408 PAPILLA AND VILLI. 



found. In other mucous membranes transitions are met with between these two 

 extremes. 



It frequently happens that in certain circumscribed places the lymphoid tissue 

 is greatly increased in amount, and becomes densely packed with lymphoid cells. 

 In this way the so-called solitary glands, lymphoid follicles or lymphoid nodules are 

 produced. If there are many lymphoid nodules adjacent to one another, so as to 

 make up a localized patch, a so-called agminated gland is formed, or if massed 

 together more thickly, a lymphoid organ like the tonsil. These collections of 

 lymphoid tissue, which may if large extend, on the one hand, down into the sub- 

 mucous tissue, and on the other, upwards into the epithelium, have been already 

 referred to (p. 388), and will be more particularly described when the several parts 

 in which they occur come under consideration. The lymph-cells of this tissue 

 migrate between the cells of the epithelium which covers the surface, and may even 

 become free in considerable numbers in the fluid which moistens the surface (Stohr). 

 The purpose served by this emigration of lymph-cells is not understood. 



Blood-vessels are abundant in most mucous membranes. The branches of the 

 arteries and veins, dividing in the submucous tissue, send smaller branches into the 

 corium, which divide to form a network of capillaries in the latter. This capillary 

 network lies immediately beneath the epithelium, or the basement-membrane when 

 this is present, advancing into the villi and papillae to be presently described, and 

 surrounding the tubes and other glandular recesses. The lymphatics also form 

 networks of cleft-like vessels in the mucous membrane, which communicate with 

 plexuses of larger valved vessels in the submucous tissue ; they commence either by 

 blind diverticula, as in the villi, or in the form of a superficial network, which is almost 

 always more deeply placed than the network of blood-capillaries. The lymphatic 

 vessels often form sinus-like dilatations around the bases of the lymphoid nodules. 



The nerves of mucous membranes seem chiefly to be distributed to the mus- 

 cularis mucosae where this exists. Before proceeding to their destination they are 

 in many parts collected together to form a gangliated plexus in the submucous 

 tissue, such as the plexus of Meissner in the alimentary canal. Some nerves pass 

 however to the epithelium and terminate between the epithelial cells ; at least this 

 has been shown to be the case in the stratified epithelium which covers the mucous 

 membranes of certain parts. 



Papillae and 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 covered with 

 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 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. Being set close together like the pile 

 of velvet, 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 corium, covered with epithelium, and containing blood-vessels and lacteals, which 

 are thus favourably 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 in which they occur. 



In some few portions of the mucous membranes the surface is marked with fine 

 ridges which intersect each other in a reticular manner, and thus inclose larger and 

 smaller polygonal pits or recesses. This peculiar character of the surface of the 

 membrane, which has been termed " alveolar," is seen very distinctly in the gall- 



