Paper - On the proportions, development and attachment of the tectorial membrane (1915)
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Hardesty I. On the proportions, development and attachment of the tectorial membrane. (1915) Amer. J Anat. 18(1): 1-70.
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On the Proportions, Development and attachment of the Tectorial Membrane
Irving Hardesty
From the Department of Anatomy, Tulane University of Louisiana
Eleven Figures
Introduction
Some years ago the writer (Hardesty '08) made an attempt to present something of the actual nature of the mammalian tectorial membrane based chiefly upon the study of teased fresh and unshrunken specimens. After learning something of the nature, shape and proportions of this structure in the fresh condition as compared with its appearance in the more usually employed sections of the cochlea, the writer became convinced that of the very numerous observations upon the membrane previously published practically all had been based upon the various more or less abnormal, rather than normal, appearances of it produced by the methods of its preparation for study. Experiments as to the action of the commonly used reagents upon the fresh tectorial membrane showed that under the usual treatment with alcohol in dehydration and the action of clearing reagents it suffers far more shrinkage and distortion than any other structure in the cochlea and perhaps in the entire body. This result of dehydration and clearing was very evident even after the action of fixing fluids which themselves were found to produce but little distortion. For this reason it was concluded that the usual sections of different embedded cochlea, and often different sections of the same cochlea, show the membrane, even in a given locality of its coil, varying greatly in its shape, size and in its position over the spiral organ (of Corti). The hope then suggested itself that something might be contributed to the knowledge of the actual character, shape and proportions of the tectorial membrane by studying it in the fresh condition, by comparing the fresh with teased out membranes acted upon bj' various fixing fluids, and by experimenting with methods of embedding for sections of the cochlea in which more normal appearances of the membrane might be retained. Cochleae of pigs were used wholly in that investigation, almost entirel> from pigs at or near 'term.' Those of one young suckling pig were obtained. The paper made no attempt to cover the process of the development of the tectorial membrane further than to give a brief review of the various publications bearing upon it and then considered as already having well covered the process, and it gave one drawing showing one stage of the development for use in describing the assumed process by which its final structure is acquired and its adult position with reference to the spiral organ is attained.
Since the publication of the above paper dealing with the membrane in young animals, the writer has been occupied in an attempt to construct a model of the auditory apparatus in which the proportions, position and environment of the tectorial membrane in the cochlea are simulated and by which its behavior when subjected to the energy imparted to it by sound waves may be illustrated. In the construction of this model, a further detailed stud}^ of the proportions and position of the actual tectorial membrane seemed necessary, and in this study it became evident that the observations should be carried to the membranes of adult animals rather than confined to those in the cochleae of embryos and fetuses. Since several of the results of this further study seem of interest, though some of them are confirmatory^ of results previously published, and since relatively so few observations upon the adult mammalian cochlea have been recorded, it is the purpose of the present paper to give certain of the findings of a somewhat detailed study of the spiral organ of the adult mammal. The shape, proportions, position and attachment of its tectorial membrane are especialh^ considered. Toward the end of the study, certain points in the question of the attachment and position of the adult membrane and the structure of its surfaces arose in comparing the findings with those recorded for the membranes in cochleae of embryos and fetuses in the paper of the writer and in papers by others appearing since. These points made it desirable to again go over from the beginning the processes by which the tectorial membrane and spiral organ are developed. As bearing upon these points, it is deemed allowable to review here certain steps in the development as now found by the writer, and some drawings illustrating these steps are given.
Another purpose for which this paper is submitted is that it is a necessary anticipation qf the study, begun earher, involving the construction of the model mentioned above, with a view to illustrating the possible action of the membrane as a vibratory mechanism in the process of hearing. A description of the results of this attempt is now in preparation.
In my previous paper, using freshly obtained cochleae of pigs at and near 'term,' it was found that by very tedious and careful procedure it was possible to remove by teasing methods parts of and even the entire tectorial membrane in the fresh state. In the fresh state, the membrane was found to be most inconceivably flexible in nature, to cohere and to adhere to the teasing instruments with a most exasperating readiness, and to have a specific gravity practically no greater than that of the fluid in which it normally lies. It is transparent and its removal was best accomplished over a black stage of the dissecting microscope. Its behavior in the fluid in which it was teased under the dissecting microscope indicated that the fresh membrane possesses barely enough elasticity against stress applied to it transversely to enable it to gradually assume its coil when floating about in the fluid l)efore coming to rest upon the bottom of the dish. Once in contact with the bottom it could not resume its coiled character. On the other hand, it appeared to possess considerably more elasticity against stress applied parallel with its length, more than enough to enable it to maintain its shape in transverse section and certainly enough to maintain its outspanning position just over the hair cells of the spiral organ, whatever the position of the head.
