The membranous semicircular canals resemble in form and arrangement the osseous canals which inclose them, but are only one third of the diameter of the latter. The ultimate filaments of the auditory nerve (q.v.) mainly go to the utriculus, to the sac culus; and to the ampulla of the canals.
The membranous labyrinth is filled by a fluid which is termed the endolymph; and in certain spots, especially at the terminations of the vestibular nerves, we observe, both in man and the lower animals, calcareous matter either in a powdered or solid form. In man and mammals generally, and in birds and reptiles, it occurs as a powder, and is termed otoconia or ear-powder, and it always consists of carbonate of lime.
We now proceed to consider the different functions or offices of the various parts of the organs of hearing.
1. Of the External Ear.—A true auricle only exists in the mammalia, and in this class it varies from little more than an irregularly-shaped cartilaginous disk, with little or no motion, as in man and the quadrumana, to an elongated funnel-shaped ear-trumpet, movable in all directions by numerous large muscles, as in the horse, the ass, and the bat.
The mode in which we sec it en:Cloyed by those animals in which it is highly developed, sufficiently indicates that its main function is to collect and concentrate the sounds which fall upon it. But the experimental investigations of Savart, with an apparatus constructed to resemble the tympanic membrane and the external apparatus, show that these parts are also adapted to enter into vibrations in unison with those of the air; and he suggested that the human auricle, by the various direc tions of different parts of its surface, could always present to the air a certain number of parts whose direction is at right angles with that of the molecular movement of that fluid, and therefore is the most favorable position for entering into vibrations with it.
2. Of the Tympanum and its Contents.—Savart's experiments show that the membrane of the tympanum is thrown into vibration by the air, and that it always executes vibra tions equal in number to those of the sonorous body which excites the oscillations in the air. He further ascertained that the malleus participates in the oscillations of the tympanic membrane, and that these vibrations are propagated to the incus and stapes, and thus to the membrane of the fenestra oxalis. The malleus has further the office of
regulating, through the tensor tympani muscle, the tension of the tympanic membrane; and to allow of the motion necessary for this purpose, we find movable joints between it and the incus, and again between the latter bone and the stapes. The contraction of the stapedius muscle similarly modifies the tension of the membrane of the fenestra oxalis; and as compression exercised on this membrane extends to the perilymph, and is propagated through it to the fenestra rotunda, the tension of the membrane of the latter opening is also influenced by the muscle in question. The ineus is much more limited in its motions than either of the other bones, and its use seems to be to complete the chain of ossicles in such a manner as to prevent any sudden or violent tension of the membranes, such as we can easily conceive might occur, if the conductor between the membranes were a single bone. The presence of air in the tympanic cavity serves a. double purpose: in the first place, it preserves a uniform temperature on the outer sur faces of the fenestral membranes, and thus supports a fixed elasticity in them, which would not be the case if they were freely exposed to ordinary atmospheric changes; and secondly, the action of the chain of ossicles as conductors of sound is materially increased by their being completely surrounded by air, as is obvious from the first prin ciples of acoustics.
3. Of the Labyrinth. —Sound is conducted to the labyrinth in three ways• first, by the chain of bones; secondly, by the air in the tympanic cavity; and thirdly, through the bones of the head.
Muller has shown, by very ingenious experiments on an apparatus constructed to resemble. on a large scale, the middle and internal E., that while the air in the tympa num conducts sound to the cochlea, through the fenestra rotunda, the chain of bones forms a much better conductor of it to the vestibule, through the fenestra oxalis (see the chapter on hearing in his Physiology). Hence, we infer that the vestibule is adapted to receive sounds from the membrane of the tympanum and the external E., while the cochlea, on the other hand, as its structure and connections indicate, may be regarded as that part of the labyrinth which is specially affected by sounds communi cated through the bones of the head.