ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 293 



of the gut must happen here, for since the action of lipase 

 is reversible it must synthesize fat from the products of the 

 fat digestion which is going on in the lumen of the intestine. 

 In other words, 



Fatty acid + Alcohol + Fat. 



It is this synthesis of fat in the epithelium of the gut which 

 gives rise to the appearance of fat droplets in this locality, 

 and which the older observers looked upon as evidence sup- 

 porting the idea that fat is absorbed as an emulsion. 



We have yet to explain the transport of the fat into the 

 lymph-channels. The fatty acid and alcohol which diffuse 

 into the cells lining the intestine do not stop here but pass 

 on into the lymph current beyond. At the beginning of a 

 digestion period this is also free from fat, but it contains lipase. 

 Evidently the same play must occur in this liquid tissue which 

 we saw take place in the cells lining the intestine. Under 

 the influence of the lipase the fatty acid and alcohol are syn- 

 thesized into fat, and, it is this. fat which evidences itself in 

 the droplets found in the lymph returning from the intestine. 

 That we are dealing with a chemical equilibrium in this case 

 also is shown by the fact that the lymph always shows the 

 presence of free fatty acid accompanying the fat. 



In order to get a conception of the process of fat absorp- 

 tion as a whole, we need only to imagine these different proc- 

 esses of analysis, diffusion, and synthesis going on simul- 

 taneously and side by side. We are in a position now to rec- 

 ognize the unimportant role played by the fat droplets which 

 appear in the epithelial cells of the intestine. They are 

 evidently transitory creations, which exist only as long as 

 fatty acid and alcohol are diffusing through these cells. When 

 the last remnants of fatty acid and alcohol diffuse out of the 

 cells into the lymph stream the fat in the cells can no longer 

 maintain itself, and the lipase which before hastened its 

 synthesis now hastens its analysis until it too has disappeared 

 as fatty acid and alcohol into the lymph stream. This leaves. 



