CHAP, i.] DIGESTION. 303 



The nature of the fat is supposed to vary with that of the food, 

 but this has not been conclusively shewn. 



The lymph taken from the duct during fasting differs chiefly 

 from that taken after a meal, in the much smaller quantity of fat, 

 the microscope shewing besides the white corpuscles only very few 

 oil-globules, and in the almost entire absence of the molecular basis. 

 Lymph in fact is, broadly speaking, blood minus its red corpuscles, 

 and chyle is lymph plus a very large quantity of minutely divided 

 neutral fat. 



It has been calculated that a quantity equal to that of the 

 whole blood may pass through the thoracic duct in 24 hours, and of 

 this it is supposed that about half comes from food through the 

 lacteals and the remainder from the body at large ; but these cal- 

 culations are based on uncertain data. 



Entrance of the Chyle into the Lacteals. The lacteal begins 

 as a club-shaped (or bifurcate) lymphatic space lying in the centre 

 of the villus, and connected with the smaller lymphatic spaces of the 

 adenoid tissue around it; it opens below into the submucous lymph- 

 atic plexus from which the lacteal vessels spring. The adenoid 

 tissue of the surrounding crypts of Lieberkiihn is by its lymphatic 

 spaces connected with the same lymphatic plexus. That the finely- 

 divided fat does pass from the intestine, through the epithelial 

 envelope of the villus, into the adenoid tissue, and so into the 

 lacteal chamber, is certain, but much discussion has arisen as to the 

 exact mechanism of the transit. Most observers agree that after a 

 meal the epithelium cells of the villus are loaded with fat and that 

 this fat is derived from the intestinal contents. Since the striation 

 of the hyaline border of the cells is not due to pores, as was once 

 thought, the particles must have entered into the cells very much 

 as foreign particles enter the body of an amoeba. The epithelium 

 may thus be said to eat the fat, and subsequently to pass it on 

 into the lymphatic spaces of the adenoid tissue of the villus and so 

 into the central lymphatic chamber. There would thus be a stream 

 of fatty particles through the cell from without inwards, a stream 

 in the causation of which the cell took an active part. In fact, 

 under this view, absorption by the cell might be regarded as a sort 

 of inverted secretion, the cell taking much material from the chyme 

 and secreting it, with little or ne change, into the villus. Other 

 observers however believe that the fat passes not through but be- 

 tween the epithelium-cells, being taken by the inter-epithelial pro- 

 cesses of the peculiar epitheloid-cells, described as forming a con- 

 tinuous protoplasmic reticulum, connecting the surface of the villus 

 with the central chamber. Along this reticulum the fat is sup- 

 posed to travel, the epithelium cells themselves having no active 

 share in absorption. 



The passage is probably assisted by the movements of the in- 

 testine, though even in the contractions of strong peristaltic move- 



