202 EPITHELIAL STRUCTURE. 



In other mammiferous animals, as far as examined, cilia have been found in 

 nearly the same parts. To see them in motion, a portion of epithelium may be 

 scraped off any ciliated mucous membrane and examined in a drop of weak solution 

 of salt (0*6 per cent.) or serum of blood. When it is now viewed with a magnifying 

 power of 200 diameters or upwards, a very obvious agitation will be perceived at 

 the edge of the detached piece of epithelium ; this appearance is caused by the 

 moving cilia, with which the surface of the membrane is covered. Being set close 

 together, and moving simultaneously or in quick succession, the cilia, when in brisk 

 action, give rise to the appearance of a bright transparent fringe along the margin 

 of the membrane, agitated by such a rapid and incessant motion, that the single 

 threads which compose it cannot be perceived. The motion here meant, is that of 

 the cilia themselves ; but they also set in motion the adjoining fluid, driving it along 

 the ciliated surface, as is indicated by the agitation of any little particles that may 

 accidentally float in it. The fact of the conveyance of fluids and other matters along 

 the ciliated surface, as well as the direction in which they are impelled, may also be 

 made manifest by immersing the membrane in fluid, and dropping on it some finely- 

 pulverised substance (such as charcoal in fine powder), which will be slowly bub 

 steadily carried along in a constant and determinate direction ; and this may be seen 

 with the naked eye, or with the aid of a lens of low power (Sharpey). 



Cilia have been shown to exist in almost eveiy class of animals, from the highest 

 to the lowest. 1 The immediate purpose which they serve is, to impel matter, 

 generally more or less fluid, along the surfaces on which they are attached ; or, to 

 propel through a liquid medium the ciliated bodies of minute animals, or other small 

 objects which are provided with cilia ; as is the case with many infusorial animal- 

 cules, in which the cilia serve as organs of locomotion like the fins of larger aquatic 

 animals. In many of the lower tribes of aquatic animals, cilia acquire a high 

 degree of importance : producing the flow of water over the surface of their organs of 

 respiration, indispensable to the exercise of that function ; enabling the animals to 

 seize their prey, or swallow their food, and performing various other offices of 

 greater or less importance in their economy. In man and the warm-blooded 

 animals, their use is apparently to impel secreted fluids or other matters along the 

 ciliated surface, as, for example, the mucus of the wind-pipe and nasal sinuses, which 

 they carry towards the outlet of these cavities. 



Structure. The cells of a ciliated epithelium contain oval nuclei, exhibiting 

 for the most part a distinct intra-nuclear network, and one or more bright 

 nucleoli. Viewed with a moderate magnifying power, their protoplasm looks 

 granular, although the free border of the cell through which the cilia pass 

 presents a clear aspect (fig. 236). The cells have most generally an elongated form, 

 like the particles of the columnar epithelium, which they resemble too in arrange- 

 ment, but they are often of greater length and more pointed at their lower end ; 

 and this is not unfrequently irregularly forked in those parts where a deeper layer 

 of cells exists below the ciliated cells (fig. 237). The cilia are attached to their 

 broad or superficial end, each cell bearing a tuft of these minute hair-like processes. 

 In some cases, the cells are shorter and cubical in figure, and when completely 

 detached may appear spheroidal. 



It has been shown by Engelmann that in large ciliated cells (fig. 238) such as 

 those which line the alimentary canal of some mollusks, e.g., the mussel and oyster, 

 it is possible to make out that the highly refracting free border of the cell to which 

 the cilia are attached is in reality formed of a number of small juxtaposed fusiform 

 or cylindrical knobs (basal Jcnobs). To each of these a cilium is attached on the 

 one side, and from the other end there passes towards the end of the cell a fine, 



1 The Arthropoda offer a singular exception, and it is remarkable that in many of them the spermatozoa 

 are also devoid of a vibratile filament. 