Under the compound microscope, study of both its whole thickness and of torn bits showed it to consist of very numerous fine filaments embedded in a seemingly gelatinous matrix. The membrane being of ectodermal origin, the matrix was considered as a jelly-like form of keratin. The varying directions, length and independent character of the filaments was described and the various other features of the membrane which seemed evident to the writer were compared as they agreed with or differed from the findings of others recorded in the literature. It was further found by experiment that the action of certain fixing fluids of themselves, those containing no alcohol and whose ingredients produced no appreciable exosmotic diffusion currents in tissue elements, caused very slight distortion of the membrane. In Zenker's fluid, for example, the endosmotic and exosmotic effects upon the membrane appeared to be quite evenly balanced and measurements of the dimensions of tectorial membranes after its action indicated that practically no differences from the dimensions of the fresh had been produced. Cochleae could be left in this fluid long enough for decalcification of the fetal bony lamina and thus allow much greater ease and precision in removing this from the membranous cochlea preliminary to clipping the latter away in order to remove the tectorial membrane. After Zenker's fluid, the membrane in the fetus at term appeared more opaque, less flexible, less fragile, and more easil}^ i-emovable entire from its attachment upon the vestibular lip of the spiral limbus. Such fixed specimens could be washed in water, but attempts to dehydrate and clear them always resulted in shrinkage and distortion. Mounts for study had to be made in glycerine or glycerine-jelly.
Measurements of both the fresh and the thus fixed tectorial membranes gave an average length of the membrane, in pigs at about term, of 25.5 mm. In width and thickness it was found to decrease gradually and evenly from its end at the apex of the cochlea, where it is much the largest, to the narrow and thin end of its basal coil, which ceases to project in width beyond the hair cells of the spiral organ. In the first or apical coil, the width of the membrane was found to be such that its outer edge projects considerably beyond the outer hair cells of the organ.
In extent, the tectorial membrane was found to occupy the cochlear duct throughout and to be strictly coextensive with the spiral organ. Thus its length is less than the lengths of the scalae on its either side, especially less than that of the scala tympani. Each of its ends was found to l)e rounded and to terminate bluntly. Its axial edge thins suddenly and knife-like and is permanentl}' attached upon the vestibular lip of the spiral limbus, being left thus attached by the ectodermal cells, \vhich, at an early stage, ceased to produce it, leaving it adherent to the fibrous mesenchymal tissue comprising 'Buschke's teeth' and the remaining portion of the vestibular lip covered by it. Its outer edge was found to be bluntly rounded and slightly scalloped, as a result of the component filaments curving apex-ward around this edge from the basal surface, and frequently in small bundles, to form the edge. The apical surface was found to be convex and smooth. The immediate convex surface consisted of a thin layer representing the result of the first activity of the then young producing cells below, in which layer or first product the matrix at least had not been so completely produced as later, allowing the first formed ends of the filaments to become tangled or washed, as it were, into an irregular arrangement resembling a reticulum. The remainder of the filaments continued into the body of the membrane, untangled and evenly embedded in the later and more completely produced matrix of the membrane. In transverse sections of the membrane, this immediate surface appears as a darker rind, or so condensed that the filaments cannot be traced in it, and it was referred to as the 'peripheral condensat'on.' A peripheral condensation likewise appears on the basal surface of the membrane in sections of embedded cochleae. There it was usually thinner and less uniform than on the convex surface but was considered as representing the last and likewise incomplete product of the activity of the cells producing the membrane — the surface left as the cells were ceasing to function and were being torn away from their product in receding from it. The appearance of condensation, or the darker rind, was considered as due to the action of the reagents upon the different character of these surfaces.
