236 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



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 bases 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. 23). On the other hand, where the 

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

 be smaller than those which are attached. Moreover, when it is very compactly 

 arranged, its sides may be flattened against each other, so as to become polygonal ; 

 and this constitutes the prismatic variety. 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 cylin- 

 ders 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 tessellated 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 ali- 

 mentary canal, from the cardiac orifice downwards ; it is found also in the larger 

 ducts of most glands which open into that canal, or upon the external surface, 

 such as the ductus choledochus, the salivary ducts, those of the prostate and Cow- 

 per's glands, the vas deferens, and the urethra. In all these situations, it comes 

 into connection with the pavement-epithelium, which usually lines the more deli- 

 cate canals of the glands, as well as their terminal follicles. There are certain 

 parts, moreover, on which the Epithelial cells retain their primitive roundness, 

 with very little modification ; such cells are said to constitute a " spheroidal epi- 

 thelium." The most important example of it is presented by the urinary pas- 

 sages, which it lines from the pelvis of the kidneys to the inner orifice of the 

 urethra ; but it is also found in the excretory ducts of the mammary, perspiratory, 

 and some other glands ; and presents itself also as the characteristic form in 

 many situations, in which the secreting process is most actively going on. This 

 form may pass by insensible gradations into either of the other two. 



231. Both the two principal forms of Epithelial cells are frequently observed 

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

 Cilia; and these, although of extreme minuteness, are organs of great import- 

 ance in the animal economy, through the extraordinary motor power with which 



they are endowed. The form of the ciliary 



Fig. 26. filaments is usually a little flattened, and 



tapering gradually from the base to the point. 

 Their size is extremely variable ; the largest 

 that have been observed being about l-500th 

 of an inch in length, and the smallest about 

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

 appears to bend from its root to its point, re- 

 vibratiie or ciliated Epithelium; a, nucie- turning again to its original state, like the 



ated cells, resting on their smaller extremi- stalks of COrn when depressed by the Wind ] 



ties; &, cilia. an( j wne n a number are affected in succession 



with this motion, the appearance of progres- 

 sive waves following one another is produced, as when a cornfield is agitated 

 by frequent gusts. When the ciliary movement is taking place in full activity, 

 however, nothing whatever can be distinguished, but the whirl of particles in 

 the surrounding fluid ; and it is only when the rate of movement slackens, that 

 the shape and size of the cilia, and the manner in which their stroke is 

 made, can be clearly seen. The motion of the cilia is not only quite in- 

 dependent (in all the higher animals at least) of the will of the animal, but is 

 also independent even of the life of the rest of the body ; being seen after the 

 death of the animal, and proceeding with perfect regularity in parts separated 



