PART III. THE INTERNAL EAR (Part 1)

PART III. THE INTERNAL EAR CHAPTER XVIII. ANATOMY OF THE INTERNAL EAR. Galen named the internal ear the labyrinth, although he did not attempt to describe its various parts. This name it continues to bear, although so much labor has been given to its exploration, that we now have the thread to guide us through its devious passages. Yet in our own time, a part of this internal ear — the cochlea — is still the subject of vig- orous research and heated discussion, and different views are yet entertained by competent authorities as to the true description of its component parts. I shall attempt to give the student such an account of its anatomy as shall serve as a basis for the study of its physiology and diseases, without entering into the discussion of the points still unsettled.* The internal ear may be conveniently studied by dividing it into the following parts : 1. The vestibule. 2. The semicircular canals. 3. The cochlea. 4. The auditory nerve. We shall first study the osseous envelope of these parts, and then consider their contents ; the latter being, of course, far more important. THE VESTIBULE. The vestibule is considered the essential part of the inter- nal ear by all authorities. A part answering to the vestibule * In compiling this anatomical sketch, the author has been at times com- pelled, in order to avoid inaccuracy of statement, to use the exact words of the writer whose work he has used. He has not inserted the quotation marks, but the authorities he has consulted will be found at the end of the chapter. The text-book of Henle has formed the basis of the description of the micro- scopic anatomy of the labyrinth. 462 VESTIBULE. is to be found in all animals in whom an auditory apparatus can be detected. It is the seat of the principal expansion of the auditory nerve upon the saccule, described on page 476. This saccule floats in the perilymph and communicates through that fluid with the membrane of the fenestra ovalis, and con- sequently Avith the air in the tympanic cavity. Horizontal Section through the Lower Half of the Left Ear. After a Photograph— Rudinger. Made from a preparation softened by hydrochloric acid and then hardened in alcohol. 1. Cartilaginous portion of the auditory canal, having a great anterior convexity. At figure 1 the posterior wall presses well into the canal, so that it is the narrowest at this point. 2. Cartilages of the anterior wall of the canal. 3. The osseous part of the canal. 4. Mem- brana tympani. 5. Cavity of the tympanum. 6. Stapes bone. 7. Stapedius muscle. 8. Section of facial nerve. 9. Tensor tympani muscle. 10. Auditory nerve. 11. Nerve of the cochlea. 12. Section of the cochlea. 13. Inferior nerve of the ampullm. 14. Section of the sacculus hemillipticus. 15. Sacculus liemisphoericus. 16. Section of membranous semicircular canal. The vestibule is an irregularly-shaped osseous cavity, the diameter of which from above downwards, as also from behind forwards, is about one-fifth of an inch. It is about one-tenth of an inch between its inner and outer wall. The semicircular canals open into it by five orifices behind the cochlea, by a single one in front. The fenestra ovalis is on its outer wall ; on its inner are several minute holes, making up the maculse cribrosae for the entrance of a portion of the auditory nerve from the internal auditory canal. At the posterior part of the VESTIBULE. 463 inner wall is the orifice of the aqueductus vestibuli, a fine canal penetrating the vestibule from the posterior surface of the petrous bone, and contains a tubular prolongation of the lining membrane of the vestibule, ending in the cranial cavity, between the layers of the dura mater. The Left Vestibule, with the Semicircular Canals, from an Adult, seen from within.— Riidinger. 1. The horizontal semicircular canal. 2. The upper semicircular caned. 3. The poste- rior semicircular canal. 4. A brisi is passed through the aqueductus vestibidi, and passes into the opening of two canals, and appears on the upper wall of the ves- tibule. 5. The mouths of the osseous am- pullae of upper* and horizontal semicircu- lar canals. 6. The opening of the lower ampulla of the posterior semicircular canal, below the numbers 6 and 1. 7. The lower opening, in which the bristle is seen, represents the opening Of the com- • two semicircular canals. The Vestibule.