236 THE TISSUES. 



tents; and on the skin the outer strata of cells become dried and 

 collapsed into solid horny scales. In the first instance, the epithe- 

 lium is termed a conoidal (or cylinder), and in the latter case, a 

 scaly epithelium. Between the conoidal and the flattened cell, many 

 varieties are found; the globular and the polyhedral form predo- 

 minating. 



Again, the conoidal cells are sometimes found surmounted by 

 cilia, 1 so called; in which case we have a ciliated epithelium. 



The conoidal (or cylinder) and the ciliated epithelium are found 

 only on the mucous membrane, in the adult human body ; the scaly 

 variety exists everywhere on the skin and serous membranes, and 

 also on certain parts of the mucous membranes, hereafter to be 

 specified. 



The granules are more numerous in epithelial cells in proportion 

 as the latter are smaller and younger, and in these also the circular 

 or oval nuclei are more apparent. Acetic acid renders them very 

 distinct. Indeed, the granular appearance and the distinctness of 

 the nuclei seem to measure the functional activity of the cells, and 

 when both disappear the cells become detached or desquamated. 



The size of epithelial cells varies according to their form, and 

 also in different parts of the body, and in the different layers in 

 the same part. The conoidal cells of the epithelium of the small in- 

 testine are yy 1 ^ to ^^ of an inch in length, and 3^5 u to ^EQ-Q of 

 an inch broad. The cells in the most superficial layer of the 

 conoidal epithelium of the larynx are ^J^ to F ^ of an inch long, 

 and 45^ to 2^-Q f an i nc h broad. In the very lowest layers of 

 the epithelium of the mouth, when the cells are arranged nearly 

 perpendicular to the basement-membrane, they are T5 Vo to TTOO f 

 an inch long; in the middle layers they are -^^ -$ to zJ-g-Q of an inch 

 broad, having become somewhat flattened ; while the most super- 

 ficial cells are large flattened plates, called epithelial plates, by Kol- 

 liker ^^ to even 3 J^ of an inch across. (Fig. 147.) 



The thickness of the epithelium must depend on the size of the 

 cells and the number of layers. The small intestine, having but a 

 single layer, will have an epithelium yy 1 ^ to ^| 5 of an inch thick, 

 as has been shown. The compound epithelium of the mouth is 

 ri2 to 5 J e f an i nc k thick; and that of the larynx is 4^ to 2 |o of 

 an inch thick. 



1 From " Cilium," an eyelash ; since they resemble fine hairs. 



