STRUCTURE OF MUCCUS MEMBRANE. 511 



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 op- 

 posite 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 mus- 

 cular coat by a very loose areolar tissue, as in the oesophagus, f 



Structure of Mucous Membrane. Mucous membrane is analogous to 

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

 sue very closely in its structure. It is composed of three layers, an epithe- 

 lium, a proper mucous, and a fibrous layer. 



The Epithelium is the epiderma of the mucous membrane. Throughout 

 the pharynx and oesophagus it resembles the epiderma, both in appear- 

 ance and character. It is continuous with the epiderma of the skin at the 

 margin of the lips, and terminates by an irregular border at the cardiac 

 orifice of the stomach. At the opposite extremity of the canal it terminates 

 by a scalloped border just within the verge of the anus. In the mouth it 

 is composed of laminae of cytoblasts, cells, and polyhedral scales (fig. 209). 

 Each cell and each scale possesses a central nucleus, and within the nu- 

 cleus are one or more nucleus-corpuscles. According to Mr. NasmythjJ 

 the deepest lamina of the epithelium appears to consist of nuclei (cytoblasts) 

 only ; in the next the investing vesicle or cell is developed ; the cells by 

 degrees enlarge and become flattened, and in the superficial lamina? are 

 converted into thin scales. The nuclei, the cells, and the scales are con- 

 nected together by a glutinous fluid of the consistence of jelly, which con- 

 tains an abundance of minute opaque- granules. The scales of the super- 

 ficial layer overlap each other by their margins. During the natural 

 functions of the mucous membrane the superficial scales exfoliate continu- 

 ally and give place to the deeper layers. In the stomach and intestines these 

 bodies are pyriform in shape, and have a columnar arrangement, the apices 

 being applied to the papillary surface of the membrane, and the bases 

 forming, by their approximation, the free intestinal surface. Each column 

 is provided with a central nucleus and nucleus-corpuscle, which gives its 

 middle a swollen appearance ; and, from the transparency of its structure, 

 the nucleus may be seen through the base of the column, when examined 

 from the surface. Around the circular villi, the columns, from being 

 placed perpendicularly to the surface, have a radiated arrangement. The 

 columnar epithelium is produced, in the same manner with the laminated 

 epithelium, in cytoblasts, cells, and columns, and the latter are continually 

 hrown off to give place to successive layers. 



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

 ports, vol. v. 



j- The spaces between the columns of the rectum become closed at the anus so as to 

 form a series of pouches represented in the accompanying cut. These pouches are 

 sometimes dilated and produce a disease first described by Dr. Physick. (See Gibson's 

 Surgery.) The mucous membrane of the rectum is connected to the muscuiar coat by 

 a very loose cellular tissue, as in the oesophagus. G. 



$ Investigations into the structure of the Epithelium, presented to the medi6al section 

 of the British Medical Association, in 1839, published in a work entitJed " Thr^e .Me 

 moirs on the Development of the teeth and epithelium," 1841. 



