TISSUES, ORGANS, AND SYSTEMS. 75 



which abound in vessels and nerves, and by the activity of its elements 

 to take part in secretion and absorption. All epidermic tissues are non- 

 vascular, and support themselves from a plasma which is yielded by the 

 deeper-seated vessels. They are very easily regenerated when their 

 superficial portions are removed, and in this case they grow chiefly by 

 the development of new elements in the deeper layers ; even when wholly 

 lost they are readily reproduced. 



The epidermic tissue takes the following forms : 



1. Corneous tissue. This always consists of compact masses of cells, 

 which are soft in the neighborhood of their vascular basis, but at a 

 greater distance become more or less solid and hard (corneous), and fre- 

 quently lose their originally vesicular constitution and nucleus, and 

 become the so-called horny scales. The following organs are formed by 

 this tissue : 



a. The Epidermis ; which invests the exterior of the body, and is 

 continuous at the great apertures of the inter- Fi-.i2. 



nal cavities with the epithelium. It consists of 

 two tolerably distinct layers ; the mucous layer 

 (rete mucosum), with soft, rounded polygonal 

 cells, which, under certain circumstances, con- 

 tain coloring matter. This layer applies itself 

 accurately to all the inequalities of the corium 

 (which nourishes the epidermis), and externally 

 passes into the polygonal scales of the horny 

 layer. 



b. The Nails. These may be regarded as a modification of the epi- 

 dermis, whose horny layer has attained a still greater density; and, 

 with its rete mucosum, lies upon a special depressed surface of the cutis, 

 the bed of the nail ; and is partly sunk in a peculiar cleft, the fold 

 of the nail. 



c. The Hairs. Filiform epidermic* structures, seated upon a vas- 

 cular papilla, in a peculiar sac, the hair sac, which is a process of the 

 corium, and is lined by a continuation of the epidermis. The structural 

 elements in the region of the papilla are soft and vesicular ; the more 

 distant are metamorphosed into three kinds of cells plates, flat fibres, 

 and more or less rounded irregular cells. 



2. Epithelium. Soft nucleated cells, nowhere densely corneous ; 

 rounded, polygonal, fusiform, cylindrical or conical in shape ; sometimes 

 possessing cilia, sometimes not, arid occurring in one or many layers. 

 Hence we have the following forms : 



FIG. 12. Plates of the horny layer in man, magnified 350 diameters. 1, without addition, 

 viewed from the surface, one, with a nucleus : 2, from the side. 



* [It is to be questioned if the hairs are truly epidermic structures, vide infra, Hair. TRS.] 



