The three senses that give us information of what is happening beyond the surface of our bodies are smell, hearing, and sight. Since smell is closely related to taste, which was talked about in the last chapter, we shall take it up first. Smell is like taste in that it is aroused by chemical substances, but to be smelled these must be in gaseous form, not dissolved in water, as for taste. The organ for smell is in the upper part of the nasal chamber. There are really two of them, one in each nostril. They are made up simply of little patches of mucous membrane, as the membrane that lines the nose is called, in which are many of the particular kind of cells that are affected by odors. An interesting thing about these patches is that they are not in the part of the nostrils through which the main current of air sweeps in breathing, but in a little pocket off this main channel. If air containing an odorous substance is breathed in or out, a little of it works its way into the side pocket and is smelled. If we wish to get more of the odor we do it by sniffing, which is changing the shape of the nostrils to throw the air current more directly against the smell organs. These organs are amazingly sensitive. It is hard to appreciate the minuteness of the amounts of material that can be smelled. Especially is this true of those animals that have a really keen sense of [Illustration: Photo, Fifth Avenue Hospital A NEW HOSPITAL DESIGNED TO SUPPLY THE MAXIMUM OF LIGHT AND AIR--CHIEF REQUISITES FOR HEALTH--TO EACH ROOM] [Illustration: Copyright, Fifth Avenue Hospital THE ARCHITECT’S PLAN WAS MADE WITH THE NEED FOR LIGHT AND AIR IN MIND] smell. The amount of substance that rubs off from a rabbit’s feet onto the ground at each step cannot be much to begin with; yet this continues for hours to give off gas into the air, and a dog coming along at any time meanwhile will get enough of the gas into his nostrils to smell it. Fortunately for our comfort the sense of smell fatigues very rapidly. An odor that is excessively disagreeable at first presently no longer troubles us. If it were not for this, it would be almost, if not quite, impossible to obtain laborers in those industries where the odor is necessarily bad. There is, however, a source of danger in this quick subsidence of smell perception. About our only method of judging offhand as to the ventilation of a room is by the smell, and this fails as a guide when we have been in the room for a time. Persons coming in from outside are often struck by the bad state of the air in rooms whose occupants are not conscious that there is anything amiss. Because of this the ventilation of schoolrooms usually is not, and never should be, left merely to the judgment of the teacher, but definite rules are laid down as to opening of ventilators or windows.