CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 547 



villus are more or less occupied by wandering cells of which we 

 spoke under the general term of leucocytes. These do not all 

 present the same appearances and most probably are not all of the 

 same kind. A number of them may be distinguished by the fact 

 that the cell body is loaded with discrete granules which stain 

 readily and deeply with certain anilin dyes, and which though not 

 of a fatty nature turn black with osmic acid. 



Some of these leucocytes wander not only through the labyrinth 

 of the reticulum but pass into the epithelium between the cells, 

 and may project processes into, or even make their way eventually 

 into the interior of the intestine ; or following the reverse course 

 may wander from between the epithelium cells into the body of 

 the villus; some of them moreover undoubtedly contain fat. 

 Hence the view has been suggested that these leucocytes are 

 important agents, indeed the chief agents in the absorption of fat. 

 It has been supposed that they, receiving the globules of fat into 

 their cell substance, in fact eating the fat exactly after the manner 

 of an amoeba, either while projecting between the columnar cells, 

 in which case they carry their burden of fat through the epithelium 

 into the villus, or while wandering in the labyrinth of the villus, 

 bear it away bodily into the lymphatic system. But the number of 

 leucocytes really containing any appreciable quantity of fat is too 

 small to account for the amount of fat absorbed ; since as we just 

 pointed out in a certain kind of these cells, and this kind is often 

 very abundant, the granules in the cell substance which stain with 

 osmic acid are not fat. Nor is the abundance of leucocytes in the 

 mucous membrane during the period of digestion a sure proof that 

 they are concerned in absorption, but rather an indication only 

 that active changes of some kind are going on, since after the 

 administration of a saline such as magnesium sulphate, which 

 produces effects the very reverse of absorption, these leucocytes are 

 present in unusual numbers. Moreover under some circumstances, 

 as in the villi of a new-born puppy after a meal of milk, they 

 are absent even when digestion of fat is rapidly going on and the 

 lacteals are filling with fat. In fact, what we stated above 

 concerning the presence of fat in the bodies of the columnar cells 

 shews that leucocytes can have little to do in transferring fat from 

 the interior of the intestine into the body of villus ; and there are 

 no adequate reasons for attributing to them any real share in the 

 transference of fat from the body of the villus into the lacteal 

 chamber. 



311. The lacteal chamber opens at the base of the villus 

 into the valved lymphatic vessels lying below, and in these the 

 flow of lymph (chyle) is being promoted by the various causes 

 detailed in 300. The pressure for instance exerted by the 

 peristaltic contractions of the intestine helps to empty the lym- 

 phatic vessel into which a lacteal chamber opens and so promotes 

 the emptying of the latter. In addition to this the plain muscular 



