The Sense of Hearing.
The nerves of the sense of hearing terminate in the inner part of the ear. The tympanum, or "ear drum," receives the sound vibrations entering the cavities of the ear, and, intensifying and adapting them, it passes them on to the ends of the auditory nerve in the internal ear, which conveys the sensation to the brain. The auditory nerve reports to the brain the degrees of pitch, intensity, quality, and harmony, respectively, of the sound waves reaching the tympanum. As is well known, there are certain vibrations of sound which are too low for the auditory nerve to register, and others too high for it to record, both classes, however, capable of being recorded by scientific instruments. It is also regarded as certain that some of the lower animals are conscious of sound vibrations which are not registered by the human auditory nerves.
The Sense of Smell.
The nerves of the sense of smell terminate in the mucous membrane of the nostrils. In order that these nerves report the odor of outside objects, actual contact of minute particles of the object with the mucous membrane of the nostrils is necessary. This is possible only by the passage through the nostrils of air containing these particles; mere nearness to the nostril will not suffice. These particles are for the most part composed of tenuous gases. Certain substances affect the olfactory nerves much more than do others, the difference arising from the chemical composition of the substance. The olfactory nerves convey the report to the brain.
The Sense of Taste.
The nerves of the sense of taste terminate in the tongue, or rather in the tiny cells of the tongue which are called "taste buds." Substances taken into the mouth chemically affect these tiny cells, and an impulse is transmitted to the gustatory nerves, which then report the sensation to the brain. The authorities claim that taste sensations may be reduced to five general classes, viz.: sweet, bitter, sour, salty, and "hot."
There are certain nerve centers having important offices in the production and expression of mental states, located in the skull and in the spinal column—the brain and the spinal cord—which we shall consider in the following chapter.
CHAPTER III.
The Great Nerve Centers.
THE great nerve centers which play an important part in the production and expression of mental states are those of the brain and spinal cord, respectively.