254 ABSORPTION OF THE FOOD. 



From all the facts known in connection with bacteria, the con- 

 clusion is inevitable that they serve a useful purpose in the econ- 

 omy ; they may, however, when in excess, produce such amounts of 

 harmful substances as to be injurious when these are absorbed. 



ABSORPTION OF THE FOOD. 



Attention has already been called to the fact that a large part 

 of the food-stuffs taken into the body are not .in a condition to be 

 absorbed by the blood, nor to be utilized by the tissues when 

 brought to them by that fluid (p. 167), and that digestion consists 

 in bringing about the changes in them necessary to effect this 

 result. It is these changes which we have studied, and which 

 will prepare us to understand the process of absorption. 



Manifestly, absorption might take place anywhere in the alir 

 mentary tract from the mouth to the anus, but it has been demon- 

 strated that in some portions of this canal very little absorption, 

 if any, takes place, and that in others the greater part of the proc- 

 ess is carried on. 



Mouth-absorption. Under ordinary circumstances there 

 is no absorption while substances are in the buccal cavity. This 

 is certainly true for the food-stuffs, though that it may occur 

 for some other substances is proved by the fact that cyanid of 

 potassium taken into the mouth and retained there will produce 

 death. 



There is likewise no absorption while food is passing through 

 the esophagus ; the time occupied in the transit is altogether too 

 brief, and the conditions generally are unfavorable. 



Gastric Absorption. The food-stuffs which enter the stom- 

 ach are: (1) inorganic, water and salts; (2) carbohydrates, starch 

 and sugars ; (3) fats or oils ; and (4) proteids. 



Inorganic Food-stuffs. Water taken into the stomach by itself 

 is not absorbed to any extent by that organ. Von Mering demon- 

 strated this in a dog in which he first established a fistula in the 

 duodenum, and then gave it by the mouth 500 c.c. of water. 

 Almost as soon as it reached the stomach it was forced out by 

 contraction of the muscular coat into the duodenum in spirts, and 

 in twenty-five minutes 495 c.c. passed out through the fistulous 

 opening in the intestine. When water contains in solution sub- 

 stances which are absorbed by the stomach-walls, some of the 

 water is also absorbed with them. 



The evidence as to the absorption of salts is very incomplete. 

 Sodium iodid in 3 per cent, solution is absorbed, but to a slight 

 extent if the solution is more dilute. Many substances, such as 

 mustard or alcohol, hasten its absorption, probably by stimulating 

 the epithelium. 



Carbohydrates. Starch is not absorbed as such, but must be 

 changed into maltose, which process, as we have seen, does take 



