The Animal Body — Digestion — Metabolism. 29 



While most of the nitrogen from tlie food protein is absorbed as 

 amino acids, it is possible tliat some of it is taken up as proteoses 

 or peptones. It is certain that in artificial digestion of proteins in 

 the laboratory, if sufficient time is given the enzyme to act, the 

 proteoses and peptones are completely broken down into amino 

 acids or similar bodies. Very probably this takes place normally 

 in the digestive tract. 



42. The large intestine. — The large intestine receives the contents 

 of the small intestine after the latter organ has ceased further ef- 

 fort at digestion. These contents consist of undigested matter, bits 

 of indigestible substances of all kinds taken in with the food, bile 

 salts which have escaped resorption, water, mineral salts, and frag- 

 ments of the mucous lining of the small intestine. Mixed with 

 these are some of the digestive juices of the small intestine. The 

 large intestine does not elaborate any digestive fluids, but its walls 

 contribute water and certain metabolic waste products, especially 

 certain inorganic salts common to the tract. It is possible that 

 some digestion may occur in the large intestine owing to traces of 

 digestive enzymes coming from the small intestine, but such diges- 

 tion is insignificant in amount. There is a constant interchange 

 of water between the contents of the large intestine and the blood 

 circulation, which results in the absorption of any soluble prod- 

 ucts, nutritive or otherwise, which may be formed in the large in- 

 testine either by digestion or bacterial action. 



43. Bacteria. — In the stomach bacteria find unfavorable condi- 

 tions for growth because of the free acid of the gastric juice, and 

 in the small intestine the presence of bile rapidly causes the death 

 of bacteria. Consequently bacteria play little or no part in diges- 

 tion in either the acid stomach or the alkaline small intestine. They 

 do act, liowever, on the woody fiber or cellulose in the first three 

 stomachs of ruminants and in the caecum of the horse. In the 

 large intestine there develops a profuse bacterial flora of various 

 forms which thrive in the absence of air. The presence of more 

 or less undigested food, together with moisture, warmth, and the 

 faint alkaline reaction, furnishes ideal conditions for bacterial 

 groAvth. Some cellulose is decomposed by the bacteria with the 

 liberation of carbon dioxid, marsh gas, and hydrogen. Sulfureted 

 hydrogen is also produced thru putrefaction of protein substances. 

 Some nitrogen is found, but this has its source in the air taken in 

 with the food. Much of the gas is doubtless absorbed into the 

 circulation and eliminated from the lungs. Products other than gas 



