DIGESTION. 



255 



alike consist of bundles of unstriped muscular tissue supported by con- 

 nective tissue. They are well provided with lymphatic vessels, which 

 form a set distinct from those of the mucous membrane. 



Between the two muscular coats is a nerve-plexus (Auerbach's plexus, 

 plexos myentericus) (Pig. 181) similar in structure to Meissner's (in the 

 submucous tissue), but with more numerous ganglia. This plexus regu- 

 lates the peristaltic movements of the muscular coats of the intestines. 



(3) Between the mucous and muscular coats, is the submucous coat, 

 which consists of connective tissue, in which numerous blood-vessels and 

 lymphatics ramify. A fine plexus, consisting mainly of non-medullated 

 nerve-fibres, "Meissner's plexus," with ganglion cells at its nodes, occurs 



FIG. 182. Horizontal section of a small fragment of the mucous membrane, including one entire 

 crypt of Lieberkuhn and parts of several others: a, cavity of the tubular glands or crypts; 6, one 

 of the lining epithelial cells; c, the lymphoid or retiform spaces, of which some are empty, and others 

 occupied by lymph cells, as at d. 



in the submucous tissue from the stomach to the anus. From the posi- 

 tion of this plexus and the distribution of its branches, it seems highly 

 probable that it is the local centre for regulating the calibre of the blood- 

 vessels supplying the intestinal mucous membrane, and presiding over the 

 processes of secretion and absorption. 



(4) The mucous membrane is the most important coat in relation to 

 the function of digestion. The following structures, which enter into its 

 composition, may now be successively described; the valvulce conniventes; 

 the vitti; and the glands. The general structure of the mucous mem- 

 brane of the intestines resembles that of the stomach (p. 241), and, like 

 it, is lined on its inner surface by columnar epithelium. Adenoid tissue 

 (Fig. 182, c and d) enters largely into its construction; and on its deep 

 surface is the muscularis mucosce (m m, Fig. 183), the fibres of which are 

 arranged in two layers: the outer longitudinal and the inner circular. 



Valvulae Conniventes. The valvulce conniventes (Fig. 184) com- 

 mence in the duodenum, about one or two inches beyond the pylorus, and 

 becoming larger and more numerous immediately beyond the entrance of 

 the bile duct, continue thickly arranged and well developed throughout 



