— 
e 
46. SKIN AND MUCOUS MEMBRANES. 
membranous surfaces of the body, and which form the 
Epidermis, or superficial layer of the skin, and the Epithelium — 
of the internal membranes. And it will be convenient here 
to consider the entire structure of the Skin, the Mucous 
Membranes, and the Serous Membranes, which are complex 
fabrics, chiefly made up of the elementary tissues already 
described. These membranes may each be considered as 
composed of three principal parts, namely, the superficial 
layer or layers of cells, the basement-membrane whereon the 
cells lie, and the subjacent texture covered by this, which — 
consists of fibrous tissue compactly interwoven and traversed — 
by blood-vessels, nerves, absorbents, and also. containing” 
glands of various kinds. The Skin and Mucous Membrane 
may, in fact, be regarded as belonging to one and the same 
type ; for they are continuous with each other wherever one 
of the open cavities of the body communicates with the 
surface, as at the mouth, nostrils, and anus; and in the 
Hydra (§ 121) it has been experimentally found that the 
membranous layer covering the body may be made to change 
places with that which lines the stomach, without any sensible 
disturbance in the functions of either. The difference between 
the two essentially consists in this; that the Skin, being 
destined especially for the reception of sensations, and for the 
protection of the soft parts beneath, is more copiously furnished 
with nerves than with blood-vessels, and has its surface © 
covered by a firm, dry cuticle ; whilst the Mucous Membrane, © 
ministering especially to the organic functions, is comparatively 
little supplied with nerves, but is abundantly furnished with 
blood-vessels, and in certain parts with absorbents, whilst its 
cellular layer is soft and easily permeable by liquids. Both 
in the skin and in mucous membrane we find a multitude of 
minute glands, for the separation of particular fluids from the 
blood ; the nature of these differs with the locality. | 
37. The fibrous mesh-work of the Cutis or Z'rue-Skin is cons) 
tinuous with that of the Areolar tissue which lies immediately 
beneath it; so that the two textures are not separated one | 
from the other by any definite boundary (as the examination. 
of a vertical section (fig. 7) clearly proves), but are dis- 
tinguishable only by the compactness of the one, as contrasted. 
with the looseness of the other. The outer surface of the 
Cutis usually presents numerous minute elevations or papille 
