I530 



HUMAN ANATOMY. 



a circular and a longitudinal. The inner surface of the stratum is often broken by 

 processes of muscular tissue which penetrate the tunica propria well towards the 

 epithelium. The muscularis mucosae belongs to the mucous membrane, and there- 

 fore must be distinguished from the muscular coat proper, which is frequently a 

 conspicuous additional layer in the digestive tract. 



Mucous membranes are attached to the surrounding structures by a submucous 

 layer of areolar tissue. The latter varies in thickness and density, consequently the 

 firmness of the union between the mucous and submucous strata differs greatly in 

 various localities. Usually the attachment is loose, and readily permits changes in 

 position and tension of the mucosa, which, in the relaxed condition, is often thrown 

 into temporary folds or rugce, as in the oesophagus and stomach. In other places 

 the folds are permanent and not effaced by distention of the organ ; a conspicuous 

 example of such arrangement is seen in the valvulae conniventes of the small intestine, 

 in which the submucous tissue forms the basis of the elevation. 



The blood-vessels supplying mucous membranes reach the latter by way of the 

 submucous tissue, in which the larger branches divide into the twigs which pass into 



Fig. 1284. 



Non-vascular epithelium 



Larger branches within 

 submucosa 



Section of injected oral mucous membrane. X 60. 



the mucosa. Within the deeper parts of the tunica propria the smaller arterial 

 branches break up into the capillaries forming the subepithelial and papillary net- 

 works, the vascular loops being limited to the connective tissue stroma and never 

 entering the epithelium. The venous stems usually follow the arteries in their gen- 

 eral course. When glands are present, the capillaries surround the tubules or alveoli 

 with rich net-works in close relation to the basement membrane. 



The lymphatics within mucous membranes are seldom present as definite chan- 

 nels, since they begin as the uncertain interfascicular clefts between the bundles of 

 stroma-tissue. Towards the deeper parts of the mucosa the lymph-paths become 

 more definite, and exist as delicately walled varicose passages which converge towards 

 the submucous tissue. Within the latter the lymph-vessels form net-works richly 

 provided with valves and the accompanying dilatations. 



The nerves distributed to mucous membranes include cerebral or spinal and 

 sympathetic branches, the latter supplying especially the involuntary muscle of the 