The basal surface of the removed membrane, viewed on the 13at, showed three lines. The axial one of these was found to be the line of the imprint of Hiischke's teeth, or the line marking the outer margin of the attached axial edge (inner zone) of the membrane. The middle line was found to be 'Hensen's stripe,' a surface marking described by Hensen in 1863. This stripe was decided to be a very shght linear elevation occurring coincident with the line of the interlocking phalanges of the pillars of the spiral organ and thus adapted to the groove or space on the surface of the organ between the hairs of the inner and outer series of hair cells. A peripheral or outer line appeared to be the line of attachment of the outer margin of an otherwise detached or detachable thin layer of the basal surface which was described as an "accessory tectorial membrane." In the apical coils, this line, or outer edge of the accessory membrane, runs considerably axial to the outer edge of the tectorial membrane. This distance however gradually decreases towards the basal coil till the line comes to coincide with the outer edge of the tectorial membrane, or, in other words, the outer edges of the two membranes gradually come to coincide toward the basal end. The evidence of the existence and the description of the structure of this accessory membrane as well as the description of the arrangement of the filaments in the membrane proper were presented in some detail.
As to the attachment of the tectorial membrane, it was shown that the membrane is purely a cell product, wholly non-cellular from its beginning; that during its formation it is of necessity attached to the cells which produce it, and, at any stage of its formation, an incompletely formed region must be and is then attached to the cells completing that region; that its thin axial edge or zone, the region whose production ceased earliest, remains permanently adherent upon the vestibular lip of the spiral limbus; but that, after its production is completed, by actual observation of it in position in teased specimens and in trustworthy sections of it, by the study of the later stages of its formation b}'^ the cells concerned, and from the change in position of the spiral organ with reference to its basal surface, the conclusion was reached that the main body or entire outer zone of the membrane does, and by necessity comes to, project entirely free over the spiral organ. Observations of others were cited both as agreeing and disputing that this outer part of the membrane is free, and it was advanced that all the disputing claims are Imsed either upon studies of mammalian cochleae in stages of development before the production of the membrane is completed or upon studies of sections of dehydrated and embedded cochleae in which, due to shrinkage and distortion produced by the treatment, the membrane was shrunken and pressed upon and thus apparently attached upon the surface of the spiral organ.
It was finally urged in the paper that the tectorial membrane and not the basilar membrane (membranous part of the spiral lamina) is the chief vibratory structure in the mechanics of hearing. The principal of the considerations upon which this suggestion was based are the following: (1) The general acceptation that the cochlea is the peripheral organ of the auditory apparatus, that auditory impulses are aroused in the hair cells of the spiral organ, about which cells the telodendria of the auditory (cochlear) nerve fibers terminate, and that these impulses are initiated by the impingement of the hairs of the hair cells against the basal surface of the tectorial membrane. (2) That the tectorial membrane is far more flexible and by structure far more capable and quahfied to serve as a vibratory mechanism than is the basilar membrane. (3) That the tectorial membrane varies in its proportions considerably more than the basilar, its thick and broad apical end gradually tapering to its narrow basal end, thus allowing greater possibilities of resonant activities and in wider range. (4) That in development and structure, the basilar membrane is not composed of individual nor independent fibers but is nothing more than a flat tendon whose component fasciculi are intimately connected with each other; and further that even if it were composed of independent fibers, being blanketed on its either side by continuous layers of tissue far thicker than itself, one of which layers is a syncytium, its fibers could not be thrown into vibration either individually or in groups. (5) That the tectoral membrane is always entirely coextensive with the spiral organ while the basilar membrane is not always so. (6) That the position of the tectorial membrane in the cochlea is more logical for the function than is that of the basilar membrane. The tectorial membrane may impart stimuli to the peripheral ends of the auditory cells and it also lies nearest the scala vestibuli, that is, to the fluid to which the foot of the stapes transfers directly the energy imparted by sound waves.
The theories in which the basilar membrane is considered the vibrating mechanism of the cochlea were, therefore, deemed untenable and an application of the 'telephone theory' to the tectorial membrane was suggested.
Since the publication of the above paper by the writer, three other papers dealing chiefly with the tectorial membrane have appeared.
