- TJie branches entering through foramina. 2. Twig passing into the modiolus. 3. Neticork in Hie osseous lamina spiralis. 4. Network on its border. L, T. Labium tympanicum. Z, I. Zona interna. Z, E. Zona externa of the membrana basilans. L, S. Ligamen- tum spirale. crus cerebelli, in the company with the portio dura, or facial nerve, from which it is partly separated by a small artery. It then passes into the meatus auclitorius in company with the facial nerve. At the bottom of the internal auditory pas- sage, in the petrous portion of the temporal bone, it divides into two branches, which are distributed to the cochlea, vesti- bule, and semicircular canals, and are called the cochlear and vestibular branches. 472 COCHLEA. The auditory nerve is remarkable for the delicacy of its structure, which caused the older anatomists to give it the name of portio mollis. It has only a very thin neurilemma. The cochlear nerve gives off a small branch, which passes to the vestibular extremity of the ductus cochlearis, and through the fourth macula cribrosa to the partition wall of the two saccules in the vestibule. From the trunk of the nerve a number of fine twigs arise, which pass through fora- mina direct to the lamina spiralis of the lower coil of the cochlea. The remainder of the cochlear nerve enters the modiolus, and is divided into anastomotic divisions. The fibres becomes separated from the trunk in a line correspond- ing to the course of the canaiis spiralis modioli, and permeate this canal. Here, by the addition of ganglion cells, they be- come gangliose sfcrise, and finally end, at almost a right-angle to the trunk, in the osseous lamina spiralis. The vestibular nerve, after a slight gangliose expansion, divides into three branches. The upper passes through the macula cribrosa superior, and ends with three branches on the utricle, and on the ampulla of the upper vertical and of the horizontal semicircular canal. A middle branch passes through the middle macula cribrosa to the saccule, while the lower passes through its own osseous canal to the ampulla of the lower vertical semicircular canal, and its fascicles are loosely held together. Todd and Bowman regard it as a direct prolongation of the white matter of the brain. In the internal auditory canal, the portio mollis forms a connection with the portio dura by means of a few fascicles of fibres, which constitute what Wrisberg called the " portio intermedia." It is not decided whether the connecting link proceeds from the auditory to the facial nerve, or from the latter to the former. Todd and Bowman believe it probable that the facial nerve sends some filaments to the blood-vessels of the labyrinth and the muscular structure of the internal ear. PERIOSTEUM OF THE LABYRINTH. The periosteum that covers the walls of the osseous canal is, with the exception of that on the outer wall of the cochlea, 473 very delicate. Henle* compares the periosteum of the laby- rinth to one of the parts of the choroid, because it is strewn with nucleated pigment cells. There are also calcareous deposits. It is very difficult, according to Henle, to separate HP Periosteum of the Labyrinth. After Henle. Periosteum of the Outer Wall of the Cochlea. After Henle. the periosteum of the labyrinth, without also detaching bits of bone. The periosteum is abundantly supplied with blood- UTRICLE AND MEMBRANOUS SEMICIRCULAR CANALS. The utricle is an elliptical tube, situated on the median wall of the vestibule. Its longest diameter corresponds to the Utricle and Membranous Semicircular Canals of the Left Side. * Lehrbucli, p. 774. 474 MEMBRANOUS SEMICIRCULAR CANALS. height of the vestibule. By means of a fine vascular and nervous network, and a very delicate connective tissue, it is fastened to the recessus ellipticus of the vestibule. The membranous semicircular canals are but the lining of the osseous canals, and, of course, of the same shape. The membranous canals open into the utriculus with five openings, just as do the osseous tubes in the vestibule. At the am- Wall of Membranous Semicircular Canals. 1. Membrana propria, artificially separated edge. 2. Epithelium. pullse, the membranous canal fills up the osseous very com- pletely ; but there is some space between the other parts. The walls of these structures are transparent, as clear as iwater, and of great delicacy. After the endolymph is re- moved, they fall together and arrange themselves in rigid folds. There is, however, a point that is firmer, called the macula acustica, situated on the median wall of the utricle, where a twig of the auditory nerve reaches this wall. The MEMBRANOUS SEMICIRCULAR CANALS. 