In finishing this topic, the subject of catching cold must be brought up, together with its relation to the regulation of body temperature. There is no doubt at the present time that common colds result from infection; in that sense they are acute diseases. It seems to be the fact, however, that the organisms which cause them are pretty constantly present in the air passages of our throats and lungs ready to invade the body whenever opportunity offers. Very recently it has been shown that rapid chilling of the skin is accompanied by contraction of the blood vessels in the linings of the throat and air passages. The effect of thus cutting down the blood supply to these linings seems to be to make them more ready of access by the organisms of colds. At any rate the fact that when the skin is suddenly chilled a cold frequently develops is best explained on this basis. Since colds are infectious, the organisms causing them may sometimes succeed in penetrating the linings of the air passages even when no change in the latter has occurred. It may therefore happen, and frequently does, that one develops a cold without being able to refer it to any time when there was a sudden chilling of the skin. Also it can easily happen that the chilling may not lead to a cold, since if the organisms do not happen to be abundant or if the general bodily resistance is high, the infection will not get a foothold, even though the linings of the air passages would permit the organisms to enter. One’s best precaution against colds would naturally be to avoid having the organisms which produce them present in the throat and lungs. Since this usually cannot be done, the remaining practical measures are to keep the general bodily health as good as possible and to avoid conditions which lead to a sudden rapid chilling, such as sitting in a cold draft when heated. CHAPTER XX THE PERPETUATION OF THE RACE