Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/953

Rh EAR.] ANATOMY 893 chain of bones across the tympanic cavity to the labyrinth. The pressure of the vibrations on the tympanic membrane forces that membrane inwards, so that its inner surface presses on the handle of the malleus, the effect of which is to rotate the hammer about its axis; but by the ligamentous attachment of the malleus to the tympanic walls and to the incus, and, as Helmholtz has shown, by the interlocking of cog-like processes connected with the articular surfaces of the two bones, the range of movement is so limited that the pressure on the malleus is transmitted through the incus upon the stapes, which presses, therefore, on the mem brane of the fenestra ovalis, so that the movements of the membrana tympani are thus transmitted to fluid within the labyrinth. The tensor tympani muscle tightens the tympanic membrane by drawing the handle of the malleus inwards, and still further adapts the structures for the transmission of sound- vibrations. An antagonistic muscle, the laxator tympani, has also been described. There is Borne difficulty in determining the action of the stapedius, but if, as is probable, it draws the stapes from the fenestra ovalis, it will diminish the pressure of the chain of bones on that membrane. iternal The Internal Ear, named the Labyrinth, from its com- x&amp;gt; plex construction, is the part of the auditory apparatus in which the nerve of hearing is distributed, and where the peripheral end-organs are situated. It is imbedded in the petrous bone, and is divided into three parts, viz., vestibule, semicircular canals, and cochlea, each of which consists of an osseous and a membranous portion (PI. XIX. figs. 8, 9, 10). The Vestibule lies immediately internal to the tympanum, between it and the bottom of the internal auditory meatus ; behind it are the semicircular canals, and in front is situated the cochlea. It is the part of the labyrinth which first appears in animals, and is therefore the most constant part of the organ. In the myxinoid fishes a single semicircular canal is superadded to the vestibule, in the lamprey two canals, but in other fishes and in the higher vertebrates three canals exist. In amphibia, reptiles, and birds the cochlea is small and rudimentary in comparison with its develop ment in mammals. The osseous vestibule is an ovoid chamber about ^-th inch in diameter. In its outer or tympanic wall is the fenestra ovalis ; in its inner are small auditory foramina, which transmit from the internal meatus the vestibular branches of the auditory nerve ; behind these holes is the opening of a minute canal, the aqueductus vestibuli; its anterior wall communicates with the scala vestibuli of the cochlea, and into its posterior wall open the five orifices of the three semicircular canals. The Semicircular Canals are named superior, posterior, and external. The superior and posterior are sometimes called the vertical canals, and the external the horizontal canal, but, as Crum Brown has shown, the superior and posterior lie in planes equally inclined to the mesial plane of the head, and the external is in a plane at right angles to the mesial plane. Further, the canals in the two ears have definite relations to each other; for whilst the superior canal of each ear is nearly parallel to the posterior canal of the other, the external canals in both ears lie nearly i.n the same plane. The canals are bent, forming nearly f ds of a circle, and would have had six openings into the vestibule had not the contiguous ends of the superior and posterior blended together to open by a common orifice. The opposite end of each of these canals and the outer end of the external canal dilate close to the vestibule to twice their usual diameter, and form an ampulla. The osseous vestibule and semicircular canals are lined by a periosteum invested by a tessellated endothelium, and contain a little fluid, the perilympJi, In this fluid the membranous laby rinth is suspended. The membranous vestibule is formed of two small sac-like dilatations, the walls of which are directly continuous with each other, though the cavities are separated by an inter mediate partition. The upper and posterior dilatation, named utriculus, is larger than the lower and anterior, named sacculus. The sacculus is continuous with the ductus cochlearis of the membranous cochlea, and both sacculus and utriculus communicate by slender tubes with a short diverticulum lodged in the aqueductus vestibuli, to which the name of ductus vestibuli may be given. The membranous semicircular canals are about ^d the diameter of the osseous. Their walls are continuous with that of the utriculus, and they open by five orifices into it. Each has an ampulla within the ampulla of the osseous canal. Both the sacculus and utriculus are in places attached to the periosteal linings of the osseous vestibule, and delicate ligamentous bands connect the membranous semicircular canals to the periosteal lining of the tubes in which they are con tained. The wall of the mem branous vestibule and canals con sists of a delicate fibrous mem brane lined by a tessellated endo thelium. The inner part of this membrane has a vitreous or hya line lustre, and gives origin in the canals to short papilla3 which pro ject into the lumen. The mem branous vestibule and canals are distended with the fluid endo- lymph. The sacculus, utriculun, and ampullse are specially modified in connection with the peripheral termination of the vestibular branches of the auditory nerve. The membranous wall forms in each of these dilatations a project ing ridge, the crista acoiistica, to which calcareous particles, the otoliths, which may be either amorphous or crystalline, are adherent. The endothelial investment of the crista is elongated into columnar cells, and intercalated between them are fusiform cells, the auditory cells, each of which, as Schultze and other observers have described, possesses a peripheral and a central process. The peripheral process projects beyond the plane of the free surface of the endo thelium into the endolymph as the auditory hair, whilst the central process extends into the sub-endothelial tissue, where the nerve plexus belonging to the terminal branches of the auditory nerve ramifies, and with which it is probably continuous. These auditory cells are, therefore, the peri pheral end-organs of the vestibular branches of the auditory nerve, and their general arrangement is not unlike that of the olfactory cells of the nose. The Cochlea is by far the most complex part of the labyrinth. It is about th inch long, and shaped like the shell of a common snail; its base lies near the internal meatus, and its apex is directed outwards. The osse ous cochlea is a tube Wound Spirally two Fie. 82. Diagram of the membranous labyrinth. onrl nlnlf ti rnna vnrm/l 1JC ductus cochlearis; dr, ductus reunicns; S, succulus; U, utriculus; dv, ductus vestibuli; a Central pillar or SC, semicircular canals. (After Wa!deyer.) modiolus. Both the pillar and the tube diminish rapidly in diameter from the base to the apex of the cochlea. The tube is imperfectly divided into two passages by a FIG, 81. el, columnar cells covering the crista acoustica ; p, peripheral, and c, central processes of auditory cells , n, nerve flbr-i s. (After Riid- inger.) DC