804 PHYSIOLOGY OF DIGESTION AND SECRETION. 



that lie between the epithelial cells, or lastly that the fat is first split 

 into fatty acid and glycerin and is absorbed by the epithelial cells in 

 these forms. The tendency of recent work is altogether in favor of 

 this last view, and we may adopt it as expressing the theory gener- 

 ally accepted at present. During digestion the epithelial cells con- 

 tain fat droplets without doubt, but it seems probable that these 

 droplets are formed in situ by a synthesis of the absorbed glycerin 

 and fatty acids. The border of the cell is said to be free from 

 fat globules, a fact which would indicate that the neutral fat 

 is not mechanically ingested as oil drops. But, granting that 

 the fat is absorbed in solution, as fatty acids and glycerin, the 

 mechanism of absorption remains unexplained. It is known 

 that the bile as well as the pancreatic juice plays an important 

 part in the process. The pancreatic juice furnishes the lipase, the bile 

 furnishes the bile salts (glycocholate and taurocholate of sodium) 

 which aid the lipase in splitting the neutral fat, and moreover aid 

 greatly the absorption of the split fats. This latter function is due 

 probably to the fact that the bile (bile salts) dissolves the fatty acids 

 readily* and thus brings them into contact, in soluble form, with the 

 epithelial cells. When the bile is drained off from the intestine 

 by a fistula of the gall-bladder or duct, a large proportion of 

 the fatty foods escapes absorption and appears in the feces. Direct 

 observation shows that the fat after passing the epithelial lin- 

 ing and entering the stroma of the villus is taken up by the 

 lymphatic vessels, the so-called lacteals. This fact is beautifully 

 demonstrated by the mere appearance of the lymphatics of 

 the mesentery after a meal containing fats. These vessels are 

 injected with milky chyle during the period of absorption so that 

 their entire course is revealed. The chyle on microscopical exami- 

 nation is found to contain fat in the form of an extremely fine 

 emulsion. In this form it is carried to the thoracic duct and thence 

 to the venous circulation. For hours after a meal the blood contains 

 this chyle fat. If a specimen of blood is taken during this time and 

 centrifugalized in the usual way, the chyle fat may be collected at 

 the top in the form of a cream. It is an easy matter to insert a 

 cannula into the thoracic duct at the point at which it opens into the 

 subclavian and jugular veins and thus collect the entire amount of fat 

 absorbed from the intestines by way of the lacteals. Experiments 

 of this kind show that, after deducting the amount of fat that escapes 

 absorption and is lost in the feces, the amount that may be recovered 

 from the thoracic duct is less than that taken in the food. It seems 

 probable, therefore, that some of the fat is absorbed directly by the 

 blood-vessels of the villi. The portion thus absorbed enters the 



*See Moore and Rockwood, "Journal of Physiology," 21, 58, 1897; 

 also Moore and Parker, " Proceedings, Royal Society," London, 58, 64, 1901. 



