250 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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

 not been worked out satisfactorily, but it is evident that our knowledge of 

 digestion will not be complete until this is accomplished. 



E. ABSORPTION ; SUMMARY OF DIGESTION AND ABSORPTION OP 

 THE FOOD-STUFFS ; FECES. 



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

 digestive secretions upon the food-stuffs 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 which take this route are distributed to the tissues by the blood, but 

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

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

 before they enter the blood-current. The products which 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 

 the portal circulation, before they reach the general circulation. During this 

 passage through the liver, as we shall find, changes of the greatest importance 

 take place. The physiology of absorption is concerned with the physical and 

 chemical means by which the end-products of digestion are taken up by the 

 blood or the lymph, and the relative importance of the stomach, the small 

 intestine, and the large intestine in this process. Leaving aside the fats, 

 whose absorption is a special case, the absorption of the other products of 

 digestion was formerly thought to be a simple physical process. The processes 

 of osmosis, and to a lesser extent of filtration and imbibition, as they are 

 known to occur outside the body, were supposed to account for the absorption 

 of all the soluble products. This belief has now given way, in large part, 

 to newer views, according to which the living epithelial cells take an active 

 part in absorption, acting under laws peculiar to them as living substances, 

 and different from the laws of diffusion, filtration, etc. established for dead 

 membranes. Since, however, it is highly probable that osmosis plays a part 

 in absorption, it will be convenient to give a brief definition of this process 

 as it occurs outside the body, in order that the use made of it in explain- 

 ing physiological absorption, as well as the objections to its use, may more 

 easily be understood. 



Diffusion and Osmosis. Certain liquids when brought into contact with 

 each other gradually mix, owing to the attraction of the molecules for each 