— After Riidinger. 1. TJie osseous lamina spiralis of the < beginning beloiv and posteriorly on the wall of the vestibule. 2. The scala tym- pani and the fenestra rotunda. 3. The scala vestibuli. 4. Fenestra oralis. 5. TJie posterior inferior wall of the lower ampulla, with the inferior macula cri- brosa, zvhich serves as a passage for the fibres of the vestibular nerve to the lower ampulla. 6. Fovea rotunda, or recessus hemisphavricus. In its centre are a num- ber of fine openings, the macula cribrosa media, through these the fibres of the middle branches of the vestibular nerve pass to the round saccule, which is the blind vestibular end of the scala vesti- bidi. 1. The upper portion of the recessus hemillipticus in which is the upper ma- cula cribrosa. 8. The lower portion of the recessus hemillipticus, which passes without any distinct dividing line into the semicircular canals. The maculae cribrosse on the inner wall of the vestibule are to be seen with the naked eye on the newly born, but in the adult they are only to be seen by means of the microscope. Henle describes four little groups, each having five openings, and each series of foramina make up what is known as a ma- cula cribrosa. Through the macula cribrosa superior, the nerves pass to the utricle and to the ampullae or flask-shaped openings of the anterior vertical and the horizontal semicircu- lar canals. The nerve fibres to the posterior semicircular 464 VESTIBULE. canals pass through the inferior macula cribrosa, and those to the saccule through the macula cribrosa media. Finally, 'through the fourth macula cribrosa passes the twig of the small branch of the cochlear nerve. The scala vestibuli of the cochlea begins on the anterior apex of the vestibule. The outer wall of the vestibule is interrupted by the fenestra ovalis, but it is so completely and smoothly closed by the base of the stapes bone, that the inner surface of this wall of the vestibule appears even. On the inner wall are two depressions, called respectively the recessus sphsericus and the recessus ellipticus. A minute elevation between them is called the crista vestibuli. Just above the recessus ellipticus opens the ampulla or flask-like orifice of the anterior vertical semi- circular canal. The two vertical canals open at the junction of the posterior and inner wall. On the same line, but a little higher in the middle of the posterior wall, is the posterior opening of the horizontal semicircular canal. The lower open- ing of the posterior vertical canal is in the angle formed by the posterior, lower, and inner wall of the vestibule. The an- terior ampulla of the horizontal canal lies on the outer wall between the fenestra ovalis and the ampulla of the anterior vertical semicircular canal. THE SEMICIRCULAR CANALS. The semicircular canals are half-elliptical or C-shaped canals which proceed from the vestibule and return to it again. They are three in number. The horizontal lies with its convex- ity directed laterally. The other two are vertical in position, forming a right-angle with each other. The two openings of the anterior vertical semicircular canal are near each other and at about the same height. The openings of the posterior vertical canals are above each other. The horizontal canal is surrounded, as it were, by the two vertical ones. There are considerable variations in different individuals, •according to Henle, in the length and curvature of the semi- circular canals, yet the general shape of these parts remains the same. The length of the anterior vertical canal, measured on the SEMICIRCULAR CANALS. 465 convex border, with the ampulla and the common eras, is about 20 millimetres ; that of the posterior is 22mm., of the horizon- P, %j^/ Osseous OocMea and Semicircular Canals, with Stapes Bone. Left Ear of Adult. —After Rudinger. Bight Osseous Vestibule, Semicircular Ca- nals, Cochlea, and Ossicula Auditus of Newly-born.— After Rudinger. tal 5mm. The part common (canalis communis) to the two vertical canals is from 2 to 3 millimetres in length. The diameter in a grown man varies from 1.3 to 1.7 millimetres. Wharton Jones makes their caliber about one-twentieth of an inch in a direction from the concavity to the convexity of their curve. The Bight Osseous Labyrinth of a newly-born Subject opened on its Posterior Surface.— After Rudinger. 1. Cochlear fenestra. 2. The osseous spiral. 