VIII. AUDITORY SENSE ORGAN.
Structure of the External Ear. — We have already discussed on page 61 part of the inner ear, namely the vestibular por- tion consisting of the semicircular canals and of the saccule and utricle. The remaining part of the ear, the cochlea, is devoted to the reception of auditory stimuli.
80 PSYCHOLOGY The structure of a large part of the eat as a whole need not here detain us since several good descriptions are to be found in the various anatomical and physiological text-books. In general, one speaks of (1) the external ear, (2) the middle ear, which consists of the drum membrane attached obliquely at the end of the auditory meatus and of the ossicles with their muscles, (3) the inner ear.
The external ear in man is quite complicated in shape. Its general function in the animal world is to collect and condense the sound waves. In view of its shape in man, of its method of attachment, and the atrophied condition of its muscles, it has little function in hearing. The canal or external meatus which extends from the concha to the middle ear serves as an avenue for the conveyance of air vibrations. It is about 22 mm. -in length and its course is somewhat tortuous. The bore varies consider- ably. The skin which lines it is provided with hairs and wax- secreting glands. The hairs and wax protect the middle and inner ear.
The Middle Ear. — The middle ear, or tympanum, is an ir- regular chamber in the temporal bone. Its outer wall consists of the membrana tympani, or drum membrane. The drum mem- brane is ellipsoid in form, tightly stretched, and has a diameter of about 10 mm. It is about 1 mm. thick and is composed of radial and annular fibers. It is stretched over the meatus in such a way that it presents a convex surface towards this1 open- ing. In the inner wall of the tympanic cavity which separates the middle ear cavity from the inner ear are situated two open- ings or windows, the fenestra ovalis and the f enestra rotunda, which are to be described/ The tympanic cavity communicates with the buccal cavity by means of the eustachian tube.1 An irregular chain of bones is stretched between the drum mem- brane and the membrane covering the fenestra ovalis. The chain of bones consists of the malleus, the incus and the stapes. Fig. 13 shows the method of articulation of the bones as well as their general form. The handle of the malleus is attached to the drum membrane. The head of the malleus (Mcp) articulates with the saddle-shaped depression in the incus. A short process of the THE RECEPTORS AND THEIR STIMULI 81 M incus (Ib) is attached by a ligament to the posterior wall of the tympanum. Its long process (II) articulates with the stapes. The stapes is attached at its stirrup-like end to the fenestra ovalis. These bones are under the control of two muscles, (a) the tensor tympani, which, when contracted, serves to tighten the drum membrane, and (6) the stapedius, which, when contracted, pulls the stapes slightly away from the fenestra ovalis, thus les- sening the tension on the whole system (and hence on the drum membrane). The stapedius may thus be looked upon as the antago- nist of the tensor tympani. When the air waves impinge upon the drum membrane, it is thrown, into a back- ward and forward motion, forward at the condensation phase, backward upon the rarefaction phase. The frequency is thus the same as that of the elastic or sounding body. This excursion system of the drum mem- brane is wide but not powerful. The ear. leverage principle upon which the Cham Of OSSlcleS WOrks IS SUCh that oranvilbone; Ib, its short process; II, j-i j (.,• •,, 3, its long process; S, the stapes or stirrup the to and fro motion transmitted to muscle (From Howeii's Text-Book the fenestra ovalis is small, but capable of overcoming strong resistance, which is necessary because of the fact that the wave motion must be transmitted by the stapes to the fluids of the inner ear. While the importance of the middle ear as a transmitting and reinforcing device must be admitted, it should be added that in pathological cases auditory sensitivity is acute in the total absence of the drum membrane and all ossicles.
