440 ANIMAL BIOCHEMISTRY 



changes that take place here are not of great importance. The mucous 

 membrane of the large intestine contains gland cells, but the secretion 

 is largely mucous and without digestive enzymes. The chemical changes 

 that do take place here are due largely to bacteria, but enzymes are 

 carried through from the small intestine, and some hydrolytic changes 

 may occur. Like the secretions in the small intestine, those in the 

 large intestine are usually alkaline. Putrefaction results in the produc- 

 tion of proteoses, peptones, amino acids, and other decomposition 

 products such as indole, skatole, and ammonia. In addition to the 

 putrefactive changes, polysaccharides, such as cellulose, are acted 

 upon by fermentive organisms, leaving behind lactic, butyric, and 

 acetic acids, alcohols, and gases such as methane, carbon dioxide, and 

 hydrogen. 



Water is absorbed in the large intestine, and the undigested mass 

 gradually takes on the characteristics of feces. Feces are composed 

 mainly of undigested food materials such as cellulose, keratin, and 

 other proteins, bacteria, and fats plus any digested nutrients that have 

 escaped absorption. 



It has been estimated that nearly one-third of the solid matter of 

 normal feces consists of bacteria. The bacterial flora of the intestine is 

 responsible for a variety of substances. The production of acids and 

 ethyl alcohol is beneficial because the body can absorb and utilize 

 these compounds. Moreover, under proper conditions, the micro- 

 organisms synthesize appreciable amounts of such vitamins as thia- 

 mine, riboflavin, folic acid, and vitamin K. Biosynthesis of vitamins 

 has been demonstrated in the paunch of ruminants and in the diges- 

 tive tracts of rats and human beings. 



DETOXICATION 



The end products of carbohydrate breakdown in the intestine are 

 usually harmless, whereas certain products resulting from protein 

 breakdown are distinctly toxic. Many of the ill effects due to con- 

 stipation are tentatively attributed to the absorption of these toxic 

 products. It is well known that intestinal putrefaction produces 

 products that are potentially poisonous and that the body possesses 

 ways and means of protecting itself against these toxic materials. 

 These defenses are known as detoxication mechanisms. 



The toxic products produced consist almost entirely of substances 

 formed from amino acids by decarboxylation to amines. Known ex- 

 amples are listed on page 441. Histamine is also formed in the body 



