402 HANDBOOK OF PHYSIOLOGY. 



it grows, and on the one hand alcohol, and on the other hand carbon 

 dioxide are produced. These substances are not the direct result of the 

 life of the cell, but probably arise from the formation of some chemical 

 substances allied to the unorganized ferments which greatly increase in 

 amount with the multiplication of the original cell. In all such fer- 

 mentative processes, organisms analogous to the yeast cell are present, 

 and it is not strange that if the ferment cell is introduced into a suit- 

 able medium, it may by its rapid reproduction have power to convert 

 an unlimited amount of one substance into another. Speaking generally 

 a special variety of cell is concerned with each ferment action, thus one 

 variety has to do with alcoholic, another with lactic and another with 

 acetous fermentation. A considerable number of species of bacteria 

 exist in the body during life, chiefly in connection with the mucous 

 membranes, particularly of the digestive tract. These bacteria are 

 unicellular organisms, devoid of chlorophyll, sometimes called fission 

 fungi or schizomycetes. They multiply chiefly by division, but many of 

 them also form spores whereas the yeast cell multiplies by gemmation. 

 The bacteria are very much smaller than the yeast cells, being only 

 from 1 to 2/j. in width. Morphologically they are classified into i. micro- 

 cocci or globular bacteria, ii. bacilli or rod-shaped bacteria, and iii. 

 spirilla or sinuous bacteria. 



Many forms of bacteria have been isolated from the mouth, a few- 

 varieties from the stomach, and a very large number from the intestines. 

 It is only in the last named locality that their multiplication has much 

 effect from a physiological point of view. The normal (hydrochloric 

 acid) acidity of the stomach usually destroys all the micro-organ isms 

 taken in with the food, but when the amount of this acid is deficient (and 

 sometimes even when it is normal) some of the spores may escape. On 

 reaching the small intestine these spores begin to develop in its alkaline 

 medium, and may increase to such an extent as to stop all pancreatic 

 and intestinal digestion ; the point where this occurs varies from day to 

 day. The large intestine always swarms with micro-organisms, though 

 the ileo-csecal valve, in some unknown way, prevents their passage into 

 the small intestine; as a consequence, both intestinal and pancreatic di- 

 gestion normally cease at this valve. The bacteria found in the intes- 

 tine are anaerobic, i.e., they do not exist in the presence of free oxygen. 



The changes induced in the intestine by the activity of micro- 

 organisms are of two kinds, fermentation and putrefaction; the former 

 of these results in the breaking down of carbohydrate matter and the 

 latter in that of proteid matter. The process of fermentation is the 

 least complex and probably occurs normally in the small intestine, to a 

 certain extent. The lactic-acid fermentation is the most important, 

 though the butyric-acid fermentation is next; under their influence the 

 carbohydrates are broken down into lactic and butyric acids, and perhaps 