The Inner Ear, or Cochlea. — The auditory portion of the inner ear, the cochlea, is shown in Fig. 8, together with its relation to the vestibule and to the semicircular canals. The cochlea is a spiral tube divided into two chambers partially by means of a bony shelf, the lamina spiralis, and partly by a membrane, mem- brana spiralis, which is attached to the shelf at one end and to g2 PSYCHOLOGY g2 PSYCHOLOGY the inner surface of the bony tube at the other. The upper cham- ber, or scala vestibuli, opens into the vestibule, while the lower chamber, or scala tympani, communicates with the tympanic cavity of the middle ear by the fenestra rotunda. At the apex of the cochlea the two divisions communicate with each other by l-'i'i. 14. — Section through cochlea (cat), dc, Ductus cochlearis; firs, ganglion spirale; Co, bony wall of cochlea; Is ligamentum spirale; ms, membrana spirale or basilar mem- brane supporting the organ of Corti; mv, Reissner's membrane; Nc, auditory nerve; sv, scala vestibuli; at, scala tympani (after Sobotta).
a small opening, the heliocotrema. Between the two scala3 is a small canal, triangular in cross section, called the ductus coch- learis. This is the membranous sac continuous with that of the vestibule. It contains the endolymph, whereas the scala vestibuli and the scala tympani are filled with peril ymph. The sides of this triangular sac are made up as follows: the bony shelf and the membrana spiralis form one side, another is formed by the mem- brane lining the bony wall, and the third by the delicate mem- brane of Reissner. These relations are shown clearly in Fig. 14.
THE RECEPTORS AND THEIR STIMULI 83 It is in this membranous sac throughout its spiral course that we find the true auditory sense organs. These are supported on the membrana spiralis (also called basilar membrane in many of the texts). The center of the cochlea as a whole consists of a spongy bone, the modiolus. The nerve fibers of the VIII, or auditory nerve, pass through the bone and are distributed to the auditory structures lying on the basilar membrane. The auditory appa- ratus in the narrow sense consists of a group of structures called the organ of Corti (Fig. 15). To understand this organ and the FIG. 15. — Partially diagrammatic representation of the organ of Corti and adjacent structures. The nerve fibers are seen passing to Corti's organ through openings in the bony spiral lamina. (From Bailey's Text-Book of Histology, Wm. Wood & Co.)
Helmholtz resonance theory of auditory stimulation, we must glance for a moment at the structure of the basilar membrane. As may be inferred, this is a spiral membrane about 0.041 mm. across at the base of the cochlea, and about 12 times wider (0.495 mm.) at the apex. If we were to detach the membrane and flatten it out, it would appear something like the stretched string system of the piano in miniature. While there are straight elastic fibers in this membrane, it must be admitted that as a whole it is largely homogeneous and that these elastic fibers are not free to vibrate. On the surface of this membrane super- 84 PSYCHOLOGY imposed upon the elastic fibers, one finds at close intervals pairs of stiff rod-like cells forming an arch. The upper ends of the cells join, while the lower ends are set wide apart upon the basilar membrane. There is thus formed a continuous row of the so- called arches of Corti. These arches support a series of modified epithelial cells, a single row on the inner side of the inner rod of Corti, and four rows beyond the outer rod of Corti. These cells are supplied with fine, stiff bristles which pierce through a fine cuticular membrane (lamina reticularis) and project out into the endolymph. There are several other supporting cells in the organ of Corti, but they are not important in this connection; since the hair cells are the true sense structures, and it is around them that the fibers of the auditory nerve end. Over the organ of Corti, extending as far out as the last row of outer hair cells one finds the teetorial membrane now looming rather large in the sensory physiology of hearing. One end of this membrane is attached approximately at the junction point between the basilar membrane and the bony shelf, the other end floats freely in the endolymph immediately above the hair cells. It is not generally recognized that owing to the position of the ear as a whole, this membrane works really in a sagittal and not in a vertical plane. Gravity thus has no direct tendency to pull it down upon the hair cells. In our opinion it can have no influence upon the gen- eration of an auditory impulse, except in so far as it may serve as an object against which the hair cells can possibly strike. Even this function is probably not a necessary one.
