Popular Science Monthly/Volume 18/April 1881/What Is a Cold?
|WHAT IS A COLD?|
To enjoy life, one must be in good health; and to remain free from disease is the desire of all. Yet there are some ailments which do not interfere very much with the pleasures of life, and therefore are not dreaded in consequence—nay, more, they are frequently treated with neglect, although in many instances they are the precursors of more serious disorders, which may in not a few cases have a fatal termination! How often, to the usual greetings which one friend exchanges with another, is the reply given, "Very well, thank you, except a little cold." A little cold, and yet how significant this may be! In how many cases do we find a "little cold" resemble a little seed, which may sooner or later develop into a mighty tree! A little cold neglected may, and frequently does, prove itself to be a thing not to be trifled with. Let me, then, pray my readers to remember that small beginnings in not a few instances have big endings, and this especially where disease exists. Let us, then, consider what is a common cold.
In the first place, we must be paradoxical, and affirm that it is not a cold at all. It is rather a heat, if I may so express myself, that is, it is a form of fever, but, of course, of a very mild type when it is uncomplicated by other diseases. It is certainly in the majority of instances due to the effects of cold playing upon some portion of the body, and reacting upon the mucous membrane through the intervention of the nervous apparatus. What is called a cold, then, is in reality a fever; and, though in the majority of instances it is of such a trivial nature as to necessitate few precautions being taken during its attack, yet in some cases it runs a most acute course, and may be followed by great prostration. Even when the premonitory symptoms of a cold are developing themselves, when, for example, what a medical man calls a rigor, or, as it is popularly designated, a shivering is felt, when we would naturally suppose that the animal temperature is below par, it is at that very moment higher than the normal, thus showing the onset of fever.
Before going at once into the symptoms and nature of the disease under discussion, it will be advisable to dip a little into that most interesting department of medical science, physiology, and, indeed, without doing so, it would be quite impossible for the majority of my readers to understand the manner in which cold acts in producing the inflammatory condition of the mucous membrane of the nose, or, as it is called, the Schneiderian membrane, which inflamed condition constitutes a cold in the head. It will be necessary to understand what a mucous membrane is, what its duties are, and how these duties are performed, before entering upon a description of a disease attacking it. To take the mucous membrane of the nose as an example. We find that it is a membrane spread out over a very large area, lining as it does a great many undulations caused by the arrangement of the bones composing the walls of the nostrils, so that a very much greater surface is required to be traversed by the air entering the lungs through the nose—the natural passage—than is required by the actual length of the canal. The object of this is obvious, when we take into account the fact that the temperature of the air is usually either below or above that of the human body, and that it is almost invariably loaded with particles of matter which would irritate the lungs did they find access to them.
The tortuous passage of the nose thus tends in the first place to equalize in some measure the temperature of the atmosphere inhaled with that of the lungs; and, in the second place, the mucus which is secreted by the Schneiderian membrane, being of a tenacious nature. tends to attract and ensnare the impurities which the air may contain. We thus see that the nostrils act as a filter to the air taken in by inhalation. If we observe any mucous surface, we can not help remarking its deep-red color, this being due to the close network of blood-vessels ramifying on its surface. In consequence of this accumulation of minute arteries and veins through which warm blood is constantly flowing, a pretty high temperature is constantly maintained in any cavity lined by mucous membrane. There is, therefore, little difficulty in understanding, how important a part the nostrils play in preparing the air for its entrance into the sensitive structure of the lungs. But the nostrils do not only temper the air—they also yield to it an amount of moisture which renders it still more bland and less irritating. We see, then, that the functions of the nostrils as regards the atmosphere inhaled are threefold: 1. In equalizing its temperature; 2. In moistening; and, 3. In filtering it. The latter function is materially aided by quite a forest of minute hairs which guard the entrance to the passages.
Having noticed how distended the blood-vessels of the mucous membrane naturally are, it will not be difficult to understand how slight a disturbance of the balance of blood-supply will be necessary to produce congestion or inflammation of the structure, and such is really the case; and it is because of this that people who have what is called an irritable mucous membrane are so susceptible of cold. They have, in fact, a chronically congested mucous membrane, which, however, is usually associated with and dependent upon a disordered digestion. Yet, notwithstanding these facts, a cold is not produced by cold air acting upon the surface which suffers. It is quite true that there are individuals with peculiar idiosyncrasies who take catarrh when they smell certain substances. For instance, many can not go into a room where powdered ipecac is exposed without immediately catching catarrh in the nasal passages; and there is reported the case of a man who could not smell a rose without being affected in a similar way.
We must now go a step further before we can understand the modus operandi by which a cold in the head, or any other region, is produced. It has been shown that one of the functions of a mucous membrane is to secrete mucus. But what is it that makes the secretion vary in quantity? Well, an irritant applied directly to the surface may produce an excessive flow, and this superabundance of mucus is thrown out by an effort of nature in its endeavor to shield the delicate membrane and remove the irritant; this may happen also when there is an excessive amount of blood in the vessels, which is the case when congestion exists, the distention of the blood-vessels acting as an irritant, and supplying in greater amount the fluid from which the mucus is extracted, thus tending to excite the secreting power to greater effort. Thus we have an explanation of the excessive discharge in catarrh of the nose. But, when the direct irritant is removed, the 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, 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, 70º 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 equilibrium of the blood-supply, and so an inflammatory condition is set up. When this exists the blood-vessels are enormously distended; consequently, an excess of blood passes through the part, the little cells which secrete the mucus being thus excited and working much more rapidly than when in health. In this way the enormous discharge of mucus, which accompanies a cold in the head, is accounted for.
Another effect of this irritation of the mucous membrane is sneezing, which is an effort of Nature to restore the equilibrium of the nervous center by another kind of reflex action. Sneezing in catarrh is a method Nature adopts to stimulate the prostrate nervous center, and thus enable it to reassert its proper control over the blood-supply to the part; indeed, it will be found that the effects of being exposed to a draught of cold air are often completely destroyed by a succession of sneezes. Of course. Nature does not always immediately succeed in these efforts; but, when she does not, the shock from which the nervous center suffers gradually passes away, and the blood-vessels again come under the control of the little nerves which regulate their caliber, and so the catarrh disappears in a few hours, or at most in a few days. It sometimes happens that the shock from the cold air acting upon the nervous center is of such severity that the consequent inflammation is intense enough to check the secretion of mucus altogether, and in consequence the mucous membrane is dry as well as inflamed, and the suffering very much intensified.
So far, we have only glanced at a cold in the head which passes away in a few hours, but this is not always the happy termination. There is a peculiar tendency which inflammation possesses of not leaving off where it commenced, but of invading the tissues in its immediate neighborhood, and more especially when the tissue is continuous with that primarily attacked, as is the case with the mucous membrane of the air-passages. A cold may commence in the head, and rapidly spread by what is technically termed continuity of tissue into the chest; and so what at the first promised to be only cold in the head may terminate in an attack of bronchitis, or even inflammation of the lungs.—Chambers's Journal.