THE ANIMAL BODY DIGESTION METABOLISM 2? 



acted upon by amylase, and the compound cane-, malt-, and milk-sugars 

 are converted by the invertases into simple glucose-like sugars. 



Thus when a human eats bread, or an animal consumes hay or corn, 

 the starch in the food must all be changed to glucose before it can enter 

 the body proper. Similarly the compound sugars in the food are con- 

 verted almost completely into simple glucose-like sugars before they 

 are absorbed, tho insignificant amounts of the compound sugars may 

 possibly be absorbed unchanged from the digestive organs. If any 

 unchanged cane sugar or milk sugar is absorbed into the blood from the 

 intestines, apparently neither can be used by the body, but it is excreted 

 without change in the urine. 



In the digestive tract no enzyme has been found which acts on cellulose 

 or on the pentosans. However, bacteria attack these substances, especially 

 in the paunch of ruminants and the caecum of the horse. Among the 

 products of such bacterial decomposition are organic compounds, such as 

 acetic acid and lactic acid, besides gases marsh gas, carbon dioxid, and 

 hydrogen. While the gases are of no value to the animal, there is little 

 doubt that the organic acids are absorbed from the digestive tract and 

 serve as nutrients, because cattle, sheep, or goats can subsist for long 

 periods on coarse straw, which consists largely of cellulose and pentosans. 

 Some digestion of cellulose may possibly be brought about by enzymes 

 contained in the food itself. When artificially digested with strong 

 acid, cellulose is converted into a gummy product and finally into 

 glucose or other simple sugars. 



49. Protein digestion. In the process of digestion the protein com- 

 pounds in the food are attacked first by pepsin in the stomach, and 

 later by trypsin and erepsin in the small intestine. The action of these 

 enzymes is to cleave the very complex protein molecules into simpler 

 ones, during which process the split molecules take up water and become 

 soluble. 



The pepsin of the gastric juice changes protein in the food into 

 soluble proteoses and peptones. This action may readily be demon- 

 strated by the following experiment: If a fragment of the white 

 part of a hard-boiled egg, which is a protein substance, is placed in 

 a dish with dilute hydrochloric acid, a little pepsin added, and the 

 whole kept at body temperature, in a short time the edges of the opaque 

 egg mass will become swollen and transparent, the change gradually 

 extending thru the whole fragment. After a time the mass will have 

 entirely disappeared, and in its stead there will remain a clear solution. 

 If this solution is evaporated to dryness there will be left a yellowish, 

 transparent mass resembling the dried white of an unboiled egg. This 

 dry digested material, which is a mixture of proteoses and pep- 

 tones, is soluble in water the same as the white of egg ; but if dissolved 

 in water it will not solidify on heating, as does ordinary white of egg. 

 This shows that the substance has been changed to something other than 

 the protein, which coagulates or solidifies on heating. 