3. The osseous spiral canal of the cochlea— canalis spiralis cochleee— divided by the spiral into two parts, scalce, or stairways, the lower the scala tympani, the vpper the scala vestibuli. 4. The basis of the internal auditory canal, ivith the entrance to the Fallopian canal and the maculae cribrosce. The latter recei.ve the fibres of the auditory nerve, and the vessels entering with it into the labyrinth. 5. Tlie osseous vestibule, opened on its posterior wall. 6. The posterior semicircular canal. 7. The upper semicircular canal. 9. Horizontal semicircular canal. Since the semicircular canals all open at both ends into the vestibule, there would be six orifices were not one of the orifices common to two of the canals. There are, conse- quently, five. These openings are called ampullae (flasks) from 30 4G6 SEMICIRCULAR CANALS. their shape, and are more than twice the diameter of the tubes. The inner extremity of the superior vertical canal has a common opening into the vestibule with the posterior vertical. According to Henle,* in the later years of life the semi- circular canals increase in length ; the horizontal canal in- creases the most, and the anterior vertical the least. The canals also increase very slightly in width ; about 0.7mm. according to Hyrtl. The functions of the semicircular canals, according to the experiments of Flourensf and Goltz,;J: are to preserve the equi- librium of the head, and consequently of the body. Goltz believes that the semicircular canals are not, so to speak, essential to the function of hearing. THE COCHLEA. This part of the internal ear is so named from its resem- blance to a common snail ; a resemblance which is very marked. It is one of the most remarkable instances in the Section through the Apex of the Eight Osseous Cochlea, parallel with the base. &. Lower surf ace of the section. t>. Upper surface of the section, c, . Canal of facial nerve. whole body of the compact packing of very important parts. Wharton Jones § remarks of its function, that the presence of a cochlea is evidence of a very advanced condition of the organ * Lelirbucli, p. 762. t Von Troltsch, second American edition, p. 505. % Knapp, Archives of Ophthalmology and Otology, vol. ii., No. 1. Brun- ner, ibid., 1. c. § Cyclopedia of Anatomy and Physiology, p. 569. COCHLEA. 467 of hearing ; "beyond tins we can arrive at no definite conclu- sion in the present state of our knowledge." Recent investiga- tions, however, render it safe to say that one of the functions of the cochlea is to discriminate between tones. The fibres of Corti connected to the cells that are to be described, beiDg the keys of an instrument of more than a thousand strings. Section of the Tempoi'al Bone, vertical to Us Long Axis— Posterior Surface of the Section. —After Rente. m. Meatus auditorius internus. c, t, t. Canal of the tensor tympani muscle, s. m. Canalis spiralis modioli. The osseous cochlea lies in front of the vestibule, and be- hind the carotid canal, and forms the promontory by pressing out, as it were, the bone towards the tympanic cavity. Inwards it strikes upon the blind end of the internal auditory canal. The cochlea is aptly compared to a tube tapering towards one extremity where it ends in a cul-de-sac, and which is coiled like the shell of a snail round an axis or central pillar. Then we must suppose this tube divided into passages by a thin parti- tion running throughout its length, and spirally around its axis. The tube of which the cochlea is formed — the canalis spiralis cochleae, is about an inch and a half long, about one- tenth of an inch in diameter at its commencement, and about 468 COCHLEA. one-twentieth at its termination. It makes two turns and a half turn, in a direction from below upwards, from left to right in the right ear, and from right to left in the left ear. The apex of the coil is directed forwards and outwards. The base of the spiral tube runs into the vestibule. The cul-de-sac at the apex forms a kind of vaulted roof called the cupola. ac Is Osseous Cochlea (Right) of the Newly-born, opened from the Outer Surface.— After Henle. b, v. Scala vestibuli. s, t. Scala tympani. 