CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 473 



singly, others forming small bundles of two or three fibres abreast. 

 These vary much in number and disposition in different animals. 

 Some of them lie close under and end in the basement membrane ; 

 others lie nearer the lymph-space, to which in some animals 

 they form a sort of muscular sheath. These fibres belong to the 

 muscularis mucosse ; at the base of the villus the fibres of the 

 muscularis mucosse take an upward course, passing between the 

 adjacent crypts of Lieberkiihn, and run into the villus, following 

 most commonly a longitudinal but sometimes a more or less 

 oblique or even a transverse direction. By the contraction of 

 these fibres the form of the villus can be changed ; but we shall 

 return to this point when we come to speak of the absorption of 

 digested material by means of the villi. 



All the space intervening between the basement membrane 

 and the central lymph-space which is not taken up by the blood 

 vessels and the muscular fibres, is occupied by the special kind 

 of reticular connective-tissue described above ( 259), the meshes 

 of which are to a greater or less extent occupied by leucocytes. 

 On the outer surface of the body of the villus this reticular 

 tissue is connected with, and indeed as we have seen forms 

 the basement membrane ; in the centre it forms around the 

 epithelioid plates of the lymph-space the walls of that cavity, 

 and supplies a similar bed for the blood capillaries ; the fine 

 connective-tissue belonging to the small bundles of muscular 

 fibres is continuous with it, and some of the muscular fibres 

 seem to end in it ; to it also is attached the connective-tissue 

 of the outer walls of the small artery and veins. The body of the 

 villus is in fact a sponge work of reticular tissue in which are 

 excavated the lymph-space and the blood channels with their 

 respective linings, into which the plain muscular fibres plunge, 

 and which is condensed on the outside into a basement mem- 

 brane. The meshes of the sponge work are further occupied, as we 

 have said, with leucocytes or with nucleated cells of an allied 

 nature ; these appear to be of more kinds than one, differing in 

 size and form, in the absence or presence of, or in the characters of 

 * granules ' in the cell-substance, and in other features. Under 

 certain circumstances they, of one or another kind, are abundant, 

 under other circumstances they are scanty. But some are always 

 present ; hence in ordinary stained specimens, not specially pre- 

 pared, the lymph-space and blood channels being collapsed, the 

 whole body of the villus appears a confused mass of nuclei; there 

 are the nuclei of the muscular fibres, the nuclei of the epithelioid 

 plates of the lymph-space and capillaries and of the other coats of 

 the artery and veins, the nuclei of the leucocytes in the meshes, 

 and lastly the nuclei belonging to the reticular tissue itself. 



The thickne'ss of the body of the villus, that is to say the 

 amount of reticular and of the other tissues lying between the 

 bases of the epithelium cells and the central lymph-space, varies 



F. n. 31 



