1146 GENERAL ANATOMY OR HISTOLOGY. 



tissue. This tissue is usually covered on its external surface by a transparent 

 basement-membrane generally composed of clear flattened cells, placed edge to 

 edge ; on this the epithelium rests. It is only in some situations that the base- 

 ment-membrane can be demonstrated. The corium is an exceedingly vascular 

 membrane, containing a dense network of capillaries, which lie immediately beneath 

 the epithelium, and are derived from small arteries in the submucous tissue. 



The fibro-vascular layer of the corium contains, besides the areolar tissue and 

 vessels, unstriped muscle-cells, which form in many situations a definite layer, 

 called the muscularis mucosce. These are situated in the deepest part of fhe mem- 

 brane, and are plentifully supplied with nerves. Other nerves pass to the epi- 

 thelium and terminate between the cells. Lymphatic vessels are found in great 

 abundance, commencing either by caecal extremities or in networks, and com- 

 municating with plexuses in the submucous tissue. 



Imbedded in the mucous membrane are found numerous glands, and project- 

 ing from it are processes (villi and papillae) analogous to the papillae of the skin. 

 These glands and processes, however, exist only at certain parts, and they have 

 been described for the sake of convenience, and with the parts as they occurred. 



SECRETING GLANDS. 



The secreting glands are organs whose cells produce, by tne metabolism of their 

 protoplasm, certain substances, called "secretions,'' of a more or less definite com- 

 position ; the material for the secretion being primarily selected from the blood. 

 The essential parts, therefore, of a secreting gland are cells, which have the power 

 of extracting from the blood certain matters, and in some cases converting them 

 into new chemical compounds ; and blood-vessels, by which the blood is brought 

 into close relationship with these cells. The general arrangement in all secreting 

 structures that is to say, not only in secreting glands, but also in secreting mem- 

 branes is that the cells are arranged on one surface of an extravascular basement- 

 membrane, Avhich supports them, and a minute plexus of capillary vessels ramifies 

 on the other surface of the membrane. The cells then extract from the blood 

 certain constituents which pass through the membrane into the cells, where they 

 are prepared and elaborated. The basement-membrane does not, however, always 

 exist, and any free surface would appear to answer the same purpose in some cases. 



By the various modifications of this secreting surface the different glands are 

 formed. This is generally effected by an invagination of the membrane in different 

 ways, the object being to increase the extent of secreting surface within a given 

 bulk. 



In the simplest form a single invagination takes place, constituting a simple 

 gland ; this may be either in the form of an open tube (Fig. 686, A), or the walls 

 of the tube may be dilated so as to form a saccule (Fig. 686, B). These are named 

 the simple tubular or saccular glands. Or, instead of a short tube, the invagination 

 may be lengthened to a considerable extent, and then coiled up to occupy less 

 space. This constitutes the simple convoluted tubular gland, an example of which 

 may be seen in the sweat-glands of the skin (Fig. 686, c). 



If, instead of a single invagination, secondary invaginations take place from 

 the primary one, as in Fig. 686, D and E, the gland is then termed a compound 

 one. These secondary invaginations may assume either a saccular or tubular form, 

 and so constitute the two subdivisions the compound saccular or racemose gland, 

 and the compound tubular. The racemose gland in its simplest form consists of a 

 primary invagination which forms a sort of duct, upon the extremity of which are 

 found a number of secondary invaginations called saccules or alveoli, as in Brun- 

 ner's glands (Fig. 686, D). But, again, in other instances, the duct, instead of being 

 simple, may divide into branches, and these again into other branches, and so on ; 

 each ultimate ramification terminating in a dilated cluster of saccules, and thus we 

 may have the secreting surface almost indefinitely extended, as in the salivary 



