EPITHELIUM FAT-CELLS. 607 



der, and such epithelium (of which that covering the villi of the 

 intestine, Fig. 328, 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 Epi- 

 thelium may be furnished with cilia; but these appendages are 

 more commonly found attached to the elongated, than to the 

 flattened forms of epithelium- cells (Fig. 321). "Ciliated epi- 

 thelium" is found upon the lining 

 membrane of the air-passages in all FlG - 321 - 



air-breathing Vertebrata ; and it also 

 presents itself in many other situa- 

 tions, in which a propulsive power 

 is needed to prevent an accumula- 

 tion of mucous or other secretions. 



Owing to the very slight attachment Cil{ated Epithelium; a, nucleated 



.- -,-. . ~ i *- ,1 . resting on their smaller extremities; 



that usually exists between the epi- 6j cilia< 

 thelium and the membranous sur- 



face 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 vigor j 1 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 subjected to microscopic examination, it will commonly be 

 found to contain a great number of ciliated epithelium-cells that 

 have been thrown off from the lining membrane of the nasal 

 passages. 



422. 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 tissue ; the cells of which are 

 distinguished by their power of drawing into themselves oleagi- 

 nous matter from the blood. Fateells are sometimes dispersed 

 in the interspaces of Areolar tissue ; whilst in other cases they 

 are aggregated in distinct masses, constituting the proper Adi- 

 pose tissue. The individual fat-cells always present a nearly 

 spherical or spheroidal form ; sometimes, however, when they 

 are closely pressed together, they become somewhat polyhedral, 

 from the flattening of their walls against each other (Fig. 322). 

 Their intervals are traversed by a minute network of blood- 

 vessels, from which they derive their secretion ; and it is proba- 

 bly by the constant moistening of their walls with a watery fluid, 

 that their contents are retained without the least transudation, 



1 Thus it has been observed in the lining of the windpipe of a decapitated 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. 



