All are familiar with the "five senses" of our elementary physiologies, sight, hearing, taste, smell, and touch. A more complete study of sensation reveals nearly three times this number, however. This is to say that the body is equipped with more than a dozen different kinds of end-organs, each prepared to receive its own particular type of stimulus. It must also be understood that some of the end-organs yield more than one sense. The eye, for example, gives not only visual but muscular sensations; the ear not only auditory, but tactual; the tongue not only gustatory, but tactual and cold and warmth sensations.
Sight.—Vision is a distance sense; we can see afar off. The stimulus is chemical in its action; this means that the ether waves, on striking the retina, cause a chemical change which sets up the nerve current responsible for the sensation.
The eye, whose general structure is sufficiently described in all standard physiologies, consists of a visual apparatus designed to bring the images of objects to a clear focus on the retina at the fovea, or area of clearest vision, near the point of entrance of the optic nerve.
The sensation of sight coming from this retinal image unaided by other sensations gives us but two qualities, light and color. The eye can distinguish many different grades of light from purest white on through the various grays to densest black. The range is greater still in color. We speak of the seven colors of the spectrum, violet, indigo, blue, green, yellow, orange, and red. But this is not a very serviceable classification, since the average eye can distinguish about 35,000 color effects. It is also somewhat bewildering to find that all these colors seem to be produced from the four fundamental hues, red, green, yellow, and blue, plus the various tints. These four, combined in varying proportions and with different degrees of light (i.e., different shades of gray), yield all the color effects known to the human eye. Herschel estimates that the workers on the mosaics at Rome must have distinguished 30,000 different color tones. The hue of a color refers to its fundamental quality, as red or yellow; the chroma, to its saturation, or the strength of the color; and the tint, to the amount of brightness (i.e., white) it contains.
Hearing.—Hearing is also a distance sense. The action of its stimulus is mechanical, which is to say that the vibrations produced in the air by the sounding body are finally transmitted by the mechanism of the middle ear to the inner ear. Here the impulse is conveyed through the liquid of the internal ear to the nerve endings as so many tiny blows, which produce the nerve current carried to the brain by the auditory nerve.
The sensation of hearing, like that of sight, gives us two qualities: namely, tones with their accompanying pitch and timbre, and noises. Tones, or musical sounds, are produced by isochronous or equal-timed vibrations; thus C of the first octave is produced by 256 vibrations a second, and if this tone is prolonged the vibration rate will continue uniformly the same. Noises, on the other hand, are produced by vibrations which have no uniformity of vibration rate. The ear's sensibility to pitch extends over about seven octaves. The seven-octave piano goes down to 27-1/2 vibrations and reaches up to 3,500 vibrations. Notes of nearly 50,000 vibrations can be heard by an average ear, however, though these are too painfully shrill to be musical. Taking into account this upper limit, the range of the ear is about eleven octaves. The ear, having given us loudness of tones, which depends on the amplitude of the vibrations, pitch, which depends on the rapidity of the vibrations, and timbre, or quality, which depends on the complexity of the vibrations, has no further qualities of sound to reveal.
Taste.—The sense of taste is located chiefly in the tongue, over the surface of which are scattered many minute taste-bulbs. These can be seen as small red specks, most plentifully distributed along the edges and at the tip of the tongue. The substance tasted must be in solution, and come in contact with the nerve endings. The action of the stimulus is chemical.
The sense of taste recognizes the four qualities of sour, sweet, salt, and bitter. Many of the qualities which we improperly call tastes are in reality a complex of taste, smell, touch, and temperature. Smell contributes so largely to the sense of taste that many articles of food become "tasteless" when we have a catarrh, and many nauseating doses of medicine can be taken without discomfort if the nose is held. Probably none of us, if we are careful to exclude all odors by plugging the nostrils with cotton, can by taste distinguish between scraped apple, potato, turnip, or beet, or can tell hot milk from tea or coffee of the same temperature.
Smell.—In the upper part of the nasal cavity lies a small brownish patch of mucous membrane. It is here that the olfactory nerve endings are located. The substance smelled must be volatile, that is, must exist in gaseous form, and come in direct contact with the nerve endings. Chemical action results in a nerve current.
The sensations of smell have not been classified so well as those of taste, and we have no distinct names for them. Neither do we know how many olfactory qualities the sense of smell is capable of revealing. The only definite classification of smell qualities is that based on their pleasantness or the opposite. We also borrow a few terms and speak of sweet or fragrant odors and fresh or close smells. There is some evidence when we observe animals, or even primitive men, that the human race has been evolving greater sensibility to certain odors, while at the same time there has been a loss of keenness of what we call scent.
Various Sensations from the Skin.—The skin, besides being a protective and excretory organ, affords a lodging-place for the end-organs giving us our sense of pressure, pain, cold, warmth, tickle, and itch. Pressure seems to have for its end-organ the hair-bulbs of the skin; on hairless regions small bulbs called the corpuscles of Meissner serve this purpose. Pain is thought to be mediated by free nerve endings. Cold depends on end-organs called the bulbs of Krause; and warmth on the Ruffinian corpuscles.
Cutaneous or skin sensation may arise from either mechanical stimulation, such as pressure, a blow, or tickling, from thermal stimulation from hot or cold objects, from electrical stimulation, or from the action of certain chemicals, such as acids and the like. Stimulated mechanically, the skin gives us but two sensation qualities, pressure and pain. Many of the qualities which we commonly ascribe to the skin sensations are really a complex of cutaneous and muscular sensations. Contact is light pressure. Hardness and softness depend on the intensity of the pressure. Roughness and smoothness arise from interrupted and continuous pressure, respectively, and require movement over the rough or smooth surface. Touch depends on pressure accompanied by the muscular sensations involved in the movements connected with the act. Pain is clearly a different sensation from pressure; but any of the cutaneous or muscular sensations may, by excessive stimulation, be made to pass over into pain. All parts of the skin are sensitive to pressure and pain; but certain parts, like the finger tips, and the tip of the tongue, are more highly sensitive than others. The skin varies also in its sensitivity to heat and cold. If we take a hot or a very cold pencil point and pass it rather lightly and slowly over the skin, it is easy to discover certain spots from which a sensation of warmth or of cold flashes out. In this way it is possible to locate the end-organs of temperature very accurately.
