794 PHYSIOLOGY OF DIGESTION AND SECRETION. 



by the histological fact that during the digestion of fats the epi- 

 thelial 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 

 further 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 epi- 

 thelium and again combined to form neutral fat. It is very prob- 

 able, 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 spht products to form neutral fat, the 

 discovery by Kastle and Loevenhart (see p. 739) 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 dur- 

 ing 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 spht 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 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 proferment or play the part of a 

 coferment (page 744). 



The Intestinal Secretion (Succus Entericus). — ^The small 

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

 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. 

 t Consult Herzog, "Zeitschrift f. physiol. Chemie," 37, 383, 1903. 



