While this subject has been in a way covered in the preceding paragraphs, there will be some value in a regrouping of the facts.

Water

Water is absorbed to a certain small extent from each segment of the alimentary canal; especially is this necessary in the absorption of solutions, as sugar solutions, where the water serves in a measure as a sort of vehicle. However, the vast preponderance of free water is absorbed from the large intestine. The object of this is sufficiently discussed above.

Salts

Salts are likewise absorbed in small quantities throughout their passage through the alimentary canal; these also in 1arge degree from the large intestine.

Sugars

While sugars are absorbed in very small amounts from the stomach, and perhaps even in slight amounts from the large intestine, the small intestine is the place where most of this process goes on.

Proteins, like the sugars, are absorbed in a small quantity in the stomach, but most of the protein absorption takes place in the jejunum and ileum. It is possible for proteins also to be absorbed from the lower portion of the large intestine and even from the rectum. This last fact is sufficiently demonstrated in clinical experience with enemata or rectal feedings. However, under ordinary conditions, the proteins that have escaped absorption in the small intestine are likely to be so modified by bacterial action as to be unfit for subsequent absorption.

Fats

Fats, perhaps more than any other foodstuff, are absorbed in vastly greater proportion from the small intestine than from any other portion of the canal. There has been much experiment and no small amount of controversy as to the exact method of absorption of fats. An early theory of the absorption of fats was that these were reduced to an emulsion in the digestive process, and in this form were taken into the epithelial cells by a sort of ameboid activity, passed from these cells into the lymph radical of the villus, from which they made their way naturally and readily into the lymphatics of the mesentery, converging toward the receptaculum chyli, thence, by way of a thoracic duct, poured into the venous system, at the point where the left external jugular vein enters the subclavian vein. A later theory was based on the observation that a large part of the fats are saponified by the sodium carbonate abundantly present in the intestinal digestive juices. If a large part is thus saponified, why not all? So it was taught that the sodium of the sodium carbonate acted as a carrier for the fats, passing into the alimentary canal as sodium carbonate, the sodium joined with fatty acid making a soluble sodium soap, readily absorbable; after absorption, and while still within the epithelium of the villus, the sodium gives up the fatty acid, which combines with glycerin, also absorbed and within the epithelial cells, forming new molecules of fat. These fat molecules collected into minutes globules are passed along by the cells and pushed out toward the lymphatic radical into which they make their way.

While this theory accounts for the appearance of the soap, it does not eliminate the ameboid action required in passing particles of matter as fat globules through the substance of the cells and the meshes of the tissue into the lymph radical.

The latest theory, while accepting the second so far as concerns the actual absorption of soaps, revives also the earlier theory in so far as that depends upon the passage of fat globules directly through tissues. However, this passage of the fat globules from the lumen of the intestines into the tissues is described as following intercellular spaces, the globules making their way through the cement substance that lies between the cells. Any globules actually seen in cell substance, then, are looked upon as being the result of the changing of soap molecules to fat molecules, as described above.