475 portion of the ampulla that contains the termination of the nerve, and which is detected by the naked eye as a whitish yellow spot, is also of firmer consistency. This point is called the crista acustica by Max Schultze. It comprises about one- third of the wall of the ampulla. It is sometimes surrounded by a pigmented line. The wall of the membranous semicircular canals is from 0.02mm. to 0.03mm. in thickness, and is composed of various layers. Q. %& A Piece of the Wall of the Utricle, with the Otoliths. After Herde. The membrana propria is of reticulate and nuclear fibrous tissue, of which the periosteum also consists. It is perforated by blood-vessels. There is a basal membrane next the mem- brana propria, and on the inner surface pavement epithelium. The macula and crista acustica that have been mentioned, are thickenings of the membrana propria caused by the min- gling of connective tissue, and the ending of the nerves. The otolith of the utriculus of the mammalia is a smooth, irregularly demarcated and uneven mass of chalky white pow- der. It was called otoconia by Breschet, ear-sand by Lincke, and ear-crystal by Huschke. The powder is held together by an almost mucous substance. The powder consists of crystals of varying shape and size. The largest are only 0.012mm. long and 0.008mm. broad. They are too small to allow the 476 DUCTUS COCHLEARIS. crystal form to be recognized. The material of which otoliths is composed is carbonate of lime. Henle says it is unknown how the otolith is fastened on to the wall of the utricle. SACCULE. The saccule is of the shape of a broad flask with a narrow neck. It lies in the recessus sphsericus of the vestibule. The neck of this bottle or flask proceeds from the lower wall, downwards and backwards, and sinks into the upper wall of the vestibular end of the ductus cochlearis, at nearly a right angle, so that a blind sac is formed at the junction of the two parts. Henle compares it to the passage of the oesophagus into the stomach, and of the small intestine into the ccecum. THE DUCTUS COCHLEARIS. (Lamina Spiralis Membranacea of THE OLD ANATOMISTS.) The ductus cochlearis begins with the blind sac in the vestibule that has been described, and passes through the whole cochlea to the apex, in which it ends again as a blind sac. The lower end rests in the recessus cochlearis, and the upper in the cul-de-sac of the cupola. The ductus cochlearis is attached on one side to the lamina spiralis ossea, and on the other to the outer wall of the osseous cochlear canal. On a transverse section the ductus cochlearis is seen to be trian- gular in shape, and has, of course, three walls, or sides. Two of these walls diverge from the edges of the lamina spiralis, and the other corresponds to the portion of the cochlear wall between which the insertion of the two others is made. The lower wall of the ductus cochlearis, which is turned towards the scala tympani is called the tympanal ; the upper, which sepa- rates the ductus cochlearis from the scala vestibuli, is called the vestibular wall. On the osseous border of the lamina spiralis is a soft struc- ture, only to be seen in the uninjured specimen of the cochlea, which lengthens the lamina spiralis towards the caliber of the ductus cochlearis. It is called by Henle the limbus laminae spi- ralis. (See Fig. 106.) It is developed from the periosteum of DUCTUS COCHLEARIS. 477 the lamina spiralis. This structure gradually decreases in breadth and height from the base to the apex of the cochlea. The edge of the osseous lamina recedes more and more at the same time from the free border of the limbus. This free bor- der becomes a furrow, called by Huschke the sulcus spiralis, having, of course, two lips. The upper lip is the labium ves- tibnlare ; the lower, the labium tympanicum. The vestibular wall of the ductus cochlearis passes off from the upper sur- face of the lamina spiralis in a line nearly corresponding to the inner attachment of the limbus laniinse spiralis, so that the latter is almost completely drawn into the ductus cochlearis. Transverse Section of a Cochlear Spiral, from a Cochlea softened in Hydrochloric Acid. After