626 SPECIAL PHYSIOLOGY. 



the digestive process, are probably specially concerned in the absorp- 

 tion of fat by the lacteals. It has been supposed that these cells 

 feed, as it were, upon the fatty matters contained in the intestine, and, 

 having become distended, discharge their fatty contents into the com- 

 mencing lacteals. A nutritive process is imagined to take place, similar 

 to that which occurs in the epithelial cells lining the commencing ducts 

 of the secreting glands, but in a reverse direction ; that is to say, not 

 by the assimilation of materials from the blood into these secreting 

 cells, to be discharged at the surface, but by the assimilation of mate- 

 rials from the surface inwards, to be discharged into the lacteals. This 

 fatty matter must enter the epithelial cells covering the villi, in the 

 form of exceedingly minute molecules, by porous diffusion or trans- 

 mission ; and the fine vertical lines or streaks, noticed by certain ob- 

 servers in these cells (p. 604), are supposed by some to be minute 

 pores or channels, through which the highly subdivided fatty matters, 

 or even fine solid particles of charcoal, enter the interior of the cells, 

 and so proceed into the lacteals. How the fat particles pass from the 

 cells into the commencing lacteals is not known. By some it is said 

 that the inner pointed ends of the epithelial cells, terminate in caudate 

 areolar tissue cells, which, in their turn, communicate with the lacteals. 

 But this is more than doubtful; and the actual transmission is proba- 

 bly by porous diffusion, through true but invisible pores, 



The onward motion of the chyle, from the commencing lacteals in 

 the villi, into and through the larger absorbent vessels on the walls of 

 the intestine, and along the mesentery, to the thoracic duct, depends 

 on several agencies. First, the chief cause is probably the vis a tergo, 

 or force from behind, originating in the continuous nature of the ab- 

 sorptive process at the commencement of the lacteals. The existence 

 of this force is proved by the distension of the whole system of vessels, 

 including the thoracic duct, even to the occurrence of rupture, when 

 that duct is tied in an animal a short time after it has been fed. This 

 pressure from behind produces a motion of the fluid in the larger ab- 

 sorbents, just as the continuous absorption of fluid by the spongioles 

 at the extremities of the roots of trees, causes the rising of the sap. 

 Even in simple dialysis, in purely physical experiments, as with the 

 endosmometer of Dutrochet, there is an ascending motion of the fluid 

 in the graduated tube, due to the energy at work in the moist mem- 

 brane. Secondly, the contraction of the non-striated muscular fibres 

 of the villi, which, when stimulated by galvanism, in living animals, 

 has been observed to shorten those processes, must compress the cen- 

 tral lacteal of each villus, and so urge on its contents into the general 

 network of absorbents. The bile may help to excite this muscular act. 

 Thirdly, the contraction of the scattered muscular fibres in the sub- 

 mucous coat, and also the peristaltic movements of the proper muscular 

 coat of the intestine, likewise excited by the food and by the bile, will 

 serve to empty the intestinal lacteals into those of the mesentery. 

 Fourthly, the lacteals and the lymphatics, as well as the thoracic duct, 

 have muscular fibres in their coats, the contraction of which moves 

 onwards their contents, and also empties them on their being exposed 

 to the air, in animals recently fed ; this explains the collapsed state of 



