Page:Popular Science Monthly Volume 18.djvu/825

Rh and immediately a message is sent back to the eyelids to shut, and exclude the particle of matter that threatens to enter the eye. All this is done so quickly that it is hardly possible to realize that there is time for reflex nervous action being brought into play.

Another instance of reflex action, but this time influencing the secretions, may be cited. Who is not familiar with the effect of a savory smell, or the sight of some luxury, upon the salivary secretion, so that, to use a common expression, "the mouth waters"? In the first, the olfactory nerve is the means by which the impression is conveyed to the nerve-center; in the other it is the optic nerve which is the transmitting agent; but in each case the impression is reflected to that nerve controlling the salivary secretion, with the effect of producing an increased flow of saliva. We thus see that the secretions can be influenced by one nerve conveying its impression to another whose filaments take origin in a common center.

Now, to come to the subject more directly under consideration in this paper, we must comprehend how cold acting on one part of the body produces catarrh of the nasal mucous membrane. Exposure to the most intense cold for a lengthened period will not produce this effect. Indeed, we find it invariably the case that severe frost in winter is, so far as catarrh is concerned, the healthiest weather we can have. During the prevalence of frost, as a rule, colds are at a minimum. The system here shows its power of accommodating itself to the circumstances surrounding it, and actually benefits by the prevailing low temperature. Let us, however, suppose a person to be sitting in a room the temperature of which is, say, 70° Fahrenheit, and that a current of cold air is rushing in at an open door or window, and playing upon the back of his head, or it may be on his legs or feet, and the probability is that he will "catch cold," and in nine cases out of ten this cold will be a catarrh in the head, and, what may appear more remarkable still, only one nostril will at first be affected. Now, if the catarrh was due to the inhalation of cold air, both nostrils would suffer; but it is not so; for, as each side of the body is supplied by its distinct set of nerves, so only that side is affected through which the reflex disturbance has been transmitted. The modus operandi is the following: The draught of cold air, acting, we will suppose, on the back of the head, conveys through the sympathetic nerve, which ramifies on the scalp, a shock to the nervous center from which these nerve-fibers proceed; but we must understand that this nerve-center sends its filaments to other portions of the body, and so the shock which this center receives by one set of nerves is reflected by another set to some surface quite remote from that primarily acted upon, and in this way a temporary paralysis of the nerves supplying the blood-vessels of the mucous membrane of the nose is brought about. In consequence, these vessels become dilated and engorged, and the shock which has brought about this congestion continuing, disturbs the