76 PHYSIOLOGY FOB DENTAL STUDENTS. 



Most of the food is now in a suitable condition for absorption. 

 Before we proceed to study the nature of this process, however, 

 there are one or two further digestive changes that we must con- 

 sider. 



The Digestive Function of Intestinal Bacteria. On account 

 of the antiseptic action of free hydrochloric acid, there is, ordi- 

 narily, no bacterial growth in the stomach, but the neutral i/a- 

 tion of acid by pancreatic juice and bile in the intestine pro- 

 vides a perfect medium for such growth. The extent and nature 

 of the bacterial growth varies very greatly according to the na- 

 ture of the diet. 



There can be no doubt that the micro-organisms are a valuable 

 aid to digestion in the case of most animals, especially of those 

 whose diet includes cellulose. Indeed, in such animals as the 

 herbivora special provision is made to encourage bacterial growth 

 by the grea^t length of the large intestine, for without bacteria, 

 digestion of cellulose is impossible. Thus if newly-hatched chicks 

 be fed with sterilized grain they succumb in about two weeks, 

 but if a. small amount of the excrement of the fowl be mixed 

 with the grain, they thrive, as ordinarily. On the other hand, if 

 the food contains no cellulose, animals may develop and grow 

 with sterile intestinal contents; thus guinea pigs have been re- 

 moved from the uterus under aseptic conditions and kept in a 

 sterile place on sterilized milk and have thrived and grown as 

 normal guinea pigs. The organisms in the intestine of man are 

 probably much more useful than harmful. No doubt they are 

 parasites, but they are useful parasites; they work for their liv- 

 ing, not only by assisting when necessary in the digestion of 

 food but also by destroying certain substances which, if absorbed, 

 would have a toxic action on the host. Thus cholin, a substance 

 produced by the digestion of lecithin, is distinctly poisonous, but 

 it really never gets into the blood because the bacteria destroy it. 



In the case of man bacterial digestion occurs both in the small 

 and large intestines, and there are varieties of bacteria capable 

 of acting on all the food stuffs. They may break up the sugars 

 into lactic acid or even further so as to form C0 2 and H. It has 

 been claimed that this formation of lactic acid in the intestine is 



