STRUCTURE OF MUCOUS MEMBRANE. 569 



Mucous COAT. The mucous membrane of the mouth invests the 

 whole internal surface of that cavity, and is reflected along the parotid, 

 submaxillary, and sublingual ducts, into the corresponding glands. It 

 terminates anteriorly upon the outer margin of the red border of the 

 lips, and posteriorly is continuous with the mucous lining of the pharynx. 

 The mucous membrane of the pharynx is continuous with the mucous 

 lining of the Eustachian tubes, the nares, the mouth, and the larynx. 

 In the oesophagus it is thick, very loosely connected with the muscular 

 coat, and disposed in longitudinal plica. In the stomach the mucous 

 membrane is thin and vascular at the great extremity, and becomes 

 thicker and lighter in colour towards the pyloric extremity. It is 

 formed into plaits or ruga?, which are disposed for the most part in a 

 longitudinal direction. The rugae are most numerous towards the 

 lesser end of the stomach ; while around the cardiac orifice they 

 assume a radiated arrangement. At the pylorus the mucous mem- 

 brane forms a circular or spiral fold which constitutes a part of the 

 apparatus of the pyloric valve. In the lower half of the duodenum, 

 the whole length of the jejunum, and the upper part of the ileum, it 

 forms valvular folds called valvulce conniventes, which are several lines 

 in breadth in the lower part of the duodenum and upper portion of 

 the jejunum, and diminish gradually in size towards each extremity. 

 These folds do not entirely surround the cylinder of the intestine, 

 but extend for about one-half or three-fourths of its circumference. 

 In the lower half of the ileum the mucous lining is without folds ; 

 hence the thinness of the coats of this intestine as compared with the 

 jejunum and duodenum. At the termination of the ileum in the 

 caecum, the mucous membrane forms two folds, which are strength- 

 ened by the muscular coat, and project into the caecum. These are 

 the ilio-C(cal valve (Valvula Bauhini). In the caecum and colon the 

 mucous membrane is raised into crescentic folds, which correspond 

 with the sharp edges of the sacculi ; and, in the rectum, it forms 

 three valvular folds,* one of which is situated near the commencement 

 of the intestine ; the second, extending from the side of the tube, 

 is placed opposite the middle of the sacrum ; and the third, which is 

 the largest and most constant, projects from the anterior wall of 

 the intestine opposite the base of the bladder. Besides these folds, 

 the membrane in the empty state of the intestine is thrown into 

 longitudinal plaits, somewhat similar to those of the oesophagus ; these 

 have been named the columns of the rectum. The mucous membrane 

 of the rectum is connected to the muscular coat by a very loose 

 areolar tissue, as in the oesophagus. 



Structure of Mucous Membrane. This membrane is analogous to 

 the cutaneous covering of the exterior of the body, and resembles that 

 tissue very closely in its structure. It is composed of three layers, an 

 epitlielium, a proper mucous, and a fibrous layer. 



* Mr. Houston, " On the Mucous Membrane of the Rectum." Dublin 

 Hospital Reports, Vol. V. 



