CHAPTER IV.
COLD, AND THE HARM IT DOES.
FROM what has already been said on the subject of animal heat it has become evident that any undue abstraction of it from the body must be followed by evil consequences.
Cold is simply the absence of heat, and when the body is surrounded by substances at a low temperature, heat is abstracted from its surface, and a certain painful and familiar sensation is imparted through the nerves of the skin.
The action of cold on the surface of the body is very complex. In the first place it prevents the flow of blood to the skin by causing contraction of the muscular fibre, which lessens the calibre of the small arteries of the skin, and so impedes the flow in the capillaries. It also produces reflex nervous action, which causes the little arteries to contract still more violently, until the circulation in the skin of the part is stopped entirely. That is what happens when on a very cold day your finger is, as you call it, "dead," and it is the first stage of frost-bite in which the part actually dies and comes away. Now when the blood does not circulate near the skin, the action of the skin as an excretory organ
