STEUCTUEE OF THE INTESTINE. 



177 



The mucous membrane is bounded next to the submucous coat by a 

 double layer of plain muscular fibres (muscularis mucosce). Bundles 

 from this pass inwards through the 

 membrane towards its inner surface 

 and penetrate also into the villi. 

 The mucous membrane proper is 

 pervaded with simple tubular glands 

 the crypts of Lieberkulm which 

 are lined throughout by a colum- 

 nar epithelium like that which 

 covers the surface and the villi. 

 The mucous membrane between 

 these glands is mainly composed of 

 lymphoid tissue, which is aggre- 

 gated at intervals into more solid 

 nodules (fig. 206), constituting when 

 they occur singly the so-called soli- 

 tary glands of the intestine, and 

 when aggregated together form the 

 agminated glands or patches of Peyer. 

 The latter occur chiefly in the ileum. 



The villi with which the whole 

 of the inner surface of the small 

 intestine is closely beset are clavate 

 or finger-shaped projections of the 

 mucous membrane, and are com- 

 posed, like that, of lymphoid tissue 

 arid covered with columnar epi- 

 thelium (fig. 207). The characters 

 of this have been already described 

 (Lesson VII.). Between and at 

 the base of the epithelium-cells 

 many lymph-corpuscles occur. The 

 epithelium rests upon a basement- 

 membrane formed of flattened cells. 

 In the middle of the villus is a 

 lacteal vessel (c.L) which is some- 

 what enlarged near its commencement. It is replaced in some animals 

 by a network of lymphatics. Surrounding this vessel are small bundles 

 of plain muscular tissue prolonged from the muscularis mucosa3. The 

 network of blood-capillaries (fig. 208) lies for the most part near the 

 surface within the basement-membrane ; it is supplied with blood by 



FIG. 208. SMALL INTESTINE, VERTICAL 



TRANSVERSE SECTION WITH THE 

 BLOOD-VESSELS INJECTED. (Heitz- 



mann. ) 



V, a villus ; G, glands of Lieberktihn ; M, 

 muscularis mucosae ; A, areolar coat; R, 

 ring-muscle (circular layer of muscular 

 coat) ; L, longitudinal layer 'of muscular 

 coat ; p, peritoneal coat. 



