EMULSION THEORIES OF FA T ABSORPTION. 449 



that existing immediately after such a discharge from the stomach, while the 

 emulsion condition is a later stage. 



In whatever form fats may be absorbed from the intestine, it is certain 

 that previous emulsincation must greatly assist the digestive fluids, by 

 exposing an infinitely greater surface to their action. It is also certain that 

 in a great many cases, if not in all, previous emulsincation does take 

 place. 



Emulsion theories of fat absorption. It was for a long time a 

 popular theory that only a small fraction of fat is split up in the intes- 

 tine into fatty acid and glycerin; and that by means of the small 

 amount of acid so formed, aided by that present in the fat as it leaves 

 the stomach, the remainder of the fat is converted into a fine emul- 

 sion which passes as such into the villi, and reaches the central lacteal. 1 

 Such a statement may be found in most text-books, but the progress of 

 recent work has had a tendency to cast grave doubts on its truth, and 

 to show that, at least as a general statement, it is erroneous. The theory 

 does not rest on any direct observation of the amount of fat which 

 leaves the intestine as emulsified fat, compared with that which leaves 

 it in other forms, such as soap, glycerin, and emulsified fatty acids, 

 such a direct observation, in the present state of our knowledge, is 

 impossible, but on indirect evidence, which is briefly as follows : 



1. The presence of a very small percentage of fatty acid is all that 

 is necessary in presence of an alkaline solution to perfectly emulsify 

 neutral fat. 



2. This small amount of free fatty acid can readily be furnished by 

 the action of the pancreatic enzyme even on neutral fats, and to aid this 

 action all fats contain already some fatty acid mixed with them. The 

 alkaline juices poured into the intestine are capable of supplying the 

 alkali necessary for emulsification. 



3. When an animal is killed during active fat digestion, the lacteals 

 invariably contain a white milky emulsion, consisting mainly of neutral 

 fats with a small percentage of alkaline soaps. 



Therefore the most natural conclusion is that a fine emulsion is 

 formed in the intestine which passes in some manner into the lacteal. 

 The greater part of the fat is only physically, not chemically, 

 altered in digestion, and passes through the whole process as a neutral 

 fat. 



The weak point in the emulsion theory of absorption always was, 

 how the fat globules got into the interior of the villus and made their 

 way to the lacteal. Although the fat granules in an emulsion are of 

 microscopic dimensions, they are still large compared to the dissolved 

 molecules of serum or egg albumin which are unable to pass into or out 

 of the intestine through the epithelial cells. If fat granules pass into 

 the epithelial cells at all, it must therefore be by means of a 

 special kind of absorption in bulk by these cells, and not by a process 

 even of selective diffusion from solution. Such an absorption by bulk is 

 easily carried out by a cell of which the protoplasm is capable of free 

 contraction, such as the amoeba, or leucocyte, but it is difficult to conceive 

 how it can take place with a fixed cell, such as those which line the 

 intestine. Impressed, perhaps, with the necessity of some such proto- 

 plasmic movement, some observers have looked earnestly for proto- 



1 This theory was first stated by Briioke, Sitznngsb. d. k. Akad. d. JFissensch., Wien, 

 1870, Bd. Ixi. Abth. 2, S. 362. 

 VOL. I. 29 



