Auditory organ

“ The organs supplied by the auditory nerve are only partly for the purpose of hearing; there is always present also an apparatus—the semicircular canals—concerned with equilibration and co-ordination of movements. Such equilibration organs are not confined to the auditory nerve, but in the water-living vertebrates are arranged segmentally along the body, forming the organs of the lateral line in fishes; the auditory organ is but one of these lateral line organs, which has been specially developed. ”

“ The human ear would seem to present greater difficulties. For the essential part of the sense-organ, in this case, is the membranous labyrinth, a bag of complicated form, which lies buried in the depths of the floor of the skull, and is surrounded by dense and solid bone. Here, however, recourse to the study of development readily unravels the mystery. Shortly after the time when the olfactory organ appears as a depression of the skin on the side of the fore-part of the head, the auditory organ appears as a similar depression on the side of its back part. ”

“ 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. ”

“ For neither the ear nor the eye receives anything but the impulses or vibrations originated by sonorous or luminous bodies. Nevertheless, the receptive apparatus still consists of nothing but specially modified epithelial cells. In the labyrinth of the ear of the higher animals the free ends of these cells terminate in excessively delicate hairlike filaments; while, in the lower forms of auditory organ, its free surface is beset with delicate hairs like those of the surface of the body, and the transmissive nerves are connected with the bases of these hairs. ”

“ 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 Sense of Hearing.—The external ear, although curiously shaped, is not the most important part of the organ whose function it is to take cognizance of sounds. In the transmission of sound to the brain, the vibrations of the air produced by the sonorous body are collected by the external ear, and conducted through the auditory canal to the drum of the ear, which is so arranged that it may be relaxed or tightened like the head of an ordinary drum. ”

“ The auditory organ [8] consists of a ‘crista acustica’ (fig. 9) , in the form of a slight prominence of columnar cells on the ventral side of the anterior cerebral vesicle; to the summit of which a spherical otolith is attached by fine hairs. In the crista is a cavity containing clear fluid. The dorsal half of the otolith is pigmented: the ventral half is without pigment. ”

“ It is the real organ of hearing. The upper part consists of three semicircular canals, the function of which is to record the position and movements of the body in space. The middle ear communicates with the pharynx by the Eustachian tube, through which air from the mouth may be introduced into the tympanic cavity, so as to permit vibrations of the drum membrane. In the external auditory canal of the ear is produced the cerumen or ear-wax. ”

“ Fig. 154.—Transverse Section through Auditory Capsules and Brain of Ammocœtes.
Au., auditory organ; VIII, auditory nerve; gl., ganglion cells of VIIIth nerve; Au. cart., cartilaginous auditory capsule; gen., cells of old generative tissue round brain and in auditory capsule; bl., blood-vessels Here there was a test of the truth of my theory ranking second only to the test of the median eyes; the strongest possible evidence of the truth of any theory is given when by its aid new and unexpected facts are brought to light. ”

“ 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. ”

“ Nevertheless, many of the animals of the depths have{30} organs to which by analogy an auditory function has been assigned. But it must not be forgotten that even in the highest land-vertebrates the ear has two functions. It is at once the organ of hearing and of balancing. Part of the internal ear is occupied with orientating the body. ”

“ As to the auditory apparatus itself, we see that the elaborate organ for hearing—the cochlea—has been evolved in the vertebrate phylum itself. In the lowest vertebrates the auditory apparatus tends more and more to resolve itself into a simple epithelial sac, the walls of which in places bear auditory hairs projecting into the sac, and in part form otoliths. Such a simple sac forms the early stage of the auditory vesicle in Ammocœtes, according to Shipley ”

“ 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. ”

“ Even in animals as high in the scale of life as fishes, there is a complete set of sense-organs—the muciparous canals, in the head and along the lateral line down the side, the function of which we can only guess. By some they are regarded as olfactory; by others, as fitted to respond to vibrations or shocks of greater wave-length than the auditory organ can appreciate ”

“ It is not the mere motion in the auditory organs which produces the sensation of sound; the nerve certainly perceives each movement in the ears, because none is possible apart from or without primary motion; only such a motion is no sound, but only a noise. What has been written in the tingling metal according to eternal laws, is transcribed or copied in the auditory nerve ”

“ If any one in the dark presses the ball of his eye, by applying his finger to the external corner of it, a luminous appearance is observed; and by a smart stroke on the eye great slashes of fire are perceived. (Newton's Optics.) So that when the arteries, that are near the auditory nerve, make stronger pulsations than usual, as in some fevers, an undulating sound is excited in the ears. Hence it is not the presence of the light and sound, but the motions of the organ, that are immediately necessary to constitute the perception or idea of light and sound. ”

“ It is sufficient to state here that this organ consists essentially of an arrangement of epithelial cells bearing hairs which are in communication with the terminal filaments of this portion of the auditory nerve, and that groups of these hairs pass through holes in a closely investing membrane, membrana reticularis, which may act as a damping apparatus, so as quickly to stop their movements. ”

“ This observation may throw some light on the medical treatment of deaf people; as it may be learnt from their dreams whether the auditory nerve be paralytic, or their deafness be owing to some defect of the external organ. ”

“ At the other end of the scale, on the other hand, slow pulsations may be appreciated—for example, by aquatic creatures—by means of what we term the auditory organs, in a way that is not analogous to the sensation of sound in us. It may be noted that auditory organs are dotted about the body somewhat promiscuously in the various invertebrates. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ In the lobster or crayfish the auditory organs are found at the base of the smaller feelers or antennules. They are little sacs formed by an infolding of the external integument (see Fig. 26, p. 259) . Beautifully feathered auditory hairs project into the sac along specialized ridges, and the sac in many cases contains grains of sand which play the part of otoliths. Hensen seems to have proved that shrimps collect the grains of sand and place them in the auditory sac for this purpose. The curious shrimp-like Mysis has two beautiful auditory sacs in its tail. ”

“ 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. ”

“ Certain small, hard bodies, free to move around, lie in the fluid of the bag. The ends of the auditory nerve of hearing are distributed around the wall of the sac, so that they are subjected to the blows of the little particles of calcareous sand, or otoconia, as they are called, whenever the fluid in the bags is disturbed. ”

“ It is a very common ear disorder. The opening into the ear is about an inch long, or a little more, and is separated from that part of the ear within, which is known as the middle ear, by the eardrum membrane. The drum membrane is a thin, skinlike membrane stretched tightly across the bottom of the external opening in the ear or auditory canal, and shuts it off completely from the middle ear within, and in this way protects the middle ear from the entrance of germs, dust, and water, but only secondarily aids hearing. ”

“ The shell or concha of the ear, a in the annexed diagram, conducts the sound into the curved auditory passage b, which is terminated by a thin membrane, the so-called tympanic membrane, e. This membrane is set in motion by the sound, and in its turn sets in motion a series of little bones of very peculiar formation, c. At the end of all is the labyrinth d. The labyrinth consists of a group of cavities filled with a liquid, in which the innumerable fibres of the nerve of hearing are imbedded. ”