CHAPTER XXX 

 THE ABSORPTION AND USE OF FOODS 



General Statement. The digestive process, as we have con- 

 sidered it in preceding chapters, is purely one of preparation. Its 

 completion finds the food still within the alimentary tract, but 

 ready for the use of the Body. It is conveyed to the tissues, as we 

 have seen (Chap. XVII), by the blood. The passage of digested 

 food from the alimentary tract, through its walls, into the blood 

 or lymph, is known as absorption. The use of the food by the 

 tissues, since it involves chemical activities on the part of the 

 tissues themselves, is spoken of as metabolism. The discussion of 

 these two processes is the purpose of the present chapter. 



Absorption from the Stomach. Although the food remains in 

 the stomach for several hours after each meal, in fact is often not 

 wholly discharged before the taking of another one, it appears that 

 absorption from the stomach into the blood normally occurs to a 

 very limited degree, if at all. The fact that the digestive process 

 is for no foods completed in the stomach affords sufficient reason 

 why absorption should not take place there. We might suppose 

 that the single great group of food-stuffs not requiring digestion, 

 the single sugars, could advantageously be absorbed from the 

 stomach, but experiment shows that even they are absorbed very 

 slightly unless in rather high concentration, 5 per cent, in which 

 case the walls of the stomach do take them up rather rapidly. The 

 presence of alcohol in the stomach is said to increase markedly its 

 absorptive power, but this is at best a doubtful benefit, since the 

 single sugars form ordinarily a minor part of the meal, and the 

 other food-stuffs are not ready for the use of the Body, and are 

 wasted, therefore, if they arc absorbed. 



Absorption in the Small Intestine. The small intestine, being 

 the chief and final digestive laboratory of the Body, is naturally 

 the place from which absorption most largely goes on. It is, in 

 fact, specially adapted structurally, as is no other region of the 



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