Theory of Auditory Stimulation, — How is an individual hair poll stimulated? Various theories are advanced. The Helmholtz theory formerly had the most adherents, but it is gradually losing ground. This theory may be stated as follows: When a tuning fork of 500 d.v. sounds, the wave is transmitted to the fluid of the ear. One of the basilar membrane fibers is attuned to that fre- quency. It begins to vibrate sympathetically. As this fiber vi- brates, it forces the hairlets of the hair cells to strike (possibly) against the teetorial membrane. This impact is sufficient to start the chemical processes in the hair cell as a whole, which results in the arousal of a neural impulse in the nerve element connected THE RECEPTORS AND THEIR STIMULI 85 with that cell. On purely logical grounds this theory accounts very well for the various phenomena of hearing as follows: (1) for the ability of the human being and lower animals to react differently to different vibration frequencies; (2) for the ab- sence of ability to react to frequencies less than 40 v.s. and greater than 405000 v.s. The theory merely assumes that there are no basilar membrane fibers of sufficient length or shortness to respond sympathetically to rates of greater or less frequency; (3) the complete inability to react to auditory stimulation, and (4) inability to respond to one definite set of frequencies while having the ability to respond to higher and lower frequencies (tonal islands).
Objections to the Helmholtz Theory. — Few physicists, how- ever, are willing to admit that the radial fibers of the basilar mem- brane, forming a part as they do of a homogeneous membrane, can vibrate as Helmholtz supposed. Ewald has advanced a theory which seems more plausible upon the physical side. He assumes that each and every tonal stimulus causes the basilar membrane to vibrate in its entire length. Since it subdivides into a series of stationary waves of definite form, he calls the pattern so impressed upon the membrane the "acoustic image." Every tone would impress a different acoustic image. These patterns can be observed if a suitable rubber membrane about the size of the basilar membrane is stretched over a frame and made to shine with oil. A pattern of stationary waves appears on this membrane when viewed under a microscope whenever a. tuning fork or other elastic body is actuated. Theoretically these pressure patterns can account for the phenomena of hearing which we have just enumerated if we grant that such patterns bear sufficient energy to arouse the auditory hair cells. Luciani suggests replacing the basilar membrane of this theory by the tectorial membrane. The tectorial membrane under the influ- ence of a given pattern would be pressed downward upon a definite set of hair cells. We have already called attention to the fact that the tectorial membrane does not work in a plane which would easily permit this.
There are many other so-called auditory theories which we 86 PSYCHOLOGY cannot enter into here. No one theory is "accepted by all in- vestigators, but all admit that there must be some mechanism in the inner ear capable of performing very complex functions, since destruction of the organ of Corti brings in its train the inability to respond to noise and tonal stimuli.
THE SENSE OF VISION.
The Stimulus to Visual Reaction. — The stimuli needed for exciting the rods and cones are very rapid, ethereal vibrations set up by some luminous body. Ether waves varying in length, depending upon the nature of the source, are generated. As in the case of sound stimuli, ether waves may vary also in amplitude, depending upon the distance away of the source, or upon its temperature. An ether wave shorter than 397/^,/x or longer than 760/i/x, (fju== one-thousandth of a mm. ) will not arouse a visual reac- tion. Nearly all light sources give out ether waves both shorter and longer than those which affect the retina, Waves longer than 760/A/x are called infra-red, or heat rays. Those shorter than 397/A/x are called ultra-violet or chemical rays. The rods and cones are insensitive to infra-red rays, but there is no evidence to show that they are really insensitive to waves shorter than 397/x/x. The reason we cannot stimulate the retina of most of the higher ver- tebrates by such short rays is due to the fact that the rays are absorbed by the lens and the fluid of the eye before reaching the retina. When we speak of ether waves from now on, we shall mean those which affect the retina — those lying between the limits 397/x/* and 760/x/x.
