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Page:Popular Science Monthly Volume 18.djvu/824

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unnaturally abundant discharge ceases. Not so, however, when the superabundance is due to the effects of cold; for, in the latter case, a diseased condition is set up, which will only disappear when the effects of the exposure upon the nervous system have passed away.

Having demonstrated that cold is not produced by the action of cold air playing upon the part affected, but that, on the contrary, it is an effect of cold acting upon a distant part of the body, it will be necessary to explain how this is brought about. If a person sits in a draught of cold air, and this draught is directed upon the back of his head, the chances are that a catarrh of the nasal passages will result, and this is produced by what is called reflex action of the nerves. Here it will be necessary to diverge a little, and explain what reflex action is. It must be understood, then, that there are numerous nervous centers connected with the spinal cord. These nervous centers send filaments of their nerves to various portions of the body. For example, a nerve-center may be placed alongside the spine in the neck, and from this point nerves may be distributed to the back of the head and the mucous membrane of the nose. One important function of these little bodies is to control the supply of blood to different surfaces and tissues and organs. This is done by a system of minute nerves which are distributed on the arteries, by which the vessels are kept in a state of contraction. Now, if these nerves are severed from the main trunk, the blood-vessels immediately expand to the full extent of their caliber, and congestion is the result; or, if these nerves are paralyzed, the same effect is produced. Sometimes a very slight shock produces a temporary paralysis of these minute nerves when a rush of blood takes place into the arteries, of which blushing is a good example; but the nerves soon recover their control over the blood-supply, and the blush passes away. Then, again, the shock may produce quite the opposite effect: this may be so severe as to cause such extreme contraction of the blood-vessels that a deadly pallor pervades the face, as, for instance, in severe shock from fear. This, however, is caused more by the effect of shock acting upon the nerve-centers which supply the heart with motor power.

But let us suppose that one extremity of a nerve arising from a particular nerve-center is irritated; this is communicated to that center, which is affected thereby, it may be slightly or more severely. The irritation may be so great as to prostrate for the time being the nerve-center, and, in consequence, all the nerves arising from it are thrown into a state of inaction. This is called the reflex action of that nerve-center, because the effects of the irritant applied to one part of the body are thereby reflected to other parts. Instances of reflex action may be seen frequently in every-day life. Take, for example, the action of the eyelid when an object threatens to enter the eye. The retina perceives the object advancing; this is telegraphed to the nervous center supplying the muscles which open and shut the eyelids,