MUCOUS MEMBRANES. 121 



as seen in the papillae of the tongue or the rugae of the vagina. The papillae 

 contain the terminal loops of the blood-vessels and many of the endings of 

 the sensory nerves. Where increase of surface is desirable, the mucous 

 membrane may be thrown into cylindrical elevations, or villi, as conspicuously 

 seen in the small intestine. In many places, particularly in the digestive 

 tract, the mucous membrane contains more or less definite accumulations 

 of lymphoid tissue, of varying size and complexity, as exemplified by 

 the lymph-nodules within the vermiform appendix and the Peyer patches 

 within the ileum. A more or less definite line separates the epithelium 

 from the subjacent tunica propria. This demarcation is the basement 

 membrane or membrana propria. Often the basement membrane appears 

 as a mere line beneath the epithelium and is derived by slight differentia- 

 tion of the subepithelial connective tissue. Where, as around glandular 

 tissue, it is well developed and appears as a definite homogeneous membrane, 

 it is a product of the tunica propria. Exceptionally a reticular structure 

 is recognizable. Sometimes the deepest stratum of the mucous membrane, 

 next the submucous layer, is occupied by a narrow sheet of involuntary 

 muscle, the muscularis miicosa. While not everywhere present, it is well 

 developed in the intestinal tract and in places consists of two distinct strata, 

 a circular and a longitudinal. The muscularis mucosae belongs to the 

 mucous membrane and must not be confounded with the muscular coat 

 proper which is often a conspicuous additional tunic. 



The submucous layer, the stratum of areolar tissue connecting the 

 mucous membrane with the underlying structures, varies in thickness and 

 density. Usually the attachment is a loose one and readily permits changes 

 in position and tension of the mucous membrane; the latter, under such 

 conditions, is often thrown into temporary folds or rugae, as in the oesoph- 

 agus and stomach. In other places the folds are permanent and not effaced 

 by distention of the organ, as conspicuously demonstrated by the plications 

 in the duodenum in which the submucous tissue forms the basis of the band- 

 like elevations. 



The blood-vessels supplying mucous membranes reach the latter by 

 way of the submucous tissue, in which the larger arterial branches divide 

 into twigs that pass into the mucosa. Within the deeper parts of the tunica 

 propria the arterioles break up into capillaries forming subepithelial and 

 papillary networks, the vascular loops being limited to the connective-tissue 

 stroma. The veins usually follow the arteries in their general course. 

 When glands are present, the capillaries surround the tubules or alveoli 

 with rich networks, in close relation with the basement membrane. The 

 lymphatics within the mucous membrane are often represented by lymph- 

 spaces between the bundles of stroma-tissue. Towards the deeper parts of 

 the mucosa, however, more definite paths exist as thin-walled channels 

 which converge towards the submucous tissue. Within the latter the 

 lymphatics form networks, provided with valves and beset with the accom- 

 panying dilatations. 



The nerves distributed to mucous membranes include medullated and 

 nonmedullated fibres derived from the cranial or spinal trunks and the 

 sympathetic ganglia. The pale sympathetic fibres are destined for the invol- 

 untary muscle of the stroma and of the blood-vessels and for the glands. 

 The immediate supply of the involuntary muscle-cell is always the efferent 

 fibre (axone) from the sympathetic neurone, which is thus the last link in 

 the chain conducting the motor impulse. The position of the sympathetic 

 cells varies, in some cases being remote and in others close to the muscle 



