EPITHELIUM-CELLS I — CILIA. 



21 



Fig. 382. 



ther, their form is changed into prisms ; and such Epithelium is 

 often known as 'prismatic' On the other hand, if the surface 

 on which it rests be convex, the bases or lower ends of the cylin- 

 ders become smaller than their free extremities ; and thus each 

 has the form of a truncated cone rather than of a cylinder, and 

 such Epithelium (of which that covering the villi of the Intestine, 

 Fig. 394, is a peculiarly -good example) is termed ' conical.' But 

 between these primary forms of Epithelial cells, there are several 

 intermediate gradations ; and one often passes almost insensibly 

 into the other. — Any of these forms of Epithelium may be fur- 

 nished with Cilia; but these appendages are more commonly found 

 attached to the elongated, than to the flattened forms of epithe- 

 lium-cells (Fig. 382). Ciliated Epithelium is found upon the 

 lining membrane of the 

 Air-passages in all air- 

 breathing Vertebra ta ; and 

 it also presents itself in 

 many other situations, 

 in which a propulsive 

 power is needed to pre- 

 vent an accumulation of 

 mucous or other secre- 

 tions. Owing to the very 

 slight attachment that 

 usually exists between the 

 Epithelium and the Mem- 

 branous surface whereon it lies, there is usually no difficulty whatever 

 in examining it; nothing more being necessary than to scrape the 

 surface of the membrane with a knife, and to add a little water to 

 what has been thus removed. The Ciliary action will generally be 

 found to persist for some hours or even days after death, if the 

 animal has been previously in full vigour ; * and the cells that bear 

 the cilia, when detached from each other, will swim freely about 

 in water. If the thin fluid that is copiously discharged from the 

 nose in the first stage of an ordinary ' cold in the head/ be sub- 

 jected to microscopic examination, it will commonly be found to 

 contain a great number of Ciliated Epithelium-cells which have 

 been thrown-off from the lining membrane of the nasal passages. 



564. Fat. — One of the best examples which the bodies of higher 

 animals afford, of a tissue composed of an aggregation of Cells, is 

 presented by the Adipose substance ; the cells of which are dis- 

 tinguished by their power of drawing into themselves Oleaginous 

 matter from the blood. Fat-cells are sometimes dispersed in the 

 interspaces of Areolar tissue ; whilst in other eases they are aggre- 



Ciliated Epithelium; a, nucleated cells, 

 resting on their smaller extremities ; b t 

 cilia. 



* Thus it has been observed in the lining- of the windpipe of a de- 

 capitated criminal, as much as seven days after death ; and in that of the 

 River Tortoise it has been seen fifteen days after death, even though 

 putrefaction had already far advanced. 



8 A 



