Sometimes the lower jaw is dislocated and when once this has occurred it is liable to occur again, the ligaments becoming stretched.
CHAPTER IV.
THE ORGANS OF SPECIAL SENSE.
The Nose.—The nose, the organ of the sense of smell, is composed of a framework of bones and cartilages, the bridge being formed by the two nasal bones, and the septum by the vomer and the triangular cartilage. It consists of two parts, the external nose and the internal or nasal fossæ, which open to the face by the anterior nares or nostrils and into the pharynx by the posterior nares. Externally it is covered with skin, internally with ciliated mucous membrane. The fossæ have the inferior turbinated bones along their outer walls and are divided into three parts known as the [superior, the middle, and the inferior meatus], the middle one connecting with the antrum of Highmore, while into the inferior meatus the lachrymal canal empties. There are many small muscles of which little use is made, although in forced respiration, as in pneumonia, where every aid to breathing is called into play, even the alæ nasi or nostrils are made to exert what muscular power they possess in order to supply more air.
Fig. 18.—The nasal cavity.
(After Sobotta.)
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.