CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 469 



The meshes of typical adenoid tissue are always crowded with, 

 and practically filled up by, leucocytes of various sizes ; it is only 

 with very great difficulty that the network can be obtained free 

 from them. 



The connective tissue occupying the spaces between and below 

 the glands of Lieberkiihn is very similar to adenoid tissue in as 

 much as it presents a network of delicate fibres ; but the meshes 

 are somewhat larger and more irregular than those of true adenoid 

 tissue, and though they contain, are not crowded with, leucocytes ; 

 the amount of cell-substance left at some of the nodal points is 

 greater, nuclei are more abundant, and some of the processes of 

 the cells forming the bars of the network are flat expansions rather 

 than fibres. It is on the whole therefore somewhat different from 

 the typical adenoid tissue of lymphatic structures, and though it 

 is often spoken of under the same name as that tissue, it will be 

 convenient to distinguish it by some term; it might be called 

 reticular tissue. 



The tissue which fills up the body of a villus differs still more 

 from true adenoid tissue; it is formed of branching cells which 

 have for the most part retained their nuclei and a larger amount 

 of cell-substance round each nucleus; the processes are partly 

 membranous, partly fibres, and some of them exhibit a tendency 

 to form minute bundles of fibrillse. It is intermediate between 

 adenoid tissue and ordinary connective tissue, and may perhaps be 

 described as forming a loose somewhat open sponge-work rather 

 than a network. 



Lying loose in the meshes of this peculiar reticular connective 

 tissue, both in the villi and elsewhere, are seen bodies having the 

 general characters of white blood corpuscles (see 31), which, 

 though they are probably not all of the same kind, we may speak 

 of under the term of leucocytes. Sometimes these are scanty 

 but often are very numerous. This reticular connective tissue 

 forms in fact a labyrinth of irregular passages which are occupied 

 by fluid but through which leucocytes can wander to and fro. 

 We shall later on point out that this labyrinth of passages is 

 associated in a particular manner with the lymphatic vessels and 

 that the fluid occupying the spaces is in reality lymph. Indeed 

 this tissue ought perhaps to be regarded as part of the lymphatic 

 system. 



The basement membrane spoken of above appears to be formed 

 largely, at least over the villi, by the expanded ends of fibres of 

 the reticulum which reaching the surface from below spread out 

 laterally beneath the epithelium, and being joined by a certain 

 number of cells lying flat on the surface, form together a sheet 

 which is not continuous but discontinuous, being broken by 

 openings through which the bases of the cells of the epithelium 

 are brought into contact with the fluid occupying the spaces of the 

 reticulum below. 



