WHAT IS A COLD P 803 



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 re- 

 marking its deep-red color, this being due to the close network of 

 blood-vessels ramifying on its surface. In consequence of this accu- 

 mulation 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 mate- 

 rially 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, how- 

 ever, is usually associated with and dependent upon a disordered diges- 

 tion. 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 ca- 

 tarrh 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 irri- 

 tant, 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 



