606 



VERTEBRATED ANIMALS. 



FiQ. 320. 



Piyirieiil-cells from 

 tail of Tadpole ; — a. a, 

 simple forms of recent 

 origin ; 6, 6, more com- 

 plex forms subseqiiem- 

 iy assumed. 



when it is subjected to a magnifying power of 200 or 300 diame- 

 ters, by light transmitted through it, with this 

 surface uppermost ; and if the epidermis be that 

 of a negro or any other dark-skinned race, the 

 pigment-cells will be very distinctly seen. This 

 under-surface of the epidermis is not flat, but is 

 excavated into pits and channels for the recep- 

 tion of the papillary elevations of the true skin ; 

 an arrangement which is shown on a large scale 

 in the thick cuticular covering of the Dog's 

 foot, the subjacent papillae being large enough 

 to be distinctly seen (when injected) with the 

 naked eye. The cellular nature of the newly- 

 formed layers is best seen, by examining a little 

 of the soft film that is found upon the surface of 

 the true skin, after the more consistent layers of 

 the cuticle have been raised by a blister. The 

 alteration which the cells of the external layers 

 have undergone, tends to obscure their character; 

 but if any fragment of epidermis be macerated 

 for a little time in a weak solution of soda or 

 potass, its dry scales become softened, and are 

 filled out by imbibition into rounded or polygo- 

 nal cells. The same mode of treatment enables 

 us to make out the cellular structure in warts and corns, which 

 are epidermic growths from the surface of papilla enlarged by 

 hypertrophy. 



421. The Epithelium may be designated as a delicate cuticle, 

 covering all the free internal surfaces of the body, and thus lining 

 all its cavities, canals, &c. Save in the mouth and other parts in 

 which it approximates to the ordinary cuticle both in locality 

 and in nature, its cells usually form but a single layer ; and are 

 so deficient in tenacity of mutual adhesion, that they cannot be 

 detached in the form of a continuous membrane. Their shape 

 varies greatly; for sometimes they are broad, fiat, 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 

 the 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, stand- 

 ing 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 clgsely 

 pressed together, their form is changed into prisms ; and such 

 epithelium is often known as "prismatic." On the other hand, 

 if the sui'face on which it rests be convex, the bases or lower ends 

 of the cjdinders become smaller than their free extremities ; and 

 thus each has the form of a truncated cone rather than of a cylin- 



