144 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



only a single one. The Cylinder-Epithelium is very differently constituted. 

 Its component cells are cylinders, which are arranged side by side ; one extre- 



Fi". 38. 



A. 



B. 



Separated Epithelium cells, a, with 

 nuclei, i, and nucleoli, c, from mucous 

 membrane of mouth. 



Pavement-Epithelium of 

 the Mucous Membrane of the 

 smaller bronchial tubes; a, 

 nuclei with double nucleoli. 



mity of each cylinder resting upon the basement-membrane, whilst the other 

 forms part of the free surface. The perfect cylindrical form is only shown, 

 however, when the surface on which the cylinders rest is flat or nearly so. 

 When it is convex, the lower ends or basements of the cells are of much smaller 

 diameter than the upper or free extremities ; and thus each has the form of a 

 truncated cone, rather than of a cylinder ; as is well seen on the cells covering 

 the villi of the intestinal canal. (Fig. 45.) On the other hand, where the cy- 

 linder-epithelium lies upon a concave surface, the free extremities of the cells 

 may be smaller than those which are attached. Sometimes each cylinder is 

 formed from more than one cell, as is shown by its containing two or more 

 nuclei ; although its cavity seems to be continuous from end to end. And 

 occasionally the cylinders arise by stalk-like prolongations, from a pavement- 

 epithelium beneath. The two forms of Epithelium pass into one another at 

 various points; and various transition-forms are then seen, the tesselated 

 scales appearing to rise more and more from the surface, until they project as 

 long-stalked cells, truncated cones, or cylinders. The Cylinder-Epithelium 

 covers the mucous membrane of the alimentary canal, from the cardiac orifice 

 downwards ; it is found also in the larger ducts of the glands which open into 

 it, or upon the external surface such as the ductus choledochus, the salivary 

 ducts, those of the prostate and Cowper's glands, the vas deferens, and urethra. 

 In all these situations, it comes into connection with the Tesselated Epithe- 

 lium, which usually lines the more delicate canals of the glands, as well as 

 their terminal follicles. 



171. Both these principal forms of Epithelial cells are frequently observed 

 to be fringed at their free margins with delicate filaments, which are termed 

 Cllio ; [from cilium, an eyelash,] and these, although of extreme minuteness, 

 are organs of great importance in the animal economy, through the extraordi- 

 nary motor power with which they are endowed. The form of the Ciliary 



filaments is usually a little flattened, and ta- 

 pering gradually from the base to the point. 

 Their size is extremely variable; the largest 

 that have been observed being about 1 -500th 

 of an inch in length, and the smallest about 

 1-13, 000th. When in motion, each fila- 

 ment appears to bend from its root to its 

 point, returning again to its original state, 

 like the stalks of corn when depressed by 

 the wind ; and when a number are aflected 

 in succession with this motion, the appear- 

 ance of progressive waves following one 



39. 



I 



Vibratile or ciliated Kpillicliutn ; n. nu- 

 cleated cells, resting on their smaller ex- 

 tremities ; 6, cilia. 



