798 PHYSIOLOGY OF DIGESTION AND SECRETION. 



by the histological fact that during the digestion of fats the epithelial 

 cells may be shown to contain fine oil drops in their interior. The* 

 tendency of recent work, however, has been to indicate that the fats 

 are completely split into fatty acids and glycerin before absorption, 

 and that the emulsification may be regarded, from a physiological 

 standpoint, as a mechanical preparation for the action of the lipase 

 rather than as a direct preparation for the act of absorption. The 

 two products of the action of the lipase, the glycerin and the fatty 

 acid, are absorbed by the epithelium and again combined to form 

 neutral fat. It is very probable, moreover, that during this 

 synthesis the fatty acids are combined with the glycerine in such 

 proportions as to make for the most part the fat characteristic 

 of the animal, fat of a high melting-point in the case of the 

 sheep, for example, and of a lower melting-point for the dog. 

 In connection with this fact of a synthesis of the split products 

 to form neutral fat, the discovery by Kastle and Loevenhart 

 (see p. 744) that the action of lipase is reversible assumes much 

 significance. It seems quite possible that the same enzyme may 

 cause both the splitting of the fat and the synthesis of the split 

 products, not only in the intestine during absorption, but in the 

 various tissues during the metabolism or the storage of fat. Lipase 

 is found in the blood and in many tissues, muscle, liver, mammary 

 gland,* etc. and during its nutritive history in the body the fat 

 may be split and synthesized a number of times. In this connection 

 it is interesting to note that the process of splitting does not involve 

 much work. Very little heat is liberated in the process, and a cor- 

 respondingly small amount of energy is needed for the synthesis. f 



The lipase as formed in the pancreas is easily destroyed, especially 

 by acids. For this reason probably it is not found usually in simple 

 extracts of the gland made by laboratory methods. It should be 

 added, also, that the action of this enzyme is aided very 7 materially 

 by the presence of bile. This latter secretion contains no lipase 

 itself, but mixtures of bile and pancreatic juice split the neutral 

 fats much more rapidly than the pancreatic juice alone. This 

 effect is now explained on the hypothesis that the bile-acids or 

 the bile-acids and the lecithin either activate a portion of the 

 lipase which is in the state of a preferment or play the part of a 

 coferment (page 749). 



The Intestinal Secretion (Succus Entericus) . The small 

 intestine is lined with tubular glands, the crypts of Lieberkuhn, 

 which in parts of the intestine at least give rise to a liquid secretion, 

 the so-called intestinal juice. To obtain this secretion recourse has 

 been had to the operation known as the Thiry-Vella fistula. In this 



* See Loevenhart, "American Journal of Physiology," 6, 331, 1902. 

 r, "Zeitschrift f. physiol. Chemie," 37, 383, 1903. 



f Consult Herzog, 



