376 
POPULAR SCIENCE REVIEW. 
therefore, depend on variations in the radiating and conducting 
power of the skin itself. The radiation may be left out of the 
question, because, though the differences of colour in the surface 
which depend on its congestion or its anaemic condition will 
have some influence, that must be quite insignificant in com- 
parison with other forces which come into play. A box, whether 
of metal or porcelain, whether black or white, whether dull or 
burnished, no doubt radiates differently, but none of the possible 
changes of the healthy skin can in any way compare in im- 
portance with any of these differences. We must therefore look 
to modifications of the conducting power of the surface for the 
mechanism by which the uniformity is arrived at, and in doing 
so it will be necessary first to take a glance at the structure of 
the skin itself. 
The skin is composed of two main layers, a superficial non- 
vascular covering — the epidermis — which protects the parts 
beneath, in the same way that lacquer does the brass-work of 
a microscope. This epidermis is nearly white, and translucent 
to a certain extent ; it is the layer most of which is raised from 
the surface when a blister 44 draws.” The second deeper stratum 
is the true skin, or corium. It consists of connective tissue 
and fat, which support innumerable minute branches of vessels 
and capillaries. Nerves and sweat-glands abound, varying in 
number in different parts. The small vessels, the arteries and 
the veins, unlike the capillaries which connect them, are pro- 
tected by walls which contain muscular fibres ; in fact they — 
especially the former — are little more than muscular tubes, 
which are capable of varying in diameter and length according 
to the extent of contraction of their coats. The muscular 
arteries are supplies with nerves, which are in direct communi- 
cation with ganglia, or miniature centres of nervous action, and 
are not capable of being influenced by the will. When the 
circular or transverse muscular fibres of the small arteries come 
into action, by reducing the calibre of the vessels they lessen 
the flow of blood to the capillaries ; the skin becomes less red ; 
and quite pale when the contraction is extreme, as in fright. 
When the small arteries relax, blood freely enters the skin, the 
crimson of the blush being the result. The calibre of these 
small cutaneous vessels is entirely regulated by the currents 
traversing the nerves which communicate with them, and it is 
by modifications in it that the conducting power of the skin is 
capable of varying to the extent required to maintain a uniform 
body temperature. 
To take an example of the manner in which this is proved 
to be the case : — The air being close upon 70°, the clothing 
being ordinary summer dress, the temperature in the mouth is 
observed to be 9fi°T 5 at 10T5. The following consecutive series 
