570 EPITHELIUM OF MUCOUS MEMBRANE. 



The EpitMium is the epidermis of the mucous membrane. Through- 

 out the pharynx and oesophagus it resembles the epidermis, both in 

 appearance and character. It is continuous with the epidermis 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 polygonal scales (fig. 160). Each cell and each scale possesses a 

 central nucleus, and within the nucleus are one or more nucleus-cor- 

 puscles. According to Mr. Nasmyth,* 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 laminae are converted into 

 thin scales. The nuclei, the cells, and the scales, are connected 

 together by a glutinous fluid of the consistence of jelly, which contains 

 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 

 continually 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 to its middle a swollen appear- 

 ance ; 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 perpendicu- 

 larly 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 contin- 

 ually thrown off to give place to successive layers. 



The Proper mucous, or Papillary layer is analogous to the papillary 

 layer of the skin, and, like it, is the secreting structure by which the 

 epithelium is produced. Its surface presents several varieties of 

 appearance when examined in different parts of its extent. In the 

 stomach it forms polygonal cells, into the floor of which the gastric 

 follicles open. In the small intestine it presents numerous minute, 

 projecting papillae, called villi. The villi are of two kinds, cylindrical 

 and laminated, and so abundant, as to give to the entire surface a 

 beautiful velvety appearance. In the large intestine the surface is 

 composed of a fine network of minute polygonal cells, more numerous 

 than those of the stomach, but resembling them in receiving the secre- 

 tion from numerous perpendicular follicles into their floors. 



The Fibrous layer (sub-mucous, nervous) is the membrane of sup- 



* Investigations into the structure of the Epithelium, presented to the me- 

 dical section of the British Medical Association, in 1839, published in a work 

 entitled " Three Memoirs on the Development of the Teeth and Epithelium." 

 1841. 



