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