332 ESSENTIALS OF PHYSIOLOGY. 



because of their property of holding fatty acids in solution, and partly 

 because they reduce surface tension and so facilitate the passage of the 

 fatty material into the epithelial cells. In the absence of bile 60 per 

 cent, of the fat of a meal remains unabsorbed. The absence of pan- 

 creatic juice also prevents the absorption of much of the fat, because 

 fat is not absorbed unless it is acted on by lipase, and the lipase of 

 gastric juice takes a comparatively small share in fat digestion. 



The soaps formed in the intestine are split up by the intestinal 

 epithelial cells into fatty acids and alkali. The fatty acids are absorbed 

 and recombined with glycerol in the cells to form neutral fats. 



Fat absorption may be studied histologically. During its occurrence 

 the epithelial cells covering the villi become filled with droplets of fat, 



which may be stained black with 

 osmic acid (fig. 125), red with Schar- 

 lach R or Sudan III, or blue by means 

 of Nile blue. The droplets consist 

 of neutral fat, so that a re-synthesis 

 takes place in the epithelial cell, 

 the bile salts which held the fatty 

 acid in solution probably passing 

 directly into the blood stream. 



FIG. 125. Mucous membrane of frog's The fat may be traced through the 



intestine during fat-absorption n j fc th f th m 8 where 

 (From Schafer's Essentials of 



Histology.} the droplets are finely emulsihed 



ep, epithelium ; st.r, striated border ; c. bv the lymph in the tissue Spaces, 

 leucocytes; I. lacteal. J J 



and are carried into the central 



lacteal. The wandering leucocytes in the villus take up fat droplets, 

 and this may account to some extent for the 40 per cent, of absorbed 

 fat which cannot be recovered from the thoracic duct. 



Alternate contraction and relaxation of the muscular fibres in the 

 villus tends to propel the contents of the central lacteal towards the 

 larger lymph vessels in the intestinal submucosa. 



It has been demonstrated that hydrocarbons such as paraffin and 

 petroleum are not absorbed in the intestine. 



SECTION VI. 

 THE LARGE INTESTINE. 



Four or five hours after each meal the contents of the small 

 intestine begin to pass through the ileo-colic junction into the large 

 intestine. An important factor in promoting the transference of 



