126 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



are dependent upon osmosis. There are fluids separated by 

 animal membranes everywhere. In the alimentary canal, for 

 instance, is a fluid containing matters fit to be absorbed; rami- 

 fying in the wall of that canal are blood and lymph capillaries 

 filled with fluid; while separating the two is an animal membrane 

 consisting of the alimentary epithelium, a little connective tissue 

 and the endothelial lining of the capillaries. These are condi- 

 tions most favorable for osmosis, but the osmotic laws of the 

 laboratory are by no means immutable in the body. 



From what has been said of osmosis in general, and consider- 

 ing variations due to conditions of circulation, etc., the following 

 facts seem clear as to absorption in the body: (i) The substance 

 must be in a liquid or gaesous state; (2) it must be diffusible; 

 (3) the membrane must be permeable; (4) the greater current 

 is toward the more dense solution; (5) the less dense the solution 

 the more quickly will it be absorbed; (6) the greater the pressure 

 in the vessels the less rapidly will absorption into them take 

 place; (7) absorption is more rapid the more rapid the blood 

 current (continually preventing "saturation" of the adjacent 

 blood) ; (8) the higher the temperature the more rapid is absorp- 

 tion; (9) the "vital condition" of the cells is the most important 

 factor of all. 



A thorough grasp of these principles and probabilities will do 

 much to clarify almost all the phenomena of vital activity, and 

 many questions of a pathological nature. 



Absorption from the Alimentary Canal. 



It has been said that all digested materials must find their way 

 into the blood. It is to be remembered that there are two ways 

 by which they reach the vascular circulation; first, by direct 

 absorption into the capillaries of this system, and second, 

 indirectly, by absorption into the lymphatic circulation and 

 passage thence to the left subclavian vein. Those lymph 



