12 ELEMENTARY PHYSIOLOGY less. 



easy to separate that tough membrane which invests the 

 whole body, and is called the skin, or integument, 

 from the parts which lie beneath it. Furthermore, it is 

 readily enough ascertained that this integument consists 

 of two portions : a superficial layer, which is constantly 

 being shed in the form of powder or scales composed of 

 minute particles of homy matter, and is called the 

 epidermis ; and the deeper part, the dermis, which 

 is dense and fibrous (Lesson V.). The epidermis, if 

 wounded, neither gives rise to pain nor bleeds. The 

 dermis, under like circumstances, is very tender, and 

 bleeds freely. A practical distinction is drawn between 

 the two in shaving, in the course of whicli operation the 

 razor ought to cut only epidermic structures ; for if it go 

 a shade deeper, it gives rise to pain and bleeding. 



The skin can be readily enough removed from all parts 

 of the extei-ior, but at the margins of the apertures of the 

 body it seems to stop, and to be replaced by a layer 

 which is much redder, more sensitive, bleeds more readily, 

 and which keeps itself continually moist by giving out a 

 more or less tenacious fiuid, called mucus. Hence, at 

 these apertures, the skin is said to stop, and to be re- 

 placed by mucous membrane, which lines all those 

 interior cavities, such as tlie alimentary canal, into which 

 the apertures open. But, in truth, the skin does not 

 really come to an end at these points, but is dii'ectly con- 

 tinued into the mucous membrane, which last is simply 

 an integument of greater delicacy, but consisting funda- 

 mentidly of the same two layers — a deep, fibious layer, 

 containing blood-vessels, and a superficial bloodless one, 

 now called the epithelium. Thus every part of the 

 body might be said to be contained between the walls of 

 a double bag, formed by the epidermis, which invests the 

 outside of the body, and the epithelium, its continuation, 

 which lines the alimentary canal. 



The dermis, and the deep, vascular layer, which answers 

 to it in the mucous membranes, are chiefly made up of 