1, s. Lamina spiralis, c, s. Crista semihinaris. a, c. Inner opening of the aqueductus cochlea., c, m. Canalis centralis, s, m. Canalis spiralis modioli. The first turn of the cochlea has a circular sweep of a quarter of an inch, and is wider than the rest. It is separated from the second turn by a soft bony substance, which extends a little way between the second and third. The axis is com- posed of the internal walls of the tube of the cochlea and the central space circumscribed by their turns, in which space are the filaments of the cochlear nerve running in small bony canals. The axis is about one-seventh of an inch in thickness at the first turn, but it becomes thinner from the second turn, on to its termination. The axis terminates within the last half coil or cupola, in a delicate bony lamella, which resembles the half of a funnel, divided longitudinally, and called the infundibu- lum (funnel). "Wharton Jones compares the appearance of the axis of the cochlea after the outer walls have been removed, COCHLEA. 409 to the ordinary pictorial representations of the tower of Babel. The cavity of the cochlea is divided into two parts or pas- sages, called scala', by a thin osseous and membranous spiral lamina, lamina spiralis ossea. The lower one communicates with the cavity of the tympanum through the fenestra rotunda, the upper with the recessus heinisphsericus (see Fig. 94, of the vestibule). The former space is therefore called the scala tym- pani, the latter, scala vestibuli. In the scala tympani, just above the membrana tympani secondaria, which closes the Bight Osseous Cochlea, opened anteriorly. m. Modiolus. 1. s. Lamina spiralis, h. Hamulus, f. c. Fenestra cochlea. +. Section of tht middle wall of the coch'ea. tt. Its upper extremity, m. d. Modiolus. fenestra rotunda, is an opening called the entrance of the aqueduct to the cochlea. The two scalse communicate at the apex of the cochlea by a common opening called the helico- trema (a twisted foramen). This communication exists in con- sequence of the want of a lamina spiralis in the last half coil of the canal. Two very small canals called aqueducts open by one ex tremity into the labyrinth, and by the other on the surface of the petrous portion of the temporal bone. One opens into 470 COCHLEA. the vestibule, and has already been alluded to, and is called the aqueductus vestibuli ; the other enters into the tympanic scala of the cochlea, and is called the aqueductus cochleae. The length of the aqueduct of the vestibule is about one-third of an inch ; that of the aqueduct of the cochlea is about one- quarter of an inch. The aqueduct of the vestibule begins by a groove immediately below and in front of the opening com- mon to the two vertical semicircular canals. From this the aque- duct turns itself around the inner wall of the common canal, and runs downwards and backwards. It gradually widens and opens under a thin osseous projection, seen a little be- hind the middle of the posterior and inner surface of the pe- trous bone, just above the jugular fossa. From the fossa there is a narrow groove running to the opening of the aqueduct. Apex of the Left Osseous Cochlea opened to shore the End of the Lamina Spiralis. After Eenle. The aqueduct of the cochlea begins by a very small open- ing in the lower wall of the scala tympani, immediately above the fenestra rotunda. It passes downwards, inwards, and forwards in the inner wall of the jugular fossa, and opens at the bottom of a triangular depression, situated towards the middle of the edge which limits the inner and inferior surfaces of the petrous bone, and below the internal auditory canal. THE MEMBRANOUS LABYRINTH. TJie Auditory Nerve (Nervus acusticus). — The auditory nerve, or portio mollis (soft part of the 7th nerve), is the COCHLEA. 471 nerve of the sense of hearing, and is distributed exclusively to the internal ear. The auditory nerve arises from numer- ous white lines, or striae (linse transversa), which come from the posterior median fissure in the anterior wall, or floor of the fourth ventricle. It is also connected with the gray matter of the medulla. The roots of the nerve are con- nected, on the under surface of the middle peduncle, with the gray substance of the cerebellum, with the flocculus, and with the gray matter at the border of the calamus scriptorus. The nerve winds around the restiform body, from which it receives fibres, and passes forward across the posterior border of the Expansion of the Eight Cochlear Nerve, seen from the Base of the Cochlea, from a Laby- rinth softened in Hydrochloric Acid. After He.rde.