ABSORPTION FROM THE DIGESTIVE CAVITY. 



439 



accomplished through the medium of the Bloodvessels, which are distributed 

 upon the walls of the digestive cavity. But in the Vertebrata, we find an 

 additional set of vessels interposed between the walls of the intestine and the 

 sanguiferous system ; for the purpose, as it would seem, of taking up certain 

 components of the nutritive matter, of which part at least are not in a state of 

 perfect solution, and of preparing them for being introduced into the current 

 of the blood. These are the Absorbents of the intestinal walls ; of which those 

 that are found, after the performance of the digestive process, to contain the 

 white opalescent fluid known as "chyle," are distinguished as lacteals ; while 

 the remainder, like the absorbents of the system generally, are known as lymph- 

 atics. The distinction is a purely artificial one ; for the " lacteals" are the 

 "lymphatics" of those parts of the intestinal walls which they supply, as is 

 shown by the fact that, during the intervals of the digestive process, they con- 

 tain a transparent fluid in all respects similar to the "lymph" of other parts. 

 The Absorbents form a minute plexus beneath the mucous lining of the ali- 

 mentary canal along its whole extent; but in the small intestine they enter the 

 villi, at the extremities of which, indeed, they may be said to commence. 

 Those only are entitled to the designation of " lacteals" which originate from 

 the intestinal canal below the point at which the biliary and pancreatic ducts 

 pour their contents into it ; for above that point, the fatty constituents of the 

 alimentary matter are not in a state of sufficiently fine division to enter them ; 

 and the absorbed fluid is consequently pellucid, instead of possessing the milky 

 aspect. Thus, then, we are to consider the lacteal portion of. the Absorbent 

 system to be that part of it which is specially adapted, by its prolongation into 

 the villi, for the reception of an oleaginous fluid ; which we shall presently see 

 to be taken up from the contents of the alimentary canal, and to be prepared 

 for entrance into the absorbents, by a set of peculiar cells developed at the 

 radical extremities of those organs ( 461). 



460. The general structure of the Villi of the Intestinal mucous membrane 

 has been already described ( 228, 234) ; but the 

 peculiar disposition of their component structures 

 must here be more minutely noticed. Each villus 

 appears ordinarily to contain but a single lacteal tube 

 which occupies its centre ; in the larger villi, however, 

 two or even more trunks are sometimes discernible 

 (Fig. 134, A). The mode in which this tube com- 

 mences, near the extremity of the villus, has not yet 

 been precisely made out ; but it seems probable that 

 it originates in a plexus, formed by the anastomosis 

 of branches into which it subdivides. This much, 

 however, is quite certain, that the lacteals do not com- 

 mence by open orifices on the internal surface of the 

 intestinal tube, as they were formerly supposed to do. 

 Each villus is also furnished with a minute plexus of 

 capillary vessels, which lies near its surface ; these 

 sometimes pass between a single arterial and venous 

 twig, as in the villus of the Hare (Fig. 133), but are 

 sometimes supplied by several distinct twigs, as in 

 the villi of Man (Figs. 131, 132) ; the particular 

 arrangement of the vessels, the form of the plexus, &c., differing considerably 

 in different animals, and even in different portions of the intestine of the same 

 individual. From the facts to be presently stated, it will be obvious that these 

 bloodvessels are not less actively concerned in the absorbent functions, than are 

 the lacteals themselves; and there is evidence, moreover, that the circulation of 



Fig. 131. 



Villi of the Human Intestine, with 

 their capillary plexus injected. 



