404 A TEXTBOOK OF PHYSIOLOGY 



protein, and this is believed to be due to the fact that they give play 

 to the growth of antagonistic lactic acid bacilli. Quite recently it 

 became fashionable to take lactic acid bacilli by the mouth. This 

 cannot be said to have been very effective as a cure, partly, perhaps, 

 because many of the preparations sold were sterile. At the best 

 it substituted the lesser evil fermentation of carbohydrate for the 

 putrefaction of protein. 



The bacteria are killed off by the action of the healthy intestinal 

 wall; while the contents swarm with bacteria, the mucous membrane 

 and blood circulating within it are sterile. Foreign bacteria, such as 

 vibrios, appear to be killed if introduced into the large intestine. 



As the result of bacterial action, the pigment of the faeces stereo - 

 bilin and one of the pigments of the urine urobilin together with 

 its precursor urobilinogen, are formed from the bile pigments. Bile 

 salts are also decomposed into cholalic acid, and either taurin or 

 glycin. Cholesterin becomes converted into an allied body copro- 

 .sterin. 



The fact that extensive bacterial action takes place in the intestine 

 does not mean that it is necessarily harmful; indeed, when kept within 

 limits, it is, if anything, helpful to the body. A certain amount of 

 gas promotes the movements of the large bowel, and assists the removal 

 of the waste material. Experiments have been made to test the 

 value of the intestinal flora to the animal. Guinea-pigs delivered by 

 Caesarian section, breathing sterile air, and fed on sterile food, pro- 

 gressed as well as the controls kept non-sterile. Sterile chicks, on 

 the other hand, did badly on sterile food. Some died in eighteen 

 days, while, in about the same time as a starving control, others 

 recovered when the bacteria of chicken faeces were added to their 

 food. The guts of Arctic animals, such as the polar bear, are said to 

 be almost sterile. Herbivorous animals obtain a great deal of the 

 nitrogenous foodstuffs from non-protein compounds, especially 

 asparagin. It is suggested that the intestinal bacteria of herbivora 

 build up proteins which are utilized out of these amides. Asparagin 

 cannot be utilized by carnivora. By the bacteria in the capacious 

 caecum of herbivora cellulose* is split into glucose, lactic, but}Tic 

 acids, etc. These are absorbed and utilized, and thus cellulose, which 

 forms so large a bulk of the food, becomes a chief source of energy. 

 Hydrogen and methane are also produced by the bacterial fermen- 

 tation of cellulose, and constitute a small part of the flatus passed 

 from the bowel. A certain amount of oxygen is set free and utilized. 

 In balancing up the metabolism of cattle, these gaseous excretions, 

 which leave the body both by way of bowel and lung, have to be taken 

 into account. 



Protein putrefaction is always going on in the large bowel, even 

 when no food is being taken. The proteins of the secreted juices are 

 then decomposed. It is only when bacterial action is allowed to get 

 beyond proper limits that it becomes harmful. Excess of food and 

 a sluggish large intestine favour the condition. It is obvious that no 

 more food, especially proteins, should be taken in than can be digested 



