

CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 519 



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 

 fibres of the villus supply a special muscular pump for the empty- 

 ing and filling of the lacteal chamber. These fibres and small 

 bundles of fibres though running in various directions ( 262) and 

 varying in number and arrangement in different animals, take on 

 the whole a longitudinal direction parallel to the long axis of the 

 villus. It has been supposed that in contracting and shortening 

 the villus they compress the lacteal and thus empty it, and that 

 when they relax and the villus elongates again, the emptied chamber 

 fills once more. But a different interpretation of their action has 

 been offered somewhat as follows. When the muscular fibres 

 contract they shorten the villus. In thus becoming shorter the 

 body of the villus becomes proportionately broader, since probably 

 no great change of bulk in the reticulum takes place ; in this 

 broadening the part to give way will be the lacteal chamber, which 

 thus becomes broader and larger. When the muscular fibres relax, 

 the reticulum, the bars of which have been put on the stretch in 

 a lateral direction, by elastic reaction brings back the villus to its 

 former length, and the lacteal chamber elongates and narrows. On 

 this view the muscular contraction expands and so fills, while the 

 relaxation narrows and so empties the lacteal chamber. Which- 

 ever view we adopt, we may at least conclude that contractions 



