720 PHYSIOLOGY OF DIGESTION AND SECRETION. 



to intestinal troubles, such as diarrhea, or to more serious interference 

 with general nutrition owing to the formation of toxins. It is, 

 however, possible that some amount of bacterial action may be 

 necessary for completely normal digestion. As a special case it has 

 been pointed out that the gastro-intestinal tract is not provided with 

 enzymes capable of acting upon cellulose, a material that forms such 

 an important constituent of vegetable foods. Bacteria, on the other 

 hand, may hydrolyze the cellulose and render it useful in nutrition. 

 Leaving aside this special case, the question as to the necessity of 

 bacterial action has been investigated directly by attempting to 

 rear young animals under perfectly sterile conditions. Nuttall and 

 Thierf elder * report some very interesting experiments upon guinea 

 pigs in which the young animals from birth were kept sterile and fed 

 with perfectly sterile food. They found that the animals lived and 

 increased in weight, and concluded therefore that the intestinal 

 bacteria are not necessary to normal nutrition. This conclusion is 

 supported by the observations of Levin,| who finds that animals in 

 the Arctic regions in many cases have no bacteria in their intestines. 

 Schottelius J reports contrary results upon chickens. When kept 

 sterile they lost steadily in weight and showed normal growth only 

 when supplied with food containing bacteria. We may conclude, 

 however, that the evidence at present indicates that the bacterial 

 fermentation is not essential, although under the actual conditions 

 of life it plays a part in the digestive history of the food. 



Composition of the Feces. The feces differ widely in amount 

 and in composition with the character of the food. Upon a diet 

 composed exclusively of meats they are small in amount and dark 

 in color; with an ordinary mixed diet the amount is increased; and 

 it is largest with an exclusively vegetable diet, especially with vege- 

 tables containing a large amount of indigestible material. The 

 average weight of the feces in twenty-four hours upon a mixed diet 

 is given as 170 gms., while with a vegetable diet it may amount to as 

 much as 400 or 500 gms. The quantitative composition, therefore, 

 varies greatly with the diet. Qualitatively, we find in the feces 

 the following things: (1) Indigestible material, such as ligaments of 

 meat or cellulose from vegetables. (2) Undigested material, such as 

 fragments of meat, starch, or fats which have in some way escaped 

 digestion. Naturally, the quantity of this material present is slight 

 under normal conditions. Some fats, however, are almost always 

 found in feces, either as neutral fats or as fatty acids, and to a small 

 extent as calcium or magnesium soaps. The quantity of fat found is 



* Nuttall and Thierfelder, " Zeitschrift f. physiol. Chemie," 21, 109, 

 1895; 22, 62, 1896; 23, 231, 1897. 



t " Skandinavisches Archiv f. Physiologie," 16, 249, 1904. 

 j " Archiv f. Hygiene," 42, 48, 1902. 