Henle. The dotted lines indicate sections of the membrana tectoria and the auditory rods. L s. Lamina spiralis. L 1 s. Limbus laminae spiralis. S v. Scala vestibule. S t. Scala tympani. D c. Ductus cochleans. L s v. Ligamentum spirale. V. Membrana vestibularis, b. Membrana basilaris. e. Outer wall of ductus cochlearis. . Bulging of membrana basilaris. The upper surface of the vestibular lip of the limbus lamina spiralis is covered by striae, which on front view resemble the anterior surface of the incisor teeth, and hence Huschke calls them the auditory teeth. These furrows, or stria?, are filled by small rounded cells. Their number may run as high as 2,500. The limbus is composed of connective tissue, running in a radiate direction in the furrows, or striae ; beneath these furrows the connective tissue is reticulate. Henle compares the labium vestibulare to a roof over the sulcus spiralis, and the labium tympanicum to a floor. Within 478 DUCTUS COCHLEARIS. the labium tympanicum run very fine nerve fibres from the tissue of the auditory nerve to the ductus cochlearis. The labium tympanicum consists of two layers, which include the nerve fibres between them, and then unite bej^ond it in a sharp border, from which the membrana basilaris proceeds. This membrana basilaris, according to Henle, appears as a process of the upper layer of the labium tympanicum. There is, however, a structure between them, which corresponds to the periphery of the nerve bundles. On the outer portion of the upper surface of the labium tympanicum are four radiate striae, which Henle considers as marks of the nerve bundles running on the lower surface of this layer. At the periphery of these there are other open- ings. The membrana vestibularis is attached to the beginning of the upper border of the ridge of the spiral and to the outer cochlear wall. There are three layers in this membrane, which by Kolliker is called Reissner's membrane. It is epi- thelial tissue, which in embryonal life seizes upon the vestibu- lar side of the cochlear canal. This membrane has a number of blood-vessels. The membrana basilaris is well shown in Fig. 106, and being the part upon which rests the organ of Corti, has at- tracted very much attention from anatomists. It is a con- tinuation of the labium tympanicum. It gradually increases in breadth from the base to the apex, in the same proportion that the lamina spiralis with its limbus decreases in size. Its breadth in the newly-born, in the middle of the first turn or coil of the cochlea, is 0.17mm. ; at the end of the second, 0.45. This space is divided into two parts or zones. The inner was called by Kolliker, the habenula tectu, and the outer by Todd and Bowman the zona pectinata. Henle gives the two parts the simple names of inner and outer zone. On the inner zone is found the structures making up what is known as Corti's organ, from their discoverer, Marchese Corti. The outer zone is rather broader than the inner. * Corti was formerly prosector to Professor Joseph Hyrtl, and made the first exact microscopic examination of the lamina spiralis ossea, and mem- branacea. DUCTUS COCHLEABIS. 479 The basis of the membrana basilaris is a structureless membrane. On the outer zone especially are peculiar knobby points. Upon this structureless membrane are the parts known in their totality as Corti's organ. The fibres of this structure are arranged along the whole length of the mem- brana basilaris. There are spaces between them, so that they have a certain resemblance to the keys of a piano. The ligamentum spirale is the means of attaching the membrana basilaris to the outer wall of the cochlear canal. The fibres of which it is composed are like those of perios- teum. The cavity of the ductus cochlearis is divided into parts by a membrane running parallel to the membrana basilaris. (See Fig. 106.) The upper part is filled with endolymph, the lower contains what Henle calls the terminal auditory apparatus. The membrane which divides the ductus cochlearis into two parts is called the membrana tectoria by Claudius, but Corti's membrane by Kolliker. The membrana tectoria is divided into three zones. The middle zone is the denser ; the inner is structureless and has numerous openings. The outer zone is made up of a very fine and friable network. It is probable, according to Henle, that the membrana tectoria is firmly fas- tened, and that it is not possible