296 KERATIN-PRODUCING EPITHELIAL TISSUES. [BOOK I. 



different the arrangement of the cells and the physical characters of 

 the tissue which they compose, the main product which characterizes 

 them is an undefined horny substance to which the term Keratin 

 has been applied. 



The epithelium, on the other hand, which covers the mucous 

 membranes and lines the interior of secreting glands, is composed of 

 cells, whose protoplasm is the seat of the most active and remarkable 

 chemical operations, tending to separate, from the blood, constituents 

 of which it has no longer need, or to build up, at the expense of 

 certain of those constituents, new bodies which are to serve important 

 functions in the organism. 



This epithelium is mainly derived from the hypoblast, though 

 in some cases it takes its origin in the epiblast (epithelium of mouth 

 and salivary glands), in others from the mesoblast (certain portions of 

 the epithelium of genito-urinary tract). In short, the epithelium of 

 the mucous membranes is possessed of diverse chemical attributes 

 and is developed in several ways; it does not therefore possess any 

 common characters which permit of a general description. 



We shall, therefore, in this chapter, confine ourselves, in the first 

 place, to a consideration of the chemistry of the keratin-forming 

 epiblastic tissues, postponing the exposition of the chemistry of other 

 epithelial tissues to future sections of this work, where they will be 

 treated of in relation to the organs in which they occur, and the 

 functions which they specially subserve. 



SECT. 1. EPIBLASTIC KERATIN-PRODUCING EPITHELIAL TISSUES. 

 THE HORNY SUBSTANCE OF CUTICLE, NAILS, HORN, HAIR, AND 

 FEATHERS. 



The cuticle or epidermis is composed of many layers 

 Structure of ~ .,, ,. , s . , T ,1 i . J i 



Epidermis. * epithelial cells which overlie the derma or true skin. 



The cells of the most superficial layers present the 

 appearance of distorted, shapeless, hardened scales; those of the 

 deeper layer are more or less spheroidal, soft, and present a well- 

 marked nucleus. 



The most superficial cells, which are horny throughout, are 

 unacted upon by acetic acid ; this reagent renders the deeper cells 

 more transparent and their nucleus more evident. 



The cells are connected together by a cementing substance which, 

 like the cementing substance of the connective tissue, is dissolved by 

 alkalies. In order to dissolve this connecting substance and effect 

 the dissociation of the epidermal cells, maceration in cold, or even 

 warm, solutions of caustic potash or soda should be had recourse to. 

 These reagents not only dissociate the cells but, in the case of the 

 more superficial cells, render their structure more evident. 



The caustic alkali, at first, leaves the nucleus and the cell sub- 

 stance intact, merely causing the latter to swell and become more 



