STRUCTURE. 



Fig. CVI. 



stance of the membrane in which these parts lie is constructed of the variety 

 of connective tissue known as cytogenous or retiform (p. Ixxix, fig. xxxvm.), 

 which is formed of ramified and reticularly connected corpuscles, with or 

 without nuclei persistent at the points whence the branches divaricate ; 

 and in the meshes of this tissue is con- 

 tained a profusion of granular bodies 

 having all the characters of pale blood- 

 or lymph-corpuscles. This structure 

 (fig. CVL), which prevails in the mucous 

 membrane of the stomach and intes- 

 tines, both large and small, is some- 

 times named lymplioid tissue from its 

 resemblance to the interior tissue of 

 the lymphatic glands and of other 

 bodies belonging to or supposed to 

 belong to the lymphatic system, and 

 especially those known as the solitary 

 and agminated glands of the alimen- 

 tary mucous membrane. The tissue 

 forming the last-named bodies, indeed, 

 is often continuous with the lymphoid 

 tissue in their vicinity. The deepest 

 layer of the alimentary mucous mem- 

 brane, from the commencement of the 

 cesophagus downwards, is formed 

 throughout by non-striated muscular 

 tissue, and is named muscularis mucosce. 

 This lies next to the submucous tissue, 

 and consists of bundles running in 

 many parts both longitudinally and 

 circularly, in others in one of these 

 directions only. Prolongations from it 

 pass up between the glands to be distributed in the villi. 



The free surface of the mucous membranes is in some parts plain, but in 

 others is beset with little eminences named papillae and villi. The papillae 

 are best seen on the tongue ; they are small processes of the corium, mostly 

 of a conical or cylindrical figure, containing blood-vessels and nerves, and 

 covered with epithelium. Some are small and simple, others larger and 

 compound or cleft into secondary papillae. They serve various purposes ; 

 some of them no doubt minister to the senses of taste and touch, many 

 appear to have chiefly a mechanical office, while others would seem intended 

 to give greater extension to the surface of the corium for the production of 

 a thick coating of epithelium. The villi are most fully developed on the 

 mucous coat of the small intestines. Beiog set close together like the pile 

 or nap of cloth, they give to the parts of the membrane which they cover 

 the aspect usually denominated " villous." They are in reality little 

 elevations or processes of the superficial part of the corium, covered with 

 epithelium, and containing blood-vessels and lacteals, which are thus favour- 

 ably disposed for absorbing nutrient matters from the intestine. The more 

 detailed description of the papillae and villi belongs to the special anatomy 

 of the parts where they occur. 



In some few portions of the mucous membrane the surface is marked 

 with fine ridges which intersect each other in a reticular manner, and thus 



o 2 



Fig. CVI. LYMPHOID OR RETIFORM 

 TISSUE OF THK INTESTINAL Mucous 

 MEMBRANE OF THE SHEEP (from Frey). 

 MAGNIFIED 400 DIAMETERS. 



Cross section of a small fragment of 

 the mucous membrane, including one 

 entire crypt of Lieberkiihn and parts of 

 several others : a, cavity of the tubular 

 glands or crypts ; 6, one of the lining 

 epithelial cells ; c, the lymphoid or reti- 

 form spaces, of which some are empty, 

 and others occupied by lymph -cells as 

 atd. 



