B. Diagram of semicircular canal to illustrate effect of rotation. The large arrows indicate the direction of the rotation. The small arrow to the left indicates the resulting flow of the inner fluid into the ampulla; that to the right, the flow of the outer fluid into the vestibule.]
From Professor Crum Brown's paper in Nature I transcribe, with some verbal modifications, his account of how the semicircular canals enable us to feel these changes of motion. Let us consider the action of one canal. If the head be rotated about a line at right angles to the plane of the canal, with the ampulla leading, there will be a tendency for the fluid within the sac to flow into the ampulla, and for the fluid around the semicircular canal to flow into the cavity in which the sac lies. These movements will conspire to stretch the membranous ampulla, and thus to stimulate the hair-cells. This stretching will not take place in that canal if the rotation be in the reverse direction. But on the opposite side of the head is another canal in the same plane, but turned the other way. In the reversed rotation the ampulla in this canal will lead, and its hair-cells will be stimulated. Thus by means of the two canals on either side of the head in the same plane, rotation in either direction can be appreciated. And since there are two other pairs of semicircular canals in two other planes, rotation in any direction will be recognized by means of one or more of the six canals.
It is thus by means of the semicircular canals that we can appreciate acceleration of rotatory motion.[FB] But we can also appreciate acceleration of movements of translation—forwards or backwards, up or down. And Professor Mach has suggested that it is through the stimulation of the hair-cells in the patch in the sac itself (the so-called macula acustica) that we are able to appreciate these changes. The otoliths, held loosely and lightly in position by the gelatinous substance in which they are embedded, may, through their inertia, aid in the stimulation of the sense-hairs.
And this naturally suggests the question whether those sense-organs in the invertebrates which contain otoliths may not be regarded with more probability as organs for the appreciation of changes of motion than as auditory organs. This for some years has been my own belief. I have always felt a difficulty in understanding how the otoliths are set a-dance by auditory vibrations. But their inertia would materially aid in the appreciation of changes of motion. In some forms the otoliths are held in suspension in a gelatinous material. In others—the molluscs, for example—the otolith (which is generally single) is retained in a free position by ciliary action. In aquatic creatures an organ for the appreciation of changes of motion might be of more service than an auditory organ. And if one be permitted to speculate, one may surmise that the sense of hearing may be a refinement of the sense through which changes of motion are appreciated. First would come a sense of movements of the organism in the medium through the stimulation of the sense-hairs by the relative motion of the otolith; then these sense-hairs, with increased delicacy, might appreciate shocks in the medium; and, eventually, those more delicate shocks which we know as auditory waves. In this way we might account for the fact that in the vertebrates the same organ, through different parts of its structure, appreciates both change of motion and auditory vibrations. And thus the organs in the invertebrata which are generally regarded as auditory, and for which has been suggested the function of reacting to changes of motion, may, in truth, subserve both purposes—may be organs in which the differentiation I have hinted at is taking place.
Sight, like hearing, is a telæsthetic sense. Through it we become aware of certain vibratory states of more or less distant objects. The medium by means of which these vibrations are transmitted is not, as in the case of hearing, the air, but the æther which pervades all space. The rate of transmission is about 186,000 miles in a second. That which answers in vision to pitch in hearing is colour. The lowest, or gravest, light-tone to which we are sensitive is deep red, where the number of vibrations per second is about 370 billions (370,000,000,000,000). The highest, or most acute, light-tone is violet, with about 833 billion vibrations in a second. If white light be passed through a prism, the rays are classified according to their vibration-periods, and are spread out in a spectrum, or band of rainbow colours. But different individuals vary, as we shall presently see, in their sensibility to the lowest and the highest vibrations. Some people are, moreover, relatively or absolutely insensible to certain colours, generally either red or green. Such persons are said to be colour-blind. When the rainbow colours are combined in due proportion, or when pairs or sets of them are combined in certain ways, white light is produced.
We saw that in the case of sound-waves, when the number of vibrations in a second is doubled, the sound is raised in pitch by an octave. Using this term in an analogous way for colour-tones, we may say the range in average vision is about one octave—that is, from about 400 billion to about 800 billion vibrations in a second. But, though these are the limits in human vision, we know of the existence of many octaves of radiant energy physically in continuity with the light-vibrations. Photography has made us acquainted with ultra-violet vibrations up to about 1600 billions per second—an octave above the violet. And Professor Langley's observations with the bolometer indicate the existence of waves with as low a vibration-period as one billion per second, and even here, in all probability, the limit has not been reached. To the vibrations more rapid than those that are concerned in the sensation of violet, the human organism is apparently in no manner sensitive. But to infra-red vibrations down to about thirty billions per second the nerves of the skin respond through the temperature-sense. We shall have to return to these limits of sensation at the close of this chapter.
Fig. 31.—The human eye. Horizontal section, to show general structure.]
