CHEMISTRY OF DIGESTION AND NUTRITION. 311 



acids, CO,, CH 4 , and H 2 S) promote the movements of the intestine, and may 

 be of value from this standpoint; on the other hand, some of them are 

 absorbed into the blood, to be eliminated again in different form in the urine 

 (indol aud phenols), and it may be that they are of importance in the metab- 

 olism of the body; but concerning this our knowledge is deficient. On the 

 whole, we must believe that the food in its passage through the alimentary 

 canal is acted upon mainly by the digestive enzymes, the so-called " unorgan- 

 ized" ferments, but that the action of the bacteria, or organized ferment-, is 

 responsible for a part of the changes that the food undergoes before its final 

 elimination in the form of feces. These two kinds of action vary greatly 

 within normal limits, and to a certain extent they seem to be in inverse 

 relationship to each other. When the digestive enzymes and secretion- are 

 deficient or ineifective the field of action for the bacteria is increased, and this 

 seems to be the case in some pathological conditions, the result being intes- 

 tinal troubles of various kinds. The limits of normal bacterial action have 

 not been worked out satisfactorily, but it is evident that our knowledge <>f 

 digestion will not be complete until this is accomplished. 



It should be stated in conclusion that, however constant and important 

 the occurrence of bacterial fermentation may be in the alimentary canal, it 

 cannot be regarded as essential to the life of the animal, since Nuttall and 

 Theirfelder, 1 in a series of ingenious experiments made upon newly-born 

 guinea-pigs, have shown that these animals may thrive, for a time at least, 

 when the entire alimentary canal is free from bacteria. 



E. Absorption ; Summary of Digestion and Absorption of 

 the Food-stuffs ; Feces. 



In the preceding sections we have followed the action of the various 

 digestive secretions upon the food-stuifs as far as the formation of the supposed 

 end-products. In order that these products may be of actual nutritive value 

 to the body, it is necessary, of course, that they shall be absorbed into the 

 circulation and thus be distributed to the tissues. There are two possible 

 routes for the absorbed products to take: they may pass immediately into the 

 blood, or they may enter the lymphatic system, the so-called "lacteals" of 

 the alimentary canal. In the latter case they reach the blood finally before 

 being distributed to the tissues, since the thoracic duct, into which the lym- 

 phatics of the alimentary canal all empty, opens into the blood-vascular system 

 at the junction of the left internal jugular and subclavian veins. The sub- 

 stances that take this route are distributed to the tissues by the blood, but 

 it is to be noticed that, owing to the sluggish How of the lymph-circulation 

 (see section on Circulation), a relatively long time elapses after digestion 

 . before they enter the blood-current. The products that enter the blood 

 directly from the alimentary canal are distributed rapidly ; but in this case we 

 must remember that they first pass through the liver, owing to the existence of 

 1 Zeittschriflfiir phyaiologische Ohemie, 18'.)-"), lid. >J1 ; 1896, Bd. 22, ami L897, Bd. 23. 



