VERTEBKATED ANIMALS. 277 



ing all the free internal surfaces of the body, and thus lining all its 

 cavities, canals, etc. Save in the mouth and other parts in which it 

 approximates to the ordinary cuticle both in locality and in nature, 

 its cells (Fig. 466) usually form but a single layer; and are so de- 

 ficient in tenacity of mutual adhesion, that they cannot be detached in 

 the form of a continuous membrane. Their shape varies greatly. Some- 

 times they are broad, flat, and scale-like, and their edges approximate 

 closely to each other, so as to form what is termed a ' pavement ' or 

 ' tessellated ' epithelium: such cells are observable on the web of a Frog's 

 foot, or on the tail of a Tadpole; for, though covering an external 

 surface, the soft moist cuticle of these parts has all the characters of an 

 epithelium. In other cases the cells have more of the form of cylinders, 

 standing erect side-by-side; one extremity of each cylinder forming part 

 of the free surface, whilst the other rests upon the membrane to which 

 it serves as a covering. If the cylinders be closely pressed together, 

 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 cylinders 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. 479, 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 in- 

 sensibly into the other. Any of these forms of epithelium 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. 467). Ciliated epithelium is found upon the lining membrane of 

 the air-passages in all air-breathing Vertebrata: 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 secretions. Owing to the very 

 slight attachment that usually exists between the epithelium and the 

 membranous surface whereon it lies, there is usually no difficulty what- 

 ever 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; 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 com- 

 monly be found to contain a great number of ciliated epithelium-cells, 

 which have been thrown-off from the lining membrane of the nasal 

 passages. 



674. Fat. One of the best examples which the bodies of higher 

 animals afford, of a tissue composed of an aggregation of cells, is pre- 

 sented by Fat; the cells of which are distinguished 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 cases they are aggregated in distinct masses, constituting the 

 proper Adipose substance. The individual fat-cells always present a 



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. 