The very instructive paper by Held ('09) deals with the development in the cochleae of the rabbit, chick and pigeon. He used embryos and various fetal stages of the rabbit before and after birth and used the fixing fluid which was used chiefly here (see below) for fixing and decalcifying. He describes the tectorial membrane of the rabbit as composed of three layers in thickness and, during development, three zones in width. In the mature stage, he thinks the third or outer zone is questionably distinguishable, and that the only attachment of the membrane is that of its axial edge, or inner zone, upon the vestibular lip of the spiral limbus, the entire outer part being free. Certain of the findings in Held's paper will be referred to in place.
Vasticar ('09 and '10) gives what purports to be a study of the tectorial membrane from fresh and osmic acid fixed cochleae of the rabbit. As far as the anatomy of the membrane is concerned, beyond the statement that it is a "cuticular membrane of extreme delicacy," his descriptions of vertically implanted, independent filaments, like a vertically placed hair brush, the ends of the filaments sometimes showing an 'olivary corpuscle,' that the ends of the filaments give the basal surface the appearance of spiral striations, etc., are so far from structure actually observed and so fanciful as to render his conclusions worthless. He agrees with Coyne and Cannieu ('95) that the outer zone of the membrane is attached to the spiral organ, united by short cuticular ligaments to its cells as far over as the cells of Hensen, and he cites Corti, Claudius, Henle and Lowenberg as supporting this attachment of the outer zone, but states that Waldeyer, Hensen, Ranvier, Retzius, M. Duval, Tafani and others claim there is no attachment of the outer zone.
Prentiss ('13) deals with the cochlea of the pig, using stages from 4 cm. in length up to fetuses at about full term. He advances one new claim. The initial purpose of his paper seems to have been to support the frequently made assumptions of Shambaugh ('07, '08, '10, and '11) and to demolish by controversy suggestions offered by others as to the actual anatomy and probable action of the tectorial membrane. He states his best results were obtained b}^ fixing with 2 per cent osmic acid and with vom Rath's osmic-picric-acetic mixture, claiming that the precipitation of the reduced osmium tetroxide gave a browning of the tectorial membrane by which "its cuticular structure" was sharply brought out. He decalcified before embedding with 5 per cent nitric acid in 80 per cent alcohol and used celloidin sections. A certain amount of decalcification must have been accomplished while in vom Rath's fluid. Later in his paper he refers to having dissected out tectorial membranes by "more favorable methods" than those employed by me and with results which did not support my observations. While in my first experiments in teasing out the fresh membranes I did "crush the bony labyrnth with a hammer," this was not done later and in doing it I doubt that the cochlea was jarred much more than by breaking the bone in another way, nor do I think that, as assumed by Prentiss, the blows necessarily disturbed the normal attachment of the membrane much more than considerably heavier blows on the skull which the function of hearing is able to sustain. While Prentiss's methods of dissection could have been easily far better than mine, one is disappointed in that he gives no description whatever of his more favorable methods and none of the results obtained with them. Through the pages of his paper, he progressively describes the structure of the tectorial membrane as attached by 'dehcate threads,' as composed of 'lamellae' (supporting Shambaugh), "parallel fibers or lamellae," "delicate parallel plates," 'a reticulum,' and finally advances the one new claim of his paper, namely, that the membrane is a honey-comb or chambered structure, each chamber corresponding to and produced by a separate cell and that thus the transverse dimensions of the chambers are the same as those of the cells of the greater and lesser epithelial thickenings or ridges of the earlier fetal stages, both of which ridges he claims produce the membrane. The producing cells being situated on the basal side of the membrane, the lengths of his chambers, curving axis ward, must extend through the thickness of the membrane. He concluded that the content of his chambers (matrix, Zwischensubstanz, etc., of others) is a fluid "resembling the endolymph."
