As a matter of fact, there is exceedingly little evidence to show that pure, fresh, open air at any reasonable temperature and humidity ever did harm when inhaled directly into the lungs. In fact, a considerable proportion of us, when swinging along at a lively gait on the country roads, or playing tennis or football, or engaged in any form of active sport, will be found to keep our lips parted and to inhale from a sixth to a third of our breath in this way, and with no injurious results whatever. Nine-tenths of all the maladies believed to be due to breathing even the coldest and rawest of air are now known to be due to invading germs.
Nevertheless, mouth-breathing in all ages has been regarded as a bad habit, and with good reason. It was only about thirty years ago that we began to find out why. A Danish throat surgeon, William Meyer, whose death occurred only a few months ago, discovered, in studying a number of children who were affected with mouth-breathing, that in all of them were present in the roof of the throat curious spongy growths, which blocked up the posterior opening of the nostrils. As this mass was made up of a number of smaller lobules, and the tissue appeared to be like that of a lymphatic gland, or "kernel," the name "adenoids" (gland-like) was given to them. Later they were termed _post-nasal growths_, from the fact that they lay just behind the rear opening of the nostrils; and these two names are used interchangeably. Our knowledge has spread and broadened from this starting-point, until we now know that adenoids are the chief, yes, almost the sole primary cause, not merely of mouth-breathing, but of at least two-thirds of the injurious effects which have been attributed to this habit.
Mouth-breathing is not simply a bad habit, a careless trick on the part of the child. We have come to realize that physical bad habits, as well as many mental and moral ones, have a definite physical cause, and that _no child ever becomes a mouth-breather as long as he can breathe comfortably through his nose_.
This clears the ground at once of a considerable amount of useless lumber in the shape of advice to train the child to keep his mouth shut. I have even known mothers who were in the habit of going around after their helpless offspring were asleep and gently but firmly pushing up the little jaw and pressing the lips together until some sort of an attempt at respiration was made through the nostrils. Advertisements still appear of sling-like apparatuses for holding the jaws closed during sleep.