LACTEAL AND LYMPHATIC ABSORPTION. 347 



enclosed in a very delicate membrane (analogous to that which lies under the 

 epidermis and epithelium in the skin and mucous membrane, ( 640) ; and 

 that when digestion is not going on, they are covered by an epithelium. The 

 space between the reticulations of the blood-vessels and lymphatics, towards 

 the extremity of each villus, is occupied, whilst the absorption of chyle is 

 taking place, by a number of spherical vesicles or cells, varying in diameter 

 from the l-1000th to 1 -2000th of an inch, and containing an opalescent fluid. 

 At the part where the vesicles approach the granular texture of the substance 

 of the villus, minute granular or oily particles are seen. When the intestine 

 contains no more chyme, the vesicles disappear almost entirely, the lacteals 

 empty themselves, and the villi become flaccid ; the epithelium, which had 

 fallen off' during the process of Absorption, is then renewed. The vesicles at 

 the extremities of the villi can scarcely be regarded in any other light than as 

 cells, whose lives have but a very brief duration, selecting from the materials 

 in contact with the surface of the villi, and appropriating these to their own 

 growth, then liberating them, by solution or disruption of the cell-wall, in a 

 situation where they can be absorbed by the lacteals. 



462. In regard to the degree in which the function of Nutritive Absorption 

 is performed by the Lacteals and by the Venous System respectively, con- 

 siderable difference of opinion has prevailed. When the Absorbent vessels 

 were first discovered, and their functional importance perceived, it was ima- 

 gined that the introduction of alimentary fluid into the vascular system took 

 place by them alone. A slight knowledge of Comparative Anatomy, however, 

 might have sufficed to correct this error ; since no lacteals exist in the Inver- 

 tebrated animals, the function of Absorption being performed by the Mesenteric 

 veins alone ; from which it is evident that the veins do possess the power of 

 absorption : and it is scarcely to be supposed that they should not exercise 

 this power in Vertebrated animals also, since their disposition on the walls of 

 the intestinal cavity is evidently favourable to it. On the other hand, the 

 introduction of a new and distinct system of vessels would seem to indicate 

 that they must have some special purpose ; and there can be no doubt that the 

 absorption of nutritive matter is that for which they are peculiarly designed. 

 The fluid found in the lacteals is almost invariably the same ; being that to 

 which the name chyle has been applied, and which may be regarded as im- 

 perfectly elaborated blood. It appears from the uniformity of its composition, 

 which forms a striking contrast with the diversity of the food from which it is 

 obtained, that the lacteals have in some degree the power of selecting the par- 

 ticles of which it is composed ; and that, whilst they take up such a proportion 

 of each class of alimentary materials, as will rightly blend with the rest in 

 the nutritious fluid, they reject not only the remainder, but also (for the most 

 part at least) any other ingredients which may be contained in "the fluid of the 

 intestines. Such may be stated as the general result of the experiments that 

 have been made to determine their function ; though it is unquestionable that 

 extraneous substances, especially of a saline nature, occasionally find their 

 way into this system of vessels. This may not improbably be due to a cor- 

 respondence in the size and form of the ultimate particles of such substances, 

 with those of the materials normally absorbed by the lacteals.* From Mr. 

 Goodsir's observations, it would appear that the power of selection is a pecu- 

 liar vital endowment of the cells at the extremities of the villi, rather than of 



* Experiments upon the function of Absorption in Plants, whose radical vessels have 

 a corresponding power of selection, appear likely to assist in elucidating this interesting 

 subject. By the experiments of Dr. Daubeny it has been ascertained, that if a plant 

 absorb any particular saline compound, it can also be made to absorb those which are 

 isomorphous with it, though it will reject most others. See Princ. of Gen. and Comp. 

 Phys. 294. 