A claim suggesting Prentiss' contention as to the origin and structure of the membrane was advanced by Coyne and Cannieu ('95), and Prentiss states that I misquoted these authors when I cited their paper as one of those in which a fibrous character of the membrane was suggested. In citing Coyne and Cannieu at the place in question, I had no reference whatever to their idea, which I thought quite erroneous, as to the process by which the membrane is produced by the (;ells basal to it, but merety to the fact that they considered the structure of the membrane as "im reseau." Vasticar ('11) quotes directly a later statement by Coyne in which "'d' aspect areolaire^' and "la forme d^un reseau'" are again applied to it. I translated reseau as meaning reticulum. A reticulum is a net and, in anatomy, is a network whose meshes extend in all planes. A net is composed of threads, not plates nor lamellae, and the word does not convey the idea of walled pockets or alveoli, nor that of a honey-comb structure. Whether, as Coyne states, the meshes (mailles) of the reticulum conform to the sizes of the cells of the organ of Corti (cellules sensorielles ciliees), which he assumed secrete the membrane, was a question, however doubtful, not in mind at the time. Prentiss uses the paper of Coyne and Cannieu in support of his claim of the chambered structure. He is rigltt in showing that my citing the paper at all in the relation I did was unfortunate. Support of the fibrous character of the membrane alone was sought, and while I find it is fibrous, the normal arrangement of the fibers is not in the form described in the paper.
Prentiss describes the walls of his chambers as coinciding with and continuous with the sides of the producing cells, the cavities of the chambers with the bodies of the cells. His figure 5, drawn from a section of the actual specimen in which the producing cells are cut longitudinal^, is given to prove his chambered structure, but in this figure nearly half of the filaments of the membrane are given off from the ends of the cells instead of from their sides. Figure 9 of my paper ('08) represented a thin section after paraffin in which the membrane was cut in a plane parallel to but splitting Huschke's teeth. It showed the fibers near the teeth necessarily cut across, since they curve apexward toward the surface of the teeth, while the plane of section through the body of the membrane was more or less parallel in places with the course of the fibers. That the fibers in this section appeared as'washed together, cohering to each other in anastomosing bundles, was explained by me as due to shrinkage and resultant agglutination. But Prentiss interprets this figure as showing his chambered structure, though the sections of the chambers he sees (the meshes of the apparent reticulum of bundles) vary from sizes for smaller than sections of the cells supposed to produce them could be, and the very various shapes of the meshes are questionably similar to what would be the shape of the cells so sectioned. Prentiss' figure 8, a section of the membrane similar to m}^ figure 9, shows some of the larger meshes (sections of his chambers) about ten times the size of the smaller meshes. The cells of neither of the epithelial thickenings or ridges vary so in size. His section merely indicates even more agglutination than mine. As to the endolymph filling his chambers, Prentiss states that I "could not demonstrate by special stains the presence of a matrix which would hold the fibers together." The presence of the seemingly gelatinous or soft keratinous matrix, in which the fibers are embedded, seemed to me one of the most evident features of my preparations of teased tectorial membranes, and its presence and apparent character was described by me as well as by others previously. That I was unable to determine by stains a definite micro-chemical character for it is true.
I described in my teased preparations from the pig a structure which I called an "accessory tectorial membrane." Prentiss states that this probably represented the reticular membrane or lamina of the spiral organ stripped off in the teasing. Whether this structure is an accessory membrane or not, I have found that it likewise appears on the basal surface of the membrane of the adult hog and in places not torn. Prentiss' statement indicates first, that he has never seen it at all in his 'dissections' and second, that his idea of the lamina reticularis is rather peculiar.
My perhaps unnecessarily tedious experience in trying to determine the normal character of the tectorial membrane and in trying to get sections of it showing this character would lead me to suggest it rather impossible to get normal preparations after fixation with osmic acid, whose power of penetration is remarkably poor, or even with vom Rath's fluid, and to get normal appearances would be certainly impossible after decalcifying with 5 per cent nitric acid before embedding. To me, all of Prentiss' figures drawn from the actual specimens show considerable shrinkage, his figures 5 and 6 less than the others. In his figure 9, the distortion of the tectorial membrane, outward and out of shape is very evident. In his figure 10, given as representing the apical coil of a pig at about term, the outer edge of the membrane is badly pressed axisward and pressed upon the spiral organ, and the spiral organ itself is pressed down upon the spiral lamina to an extent rendering it unrecognizable for this region of the coil. Except that it is pressetl down in contact with a part of the spiral organ, there is no evidence whatever in this figure that the membrane is attached to the organ, other than that the hairs of the hair cells, represented in the figure as one simple filament from each hair cell and much longer than normal, are shown continuous into the membrane. I do not think that an insertion of the hairs into the tectorial membrane is ever seen in any stage of development except when the membrane has been crumpled or compressed upon the hairs, sticking the hairs into it. So far as I know, the only recent papers which claim this insertion as a normal attachment of the membrane are those of Prentiss and Shambaugh. Prentiss states that in all his preparations of the older cochleae the membrane was badly shrunken. To me they prove nothing as to the membrane beyond its mere existence, certainly nothing for argument as to an attachment of its out-spanning zone.
