Not only is most of the air breathed in through the nose and warmed in its passage through, but the nose is the organ of smell and by means of the peculiar property of its nerves protects the lungs against deleterious gases and helps the taste discriminate. The _olfactory_ or _first cranial nerves_, after emerging from the brain, lie on the under surface of the frontal lobe and rest on the ethmoid bone in what is known as the _olfactory tract_. Each nerve ends in a bulb-like termination called an _olfactory bulb_, which rests on the cribriform plate and sends little terminal fibers down through to be distributed to the nasal cavities, especially to the upper half of the septum of the nose, the roof of the nose, and the anterior and middle turbinated bones. For in the mucous membrane of the upper nasal cavity are specially modified epithelial cells called _olfactory cells_, which play an important part in the conduction of smell. Hence when one wishes to smell anything especially well he sniffs it up. Probably the _sensation of smell_ is caused by odoriferous particles in the atmosphere being breathed into the nose, where they affect the olfactory cells, which transmit the impulses to the olfactory nerve and so to the brain. Whereas a certain amount of moisture in the nasal cavity seems to be essential for accuracy of smell, the presence of too much or too little interferes with it. The mucous membrane has a certain power also of distinguishing different smells at the same time, though this power varies greatly in different people, one smell often wholly overpowering all others. The cartilage below the bridge of the nose is sometimes attacked in _syphilis_ and _cancer_, and _lupus_ often begins on the nose. _Deviation of the septum_ may occlude all air from one side of the nose, an effect also produced by _polypi_, generally of the turbinated bone. Either condition is easily remedied. _Nosebleed_, though generally unimportant, may be serious in adults. =The Mouth.=--The mouth is of great importance as an entrance for fresh air to the lungs when the nasal passages are for any reason impeded and as the resonant chamber from which proceeds the voice, man’s chief means of communication with his fellows. Its chief value may be said, however, to reside in the fact that it is the vestibule of the alimentary canal. It is an ovoid cavity lined with mucous membrane and is bounded in front by the _lips_, at the sides by the _cheeks_, below by the floor and tongue, and above by the _hard palate_ anteriorly and by the _soft palate_ posteriorly, the _uvula_ depending from the latter like a curtain between the mouth and the pharynx. Shape is given to the mouth by the bones of the upper and lower jaw and its size is altered by the lowering and raising of the latter, which is quite freely movable. [Illustration: FIG. 19.--The hyoid bone. (Toldt.)]