CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 417 



dog. It is difficult to suppose that this wide chamber is in- 

 tended solely for the absorption of the relatively small amount 

 of fat present in vegetable food. The question which we are 

 discussing is clearly at present to be regarded as by no means 

 settled. 



The Mechanism of Absorption. 



251. The Absorption of Fats. We have now to consider 

 the manner in which these several substances pass into either 

 the lacteal radicle or the capillary blood vessel. It will be con- 

 venient to begin with the absorption of the fats. 



We have seen reason ( 230) to think that the fats, remain- 

 ing chiefly as neutral fats, are emulsified in the intestine, by 

 means of the bile and pancreatic juice, the small quantity of 

 soap which is formed probably serving simply the purpose of 

 facilitating the emulsification. 



The neutral fats so emulsified pass in the first instance into 

 the bodies of the columnar cells of the villi. It has, it is true, 

 been maintained by some that they pass between the cells and 

 not into them ; but the evidence is against this view. Since 

 no collections of fat globules are seen in the cubical cells of 

 the glands of Lieberkiihn we infer that these have nothing to 

 do with the absorption of fat. 



How the fat enters into the substance of the cell we do not 

 know. We may presume that the striated border plays some 

 part, but what part we do not know. Though the rods mak- 

 ing up the border appear able to move so far, at least, as to 

 change their form, we have no evidence that the fat is intro- 

 duced into the cells by means of any movements of these rods. 

 We may imagine that the globules pass into the cell substance 

 by help in some way of these rods, through amoeboid move- 

 ments comparable with the ingestive movements of the body of 

 an amoeba ; but we have no positive evidence to support this 

 view. We said ( 208) that bile promotes the passage of fat 

 through membranes, possibly by in some way promoting a 

 closer contact between the particles of fat and the substance of 

 the membrane ; but even if bile has this effect on the surface 

 of the cells, its action in this respect can be subsidiary only. 

 When an animal is fed not on neutral fats but on fatty acids, 

 the chyle in the thoracic duct contains a large quantity of 

 neutral fat. We may infer that a synthesis of the fatty acids 

 into neutral fats has in such a case somewhere taken place. 

 And indeed it has been urged by some that even the neutral 

 fats are not absorbed by the epithelium cells as merely emulsi- 

 fied fat, but that they are split up within the canal, absorbed by 

 the cells as fatty acids, and immediately synthesized again into 

 neutral fats. 



27 