To those of us suffering under the misapprehension that the apex of the cochlea is always directed toward the zenith, regardless of the position of the head, while the sense of hearing is being exercised, Prentiss devotes a drawing and some of his text to show that in the pig, while feeding for example, the tectorial membrane may be below rather than above the spiral organ and therefore, must fall away from the spiral organ if not attached to it, and render the animal deaf while feeding. In my teasing out of the membrane, it appeared, as I stated, to possess a specific gravity but little greater than the fluid in which it lies and that it manifested a transverse elasticity amph' sufficient to hold it in its position spanning over and close to the spiral organ, its broad axial edge alone being attached. Prentiss' dissected preparations, made better than mine, no doubt showed the same qualities.
Material and Methods
This study is made almost wholly upon cochleae of the adult pig and pig fetuses. Some cochleae of the adult ox were used for comparison and also some isolated sections of cochleae of the rat and guinea-pig were referred to. A few adult human cochleae were used but none could be obtained in sufficiently fresh condition for other than general comparison with those of the pig.
As is well known, the structures of the cochlear duct, especially the elements of the spiral organ, suffer maceration very quickly after death. Pig material obtained as promptly as three hours after death shows beginning maceration or digestion of the organ to an extent usually rendering it very unsatisfactory. Pig embryos and fetuses kept in the uterus and therefore in the amniotic fluid, preserve somewhat longer, but in the older stages of these digestion of the elements of the organ begins remarkably quickly. Therefore, it was found very necessary to place the cochleae in the fixing fluid, directly from the freshly killed animals.
The human specimens were obtained at autopsy but were found even in the freshest case to which access could be obtained, to have suffered too much dissolution for trustworthy detailed study of either tectorial membrane or spiral organ. All the pig material and the cochleae of the ox were obtained and placed in fixing fluid at the slaughter-house. Fortunately, , in the slaughter-houses of New Orleans, the hog is always split sagittally along the vertebral column and through the head immediately after evisceration. One has but to stand near the line of the passing carcasses, beyond the man who removes the brains, likewise split, and an abundant supply of adult labyrinths may be obtained from them within a few minutes after death. The bony labyrinth of the pig does not become fused to nor embedded in the petrous portion of the temporal bone as it is in man. In the pig fetus, it may be 'shelled out' with the fingers. In the adult hog it has developed considerably more bone than in the fetus, some of which excess is in the form of flattened bony processes extending adjacent to and parallel with the cranial surface of the temporal bone. The most evident of these processes extend from the region of the semicircular canals and, after pulling away the dura mater that may be left, a sharp screw-driver may be inserted under these processes, given a twist and the entire bone labyrinth is readily removed. The mesial surface of the bony labyrinth of the ox is likewise visible in the cranial wall but is more firmly planted in the temporal bone, though not wholly fused to it. Its removal requires a stronger screw-driver and greater effort, or even a chisel and mallet.
Vials, with blank labels on them, and a supply of fixing fluids were taken to the slaughter-house and the cochleae were dropped into fixing fluid as they were removed, the name of the fluid and the stage of the specimens being then indicated on the label.
A series of developing cochleae were finally obtained, beginning with those of fetuses of 3.5 cm. in length (crown rump measurement) and increasing by from 1 to 3 cm. up to fetuses at term. The latter stage varies in length between 20 and 30 cm. and had to be judged by the appearance of the fetus, chiefly ])y the amount of the hair on the body and the condition of the eyelids. The head of the fetus was removed and split sagittally, the brain cleaned out and, in the older specimens, the cochleae were broken out with the fingers. In the younger specimens, a square of the skull containing the cochleae was cut out, the surplus external tissue removed and the square dropped into fixing fluid. From certain of these, the cochleae could be removed under the dissecting microscope upon return to the laboratory; others, the youngest, were best carried through the procedure and sectioned entire, the plane of section determined by the landmarks of the cranial wall.
