Auditory system

“ The pulses created in the air by a sounding body are received by the ear and the impulses which they impart to the auditory nerve pass to the brain and we become conscious of a sound. The ear is capable of marvelous discrimination and accuracy. "In order to form an idea of the extent of this power imagine an auditor in a large music hall where a full band and chorus are performing. Here, there are sounds mingled together of all varieties of pitch, loudness, and quality ”

“ The function of the organs of hearing, therefore, is to impart faithfully to the auditory nerve, every condensation and rarefaction occurring in the surrounding medium. The function of the auditory nerve is to bring to our consciousness the vibrations of matter resulting at the given time, both according to their number and their magnitude. Here, first, certain combinations acquire a distinct name: here, first the vibrations become musical tones or discords (Misstöne) . ”

“ This end-organ in the cochlea may be compared very fitly to the telephone which receives the message, and that portion of the brain where the auditory tract ends, to the telephone at the distant end of the path, the listener there representing consciousness. The auditory path within the brain is long and complicated, there being, in fact, many way-stations through which the message passes before it reaches the final one. ”

“ Then 800 pulses of air will reach the ear each second, and the ear drum, being flexible, will respond and will vibrate at the same rate. The vibration of the ear drum will be transmitted by the three bones and the fluid to the fibers of the auditory nerves. The impulses imparted to the auditory nerve reach the brain and in some unknown way are translated into sound. ”

“ A distinction is generally made in physics between sound and noise. Noise affects our tympanic membrane as an irregular succession of shocks and we are conscious of a jarring of the auditory apparatus; whereas a musical sound is smooth and pleasant because the tympanic membrane is thrown into successive periodic vibrations to which the auditory receptor (sense organ of hearing) has been attuned. To produce musical sounds, a body must vibrate with the regularity of a pendulum, but it must be capable of imparting sharper or quicker shocks to the air than the pendulum. ”

“ A description of sonorous vibrations is given in the article Sound; here we shall consider the transmission of such vibrations from the external ear to the auditory nerve, and the physiological characters of auditory sensation.
1. Transmission in External Ear.—The external ear consists of the pinna, or auricle, and the external auditory meatus, or canal, at the bottom of which we find the membrana tympani, or drum head. In many animals the auricle is trumpet-shaped, and, being freely movable by muscles, serves to collect sonorous waves coming from various directions. ”

“ They are probably acquainted with a whole world of physical facts of which we are utterly ignorant. Our auditory apparatus supplies us with a knowledge of sounds. What are these sounds? They are vibrations of matter which are capable of producing corresponding or sympathetic vibrations of the drums of our ears or the bones of our skull. When we carefully examine the subject, and count the number of vibrations that produce our world of sounds of varying pitch, we find that the human ear can only respond to a limited range of such vibrations. ”

“ The auditory center, A, receives the nervous impulses started by sound. When it is aroused by impulses coming from the ears, the sensation of sound occurs; but when it is aroused by nerve-currents, not from the ears, but from other parts of the brain, we have only the memory of sound. For a word to be understood, the auditory center alone is not sufficient. The sound must awaken the memories of other sensations. ”

“ The structure of the human ear is much more complicated than is generally supposed. It is an apparatus constructed to respond to the waves of sound. As a whole, it may be considered a peculiar form of nerve-ending.
The external ear forms only a part of a most elaborate apparatus whereby sound waves may be transmitted inwards to the real organ of hearing. The really sensitive part of the ear, in which the auditory nerve ends, is buried for protection deep out of sight in the bones of the head ”

“ It is owing to them and to their mechanism that we ought to feel, from the same excitant, a sensation of sound or one of colour, that is to say, impressions which appear, when compared, as the most different in the world. Now, so far as we can make out, the histological structure of our auditory centre is the same as that of our visual centre. ”

“ Like the sense of sight and the sense of smell, that of hearing, even in its simplest grades, serves to bring the organism into relation with more or less distant bodies, so long as they are sufficiently sonorous to transmit the so-called "sound" vibrations through water or air to the sensitive organs which become attuned to receive such impressions.
An auditory organ does not seem to be present at all—certainly none has as yet been detected or inferred to exist—in many of the lower forms of life; while in other animals, though inferred to exist, it remains as yet unrecognized. ”

“ Communication exists externally with the middle ear by the round and oval windows and internally with the internal auditory canal, through which passes the eighth cranial or auditory nerve, the special nerve of hearing, which is distributed to the inner ear only. ”

“ If we had an ear for height and an ear for depth, just as we have an eye for the right and an eye for the left, we should also find that symmetrical sound-structures existed for our auditory organs. The contrast of major and minor for the ear corresponds to inversion for the eye, which is also only symmetry for the mind, but not for sensation. ”

“ If the person who wishes to hear by means of the Audiphone will hold the fan against the upper teeth,—the convex side of the fan outward,—he or she will hear distinctly, for the vibrations of sound are collected and strike upon the teeth and bones, and act upon the auditory nerves from within, precisely as the vibrations act from without through the auricle. ”

“ The internal ear is formed by an involution of the integument, and not by an outgrowth of the brain. But tissue, in connexion with it, becomes in part changed, thus forming the auditory nerve, which places the tegumentary sac in direct communication with the brain itself. ”