for it to press closely upon the parts covered by it. TERMINAL AUDITORY APPARATUS. Henle terms the important structures of the lower chamber of the ductus cochlearis the terminal auditory apparatus. They consist of rod-like bodies, a perforated membrane, and nuclear cells of various shapes. A fourth part, whose exist- ence Henle thinks is doubtful, are fibres, in which connective tissue and the ultimate fibres of the auditory nerve are found. AUDITORY RODS. The most important, physiologically speaking, of this termi- nal apparatus are the auditory rods, called also Corti's teeth, 480 COKTI8 ORGAN. or Corti's fibres. They are arranged in regular order, very like the cords, hammers, or keys of a piano. It is probably their vibrations that cause us to perceive what we call tones. There From the Terminal Auditory Apparatus of a Cat. After Eenle. i. Outer ends of 'the inner fibres, e. Outer fibres. 3. Outer covering cells. 4. Epithelial cells. are two rows of these fibres, an inner and an outer. The inner rods arise from the membrana basilaris, on which their inter- nal extremities are fastened, more or less abruptly, towards the membrana tectoria, without, however, being united to the latter. The outer rods or fibres join, with their inner extrem- Profile View of Outer and Inner Bods. B. Membrana basilaris (b), with the terminal nerve fibres (n) and the inner and outer rods, i, e. 1. Inner. 2. Outer floor cells. 4. Attachment of the roof cells. Vc. Epithelium. ities, the outer end of the inner fibres. Their external ter- minations rest on the membrana basilaris. There are two varieties of the inner row of fibres or rods ; one is smooth and elliptical in shape, the other cylindrical and broader at each end. The outer row of rods is cylindrical in shape, and they stand at a greater distance apart than the inner. They have MEMBRANA RETICULARIS. 481 a tortuous course sometimes, like the letter S. The inner row of fibres is always shorter than the outer. They join together and form a roof over the inner zone of the membrana basil- aris. The base of this roof is 0.1mm. in breadth. The struc- ture of these rods, as shown by the action of reagents, is a tissue as hard as cartilage. Henle calls the terminations of the two rows of rods upon the membrana basilaris, the lower extremities ; and the extrem- ities which join to make the roof, the upper extremities. MEMBRANA RETICULARIS. This is the second of the component parts of the terminal auditory apparatus. It arises from the articulation of the rods or fibres, and extends to the outer wall of the cochlea parallel to the lamina basilaris. It is supposed to be a liga- ment to bind the rods together. The tissue of the lamina reticularis is not less firm than that of the rods, but it is delicate. AUDITORY CELLS. These are cylindrical and spherical elements which are called cells, because they contain nuclei. They may be con- sidered, according to Henle, as epithelial or ganglion cells. Some of these cells are called hair cells, stachel cells, and in them are probably the terminal filaments of the cochlear nerve. Henle divides these cells which are not epithelial into two classes : the roof-cells (deck-zellen), and floor-cells (boden- zellen). Gottstein calls these hair cells. The roof-cells are found on the convex side of the roof formed by the union of the two rows of arches. The floor cells are found at the angle which the base of each rod makes with the membrana basilaris. Henle divides the roof cells into three varieties : an inner, an outer, and a lower outer. After the exit of the nerve fibres from the canals of the 31 482 AUDITORY CELLS. labium tympanicum, the bundles of nerve fibres take two different directions. One part maintains the original radiate direction, the other proceeds spirally. These fibres take dif- ferent directions with reference to the fibres of Corti ; at the p -h Diagrammatic Representation of the Terminal Auditory Apparatus. After Henle. Teeth of the labium vestibulare, b. Epithelial cells of the labium tympanicum. c. <?pere- ings of these cells. A. Inner rods. e. Outer rods. f. Connecting fibres, g, h, i, k, k'. .Firsi to fourth bundle of spiral nerve fibres. 1. Radiate bundles, m. Cpper «e»t« roo/ ce/te. n.- Epithelial cells, o. Supporting fibres of rods. p. Eadiate nerve fibres upon membrana basilaris. q. JVerw /jore running
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