FORMATION OF THE TISSUES EPITHELIUM. 471 



racter. The Nails may be considered as nothing more than an altered form 

 of Epidermis. When near their origin, they are found to consist of cells, 

 which gradually dry into scales. A new production is continually taking 

 place in the groove of the Skin in which the root is imbedded, and probably 

 also from the whole subjacent surface. It will be presently seen, that hair 

 also originates in the Epidermis ( 623). 



(521. The internal free surfaces are also covered with a kind of cuticle, to 

 which the name of Epithelium is given. The existence of an Epithelium, 

 covering the Mucous membrane of the first part of the alimentary canal, has 

 long been known ; but it is only of late that any thing analogous to it has 

 been supposed to exist elsewhere. The Epithelia are always in contact with 

 fluids, and remain of a soft and pliant nature ; some of them undergo exfolia- 

 tion in a less degree with the Cuticle ; being less exposed to external influences, 

 whilst others are being continually thrown off and renewed ; in all instances, 

 however, when the surface is denuded, they are restored in the same manner. 

 The forms presented by the Epithelial cells are various. The two chief, 

 however, are founded in the tessdaied or pavement-epithelium, and the cylin- 

 rfer-epithelium. The free edges of the cells are sometimes fringed with cilia ;* 

 and the epithelium is then said to be ciliated. The Tesselated Epithelium 

 covers the serous and synovial membranes, the lining membrane of the blood- 

 vessels, and the mucous membranes (with their glandular prolongations), ex- 

 cept where the cylinder-epithelium exists. The cells composing it are usually 

 polygonal, and their number of layers small ; in many instances there is but 

 a single stratum. A very good example of it will be shown in Fig. 152. The 

 tesselated Epithelium which covers the delicate pia mater that lines the cere- 

 bral cavities, not even excepting the infundibulum and the aqueduct of Sylvius, 

 supports an abundance of very active cilia, which are attached along the 

 edges of its cells. The cells of the Cylinder-Epithelium have the form of Jong 

 cylinders, or rather truncated cones, arranged side by side, having one extre- 

 mity free, and the other seated upon the subjacent membrane ; frequently 

 these cylinders seem to arise, by a stalk-like prolongation, from a tesselated 

 epithelium beneath. Sometimes each cylinder is formed from more than one 

 cell ; as may be distinguished by the number of nuclei which it contains. Vari- 

 ous transitional forms may be detected at the points at which the cylinder and 

 pavement-epithelia pass into one another ; the tesselated scales appearing to 

 rise more and more from the surface, until they project as pedunculated cells. 

 The cylinder-epithelium is found in the intestinal canal, beyond the cardiac 

 orifice ; in the larger ducts of the salivary glands ; in the ductus communis 

 choledochus ; in the prostate, Cowper's glands, vesiculaB seminales, vas defe- 

 rens, tubuli seminiferi and urethra. In all these situations it is continuous 

 with the tesselate epithelium ; the latter lines the more delicate canals of the 

 various glands. The cylinders are often fringed with cilia at their extremi- 

 ties ; and the motions of these are towards the natural outlets of the cavities 

 or canals they cover. A Ciliated Epithelium is found lining the nasal cavi- 

 ties, the frontal sinuses, maxillary antra, lachrymal ducts and sac, the posterior 

 surface of the pendulous velum of the palate and fauces, the Eustachian tube, 

 the larynx, trachea, and bronchi, to the finest divisions of these last (where it 

 becomes tesselate, but still ciliated), the upper portion of the vagina, the uterus, 

 and the Fallopian tubes. The function of the cilia is probably to propel the 

 viscid secretions, that would otherwise accumulate on these membranes, to- 

 wards the exterior orifices, from which they may be removed. 



* Cilia are minute hair-like filaments, which are, during life, and for some time after 

 death, in a state of continual vibration. See Art. Cilia in Cyclop, of Auat. and Phys. ; 

 and Princ. of Gen. and Comp. Phys., 146. 



