. The feeding of animals. uble food is absorbed has been explained in part on common physical two solutions of different densi-ties, containing diffusible compoimds,are separated by a permeable mem-brane, diffusion through this membranefrom the denser to the lighter liquidwill always occur. Such a condition^ as this prevails in the intestines, wemay believe. The intestinal solution,the denser one, is separated from aless concentrated liquid, the blood,which is constantly flowing on theother side of a thin dividing mem-brane. Under these conditions thereoccurs the passage into the
. The feeding of animals. uble food is absorbed has been explained in part on common physical two solutions of different densi-ties, containing diffusible compoimds,are separated by a permeable mem-brane, diffusion through this membranefrom the denser to the lighter liquidwill always occur. Such a condition^ as this prevails in the intestines, wemay believe. The intestinal solution,the denser one, is separated from aless concentrated liquid, the blood,which is constantly flowing on theother side of a thin dividing mem-brane. Under these conditions thereoccurs the passage into the blood ofcertain parts of the digested food. Itis held that in this way water, solublemineral salts, and sugar pass directlyinto the blood vessels, chiefly fromthe small intestine. 171. Changes in the walls of the intestinal tract.—In the absorption of peptones and fats, at least, forces areencountered other than the osmotic transference of sub-stances in solution, the operation of which is still more orless Fig. 5. Intestinalvillus, showing: a,epithelium; 6, capil-laries; c, lacteal ves-sels. THE DIGESTION OF FOOD 117 The ingested proteins are changed in the stomach andintestines to peptones, and in part, perhaps mainly, toamino acids resulting from the cleavage of peptones. Thefats are split partly, or entirely, into fatty acids and glyc-erin, with the subsequent formation of soaps by the unionof the free acids with alkaline bases. It has been held thatin the passage of these new compounds through the wallsof the intestine changes occur of a synthetical character,with a partial or total reconstruction of the proteins andfats into forms similar to those in the ingested food. Thisview as to the proteins has been modified somewhat bythe demonstration of the existence of amino acids in theblood showing that, if a s;vmthesis of proteins occurs in theintestinal w^alls, it is at least not complete. Aminoacids may exist in the blood even if synthesis occurs inthe intes
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