Two Classes of Visual Stimuli. — Investigation has shown that we may classify light stimuli under two headings: (1) non- homogeneous, or white light. Here the light is made up of ether waves of all lengths. We see such light in sunlight and in objects which reflect sunlight, such as the moon, various objects in the environment, etc., in the electric light and gas lights, which emphasize, however, certain of the rays. There is no exact physi- cal measure or definition of a white light. (2) Homogeneous, or monochromatic light. In this case, the ray has one frequency, 'for THE RECEPTORS AND THEIR STIMULI 87 Monochromatic Light: How Obtained. — We obtain mono- chromatic light by passing non-homogeneous light (1 above) through a series of lenses, slits and prisms. Such an instru- ment is known as a spectrometer. The white light is refracted in such a way that the longest waves are brought to a focus at one end of the illuminated area, the shortest waves at the other end. The intermediate wave-lengths take up intermediate positions be- tween the two extremes, the whole forming the spectrum. It becomes possible to select out from the spectrum of any source a narrow band of approximately homogeneous light. The above is a laboratory, or exact, method of obtaining monochromatic light. In daily life we are rarely stimulated by pure or mono- chromatic light. We are stimulated by a visual object, which is illuminated by some primary source of light, such as the sun, or electric light. Many of these objects absorb ("dampen" or "kill ") certain of the ether waves and reflect only a portion of the rays. They thus become monochromatic stimuli, or rather they approximate it. We speak of colored flowers, the yellow book, etc. Spectroscopic examination shows that only rays from defi- nite regions are reflected from such objects. The pigments used by the artist are the best examples of this. The result of mixing pigments is merely a final compound which will reflect only the rays sought and will dampen or kill all other rays. On account of their convenience and cheapness, a great deal of student train- ing work in the laboratory has been done with these approxima- tions to monochromatic light. They take the form of colored papers, colored glass, gelatine sheets and liquid filters. The student is earnestly advised where possible to go to the spectrum for the testing of the various phenomena of monochromatic light vision, and to express his results by exactly defining the wave length of the light, giving the radiometric value of each stimulus so used, its dimensions, and the part of the eye it falls upon.6 Only in this way does our stimulus become exactly defined 8 Two spectrometers sufficiently good to illustrate all of the phenomena discussed in the text can be constructed for less than the cost of a triple color wheel, motor and discs. The energy measuring device is more of a problem both as to cost and as to manipulation.
88 PSYCHOLOGY and only in this way can that stimulus b§ duplicated in some other laboratory.
Is Wave Length a Factor in Human Adjustment? — If we take a normal human subject and stimulate his retina with mono- chromatic light, beginning with 760m and then advancing to shorter wave lengths, and ask him to react to them verbally in order, he will say, "I see red" from 760/x/x to 647m "orange" from 647/A/x to 586^, "yellow" from 586^* to 535m "green" from 535/x/x to 492^, "Hue" from 492/t/x, to 456/^, "indigo" from 456/A/u to 424/^, "violet" from 424^ to 397/ift. If our subject is a mute, we must choose some other form of response than the verbal. If an animal, possibly still another form. Neither in the case of the mute nor in that of the animal can we get a vocal reaction, but we can by one or another means prove tljat wave length as such is a factor in their adjustments, or the contrary. We can determine whether all wave lengths are factors or only certain ones. We can get the observer to do more than name the colors. Given the proper conditions, we can establish various habits of reaction to fine differences in wave lengths and in ampli- tude. In certain professions, those of the artist and of the archi- tect, and in the trades, for example, those of the dyer, of the silk buyer, of the wine tester, a large part of the life of the individual is devoted to reactions to just such differences.
Differential Sensitivity to Wave Length Difference. — The question next arises by how much must one ray differ from another in wave length before it can call out differential reactions. The value differs in different parts of the spectrum, but in general it is a fraction of a wave length when determined by the verbal reaction method. WTien determined by the conditioned reflex method the value is not so small.
Differential Sensitivity to Energy Differences.— We know that monochromatic lights differ in their energy content (ampli- tude of vibration). The monochromatic light 700/x/i, obtained by passing sunlight through a spectrometer is a bearer of far greater energy than the 700^ obtained from the tungsten lamp. In order to determine the difference between two such lights we must measure their energy by means of some kind of a thermal couple.
THE RECEPTORS AND THEIR STIMULI 89 The values necessary to produce a differential reaction varies so with the different methods of obtaining them, and varies so with; the state of adaptation of the eye, that we shall not attempt to discuss them.7 The difference in energy necessary to afford a basis for a differential reaction is extremely small in any event, and it is enough to say that many of our reactions to monochro- matic light are based upon such differences.