“ There is every reason then to believe that auditory tonal images are the sole primary and essential guides to the minute alterations of tension in the muscles of the larynx necessary for the production of corresponding vocal sounds. By humming a tune we concentrate our attention and thereby limit the activity of neural processes to systems and communities of neurones employed for the perception of tonal images and their activation in motor processes ”

“ The influence of resonance and pitch, echo and absorption of tones, i.e., the location of the sound, is of great importance. Finally, it must not be forgotten that people’s ability to hear varies with the weather. Colds reduce the power, and not a few people are influenced by temperature, atmospheric pressure, etc. These considerations show the degree in which auditory illusions can be of importance even in tests of their nature and existence. They show above all that the same object of comparison under the same circumstances must be used in every test. ”

“ Each of the simple sounds which, taken together, constitute the whole sound, causes the vibration of a special group of fibres in the auditory nerve. This fact, often repeated, generates a kind of organic predisposition which is confirmed by heredity. If from any cause one of these groups is set in motion, the other groups will have a tendency to vibrate. ”

“ In left-handed persons, on the contrary, it seems to attach to the right hemisphere.
Auditory Region of the Cortex.—Besides the two great organs of distance-receptors, namely, the nose and eye, whose cerebral apparatus for sensation has just been mentioned, those of a third great distance-receptor have to be considered. The agents of stimulation of the two former are respectively chemical (olfactory) and radiant (visual) ; the mode of stimulation of the third is mechanical, and the sensations obtained by it are termed auditory. Their cerebral localization is very imperfectly ascertained. ”

“ The organ of hearing is without any external ear and without an air-containing external auditory meatus. The aerial vibrations do not pass, as in other mammals, from the external auditory passage to the tympanic cavity and thus to the nerve-terminations of the inner ear; but they reach the tympanic cavity by direct transmission through the bones of the skull, which possess a special structure and contain abundant air-cavities. ”

“ The outer ear collects the vibrations, the middle ear conducts them, and the internal converts them into a special physiological condition of the hair-cells and the auditory nerve. This condition is communicated to the other links in the anatomical hearing chain, until the highest part of the brain, or cortex, is reached. Hearing, from the physiological point of view, is the outcome of a series of processes having their development in a corresponding series of centres, or collections of nerve-cells. ”

“ We can thus understand to a certain extent why the rabbit has 7,800 auditory cells in his organ; a number that represents a wonderfully delicate refinement in his hearing, even if we do not suppose each of these 7,800 cells to correspond with a different tone, as, if we regard each cross-shaped group of four cells as representing a single tone, this would give an exceedingly large number—about two thousand—of tone-perceptions. We may realize how delicate must be the hearing that appreciates even a thousand tones when we recollect that our concert-piano scales give only eighty-seven tones. ”

“ By every condensation the tympanum of our ear is pressed inwards, by every rarefaction it is pressed outward and thus the tympanum performs oscillations like a pendulum. The smaller or greater number of the oscillations made in a second gives us by help of the small bones in our ear and the auditory nerve the idea of a higher or lower tune. ”

“ Vibrations which cause hearing are communicated by the chain of bones. When the base of the stirrup is pushed into the oval window, the pressure in the labyrinth increases, and, as the only mobile part of the wall of the labyrinth is the membrane covering the round window, this membrane is forced outwards; when the base of the stirrup moves outwards a reverse action takes place. Thus the fluid of the labyrinth receives a series of pulses isochronous with the movements of the base of the stirrup, and these pulses affect the terminal apparatus in connexion with the auditory nerve. ”

“ The sonorous figures are formed in the auditory organ, and even in the auditory nerves, just as they have been represented upon an infinitely small scale in the air. The nerve becomes in hearing a sonorous figure. ”

“ In hearing, the minute auditory nerves merely receive an impulse from the vibrations or wavelets of the elastic atmosphere, and this constitutes what we call noise, tones, music, voice. But these tones or noises are in the mind only; they are not appreciated by the exercise of any other sense. ”

“ The auditory center of the brain often spoken of is not simply theoretical, but is defined with fair certainty. If, in a dog or an ape, a particular spot in the temporal fold of the cerebrum is destroyed on both sides, the animal will be made deaf, although his ear-organs have not been disturbed. ”

“ Auditory phenomena may be divided into two categories: provoked automatisms, and spontaneous automatisms or clairaudience. The first may be considered as hallucinations induced by diverse methods. The [182] simplest method consists in the use of certain shells, horns, trumpets, or, in a word, any object capable of augmenting and allowing the perception of those external or internal sounds, which are not usually perceptible to the hearing. This is what is observed particularly with some sea-shells. When we apply them to the ear, we hear a murmur or a slight rumbling sound. ”

“ We all have our limits, and that limit may be quickly found by these whistles in every case. The facility of hearing shrill sounds depends in some degree on the position of the whistle, for it is highest when it is held exactly opposite the opening of the ear. Any roughness of the lining of the auditory canal appears to have a marked effect in checking the transmission of rapid vibrations when they strike the ear obliquely. ”