. 



An important proportion of the dietary of herbivora, however, 

 is furnished by Pentosans, or polysaccharides derived from pentoses. 

 We have no evidence of the existence, either in the digestive juices or 

 in the epithelial wall of the intestine, of any enzymes capable of trans- 

 forming these substances into glucose. Yet the experimental fact 

 remains that pentoses can be utilized by animals, and energy derived 

 from them for the performance of work and the maintenance of the 

 body. Whether they are absorbed and oxidized in the tissues as such 

 or as glucose is not known, but the administration of Rhamnose to a 

 diabetic whose urine has been made sugar-free by the exclusion of 

 carbohydrates from the diet, leads to reappearance of glucose in the 

 urine. 



The Celluloses in the dietary are indigestible by any of the enzymes 

 produced by the digestive glands or the intestinal epithelium. Never- 

 theless they are partially utilized, as much as forty per cent, of young 

 and tender cellulose, such as that occurring in lettuce, being utilizable 

 for the production of energy by human beings. We owe this ability 

 to the digestive activities of the bacteria which inhabit the lower 

 intestine. The bacterial flora is particularly abundant in the lower 

 intestine of the herbivora, and as much as seventy per cent, of cellulose 

 may be dissolved in vitro by the intestinal juices of a horse. The 

 products of this digestion are not monosaccharides, but carbon dioxide, 

 methane and fatty acids of which the latter only, of course, are avail- 

 able for nutrient purposes. 



The most important function of the celluloses in the diet, however, 

 is that of communicating bulk to the intestinal contents, and promot- 

 ing peristalsis by affording a favorable consistency and volume for 

 propulsion with a minimum of muscular effort. This function of the 