Specimens for the study of the adult tectorial membrane in the fresh conditions were placed in amniotic liquor, brought to the laboratory and used immediately. Occasionally physiological salt solution was used to which had been added a few drops of saturated solution of the bichloride of mercury, to check maceration. However, only sufficient fresh specimens were used to obtain a few sets of observations of the fresh tectorial membrane, it having been found in the previous study and verified here that certain fixing fluids, in themselves, distort the membranes very little if at all, and that the membranes may be removed much more easily and with less injury from fixed and partially decalcified than from fresh cochleae.
It was early found that with cochleae whose bony labyrinth was advanced, a greater percentage of normal appearances could be obtained if a small hole was made through the bone at the apex, before or within two hours after placing them in the fixing fluid. In doing this, very great care had to be taken not to deeply penetrate the membranous labyrinth. A pair of very fine pointed, small bone forceps was found better for this than a drill-pointed needle. Even with fluids which decalcify as slowly as those used, the carbonic acid gas usually forms within the labyrinth faster than it can transfuse, and the resulting pressure of the confined bubbles will press the tectorial membrane upon and over the spiral organ, often pressing it in places beyond semblance of its normal shape and position. Such pressure may be relieved through a very small hole in the bone. Here, the holes were usually made immediately after return from the slaughterhouse, holding the cochlea down in a Petri dish under the dissecting microscope and in sufficient of the fixing fluid to cover it.
Of the several fixing fluids tried, two were found to give the best results both with cochleae to be teased and cochleae to be embedded for sections. A fluid was desired which would serve both as a fixing and a decalcifying fluid and which would produce neither shrinkage of the tectorial membranes nor swelling. Gilson's mercuro-nitric mixture, for example, invariably gave distorted membranes in cochleae of late fetuses and of the adult, though fair preparations in sections of cochleae from quite young fetuses were obtained after it. The very small percentage of alcohol contained in this mixture was not deemed sufficient to produce the distortion by extraction of water from the membrane, but some other of its actions did prove unsatisfactoiy with the older stages. Decalcification after fixation by various fluids was tried with discouraging results. With both the nitric and hj^drochloric acid decalcifying fluids, proven with other material, either maceration or distorting shrinkage efi"ects resulted in the preparations. Decalcification after embedding the fixed cochlea in celloidin gave better but not satisfactory results. The celloidin imprisons bubbles of gas both within as well as outside the cochleae and those imprisoned within distorted the tectorial membrane in numerous places along its extent.
Zenker's fluid and the fluid employed by Held ('09) gave the best results. In Zenker's fluid, the swelling action of the acetic acid in it, causing the tissues to take up water, seems to be counterbalanced by the action of the potassium bichromate and bichloride of mercury contained, and both these, in addition to the acetic acid, are slow decalcifiers, as well as fixing agents.
The mixture employed by Held was here made by adding 40 cc. of the commercial (40 per cent) formaldehyde and 50 cc. of glacial acetic to 1000 cc. of a 3.5 per cent aqueous solution of potassium bichromate. This mixture, is at first the color of the bichromate solution, but by warming or after standing a few hours it turns a greenish brown as the result of oxidation processes. It is recommended as best applied after this change })egins. While the reaction between the formaldehyde and potassium bichromate must set free some formic acid, and while formic acid, acetic acid and even formaldehyde to a less extent, acting alone, cause the tissues to take up water and thus produce swelling, the mixture with the excess of bichromate seems to produce no swelling of the tectorial membrane nor of the elements of the organ of Corti.
An advantage of both this fluid and Zenker's is that specimens may remain in them for a long period without injury, and it is necessary for the cochleae to remain subjected till the desired decalcification has occurred. Here, the cochleae, brought from the slaughter-house in vials of fixing fluid, were suspended in large amounts of the fluid contained in low cylinder jars with cap covers. The jar, holding about one liter, was filled twothirds full of the fluid and the cochleae suspended in it near the surface that they might be surrounded by fluid more free from the salts resulting from the decalcification, which salts sink to the bottom. Also they were suspended that they might be subjected to less pressure than if lying on the bottom. The string by which a cochlea was suspended hung over the edge and outside the jar, held in place by the cover, and to the outer end of the string was attached a label when necessary