The Effect of Simultaneous Exposure of the Retina to Two or More Monochromatic Lights: The Balancing of Monochro- matic Light Stimuli. — Some characteristic differences in response appear if we expose the retina simultaneously to two or more monochromatic light stimuli. To obtain these results in the most satisfactory way one should have a spectrometer from which two or more monochromatic bands can be made to fall simultaneously upon a test object, e.g., a plaster of Paris surface. A second spectrometer should be at hand so that the subject himself can throw any band whatsoever upon a second plaster surface adjoining the first. In addition to the two spectrometers a source of faint white light should be under control and so arranged that it can be thrown upon either plaster surface. If the experimenter with his spectrometer allows 665.2/x/x (red) of a certain energy and 492. 1/*/* (green-blue) of a certain energy to fall simultaneously upon the plaster surface, the subject reacts as to a white light of a certain photometric value. The two stimuli are said to be "complementary" or to balance or cancel one another. Again, exactly the same phenomena appear if the following pairs are used: 607.7/>t/x, (orange) and 489.7/x,/x, (blue); 567.1/A/A (yellow) and 464. 5/*/* (indigo); 563.6/x/x, (green-yellow) and 433/A//, (violet). If 564/x/x to 492//,/* (greens) are used, no corresponding single monochromatic band can be found which will complement them. If, however, two bands chosen from the extreme ends of the spectrum are superimposed upon the plaster of Paris surface along with a band from this region, the triple stimuli again balance. The subject reacts to it as to a non- homogeneous or white light. These phenomena can be obtained 7 As a matter of fact the author is not aware of a very reliable report of work where the D.L, has been obtained by such exact methods.
90 PSYCHOLOGY very simply by rapidly rotating colored discs reflecting approxi- mately the above wave lengths. The separate stimuli fall upon the retina so rapidly that the elements cannot react to them separately. That a balance occurs can be verified by rotating a smaller pair of black and white discs (black and white are used to adjust conveniently the intensity of the non-homogeneous or white light) on the same axis. The black and white discs can be so adjusted that they offer the same stimulating value as the two colored discs.
Effect of Superposing Non-Complementary Lights. — What reaction differences appear when one half of the test field is illuminated by a mixture of homogeneous light, the difference between the wave lengths of which is either too small or too great to permit of an acceptable match with white light? If the difference between the two mixed wave lengths is less than the difference between either of them and its complementary, the mixture can be matched by an intermediate wave length to which white light of a certain brightness has been added. If both of the superimposed lights lie between 760/^ju. and 540^ no white light need be added to the single monochromatic light in order to complete the match. As an example the experimenter may superimpose on one side of the test object a wave length in the red and one in the yellow and on the other side of the test object he may allow a single beam from the orange to fall; the subject calls the two a "match"— i.e., there is no basis at hand for a differential reaction.
If the difference in wave length of the two mixed lights is greater than the difference between either of them and its com- plementary, e.g.j red and blue, a single homogeneous light even if mixed with white light will not present a satisfactory match provided the intensities of both the homogeneous lights in the mixture are above the subject's threshold. Such a mixture can be matched by mixtures of red and violet in suitable proportions with an addition of white.
The above statements hold for human observers with normal color vision. They do not hold for the color blind. The typical dichromat will match a mixture of any two wave lengths with THE RECEPTORS AND THEIR STIMULI 91 an intermediate wave length the value of which depends on the relative intensities of the two components of the mixture and without requiring an addition of white light to the single light to complete the match. It is necessary only to permit him to vary the intensity of the intermediate light if the intensity of the two mixed lights remains as given.
The After-Effects of Monochromatic Light Stimulation. — One of the most interesting sets of phenomena to be met with in the whole of sensory physiology appears in the after-effects of mono- chromatic light stimulation. After the eye has been stimulated for a time by a monochromatic light which is then removed, one of two things may be reported by the subject: The subject may react as though he were stimulated anew by the original light, the so-called ' * positive after-image "; or, as though 'he were stimu- lated by light the wave length of which is complementary to the original light, the ' ' negative after-image. ' ' We can illustrate this by data obtained by the verbal report method. If we stimulate with 461/x/x (indigo blue) and the subject then looks at a gray screen, he will say, "I see yellow" (564^). He will give the same report if his eyelids are closed and covered. Under certain conditions which cannot be entered into here he may report that he sees "indigo blue" (positive after-image). Under most con- ditions, though, the positive after-image is difficult to obtain. Stating these phenomena in physiological terms, we may say that under certain conditions of stimulation (depending upon the length of stimulation, the condition of the eye and the energy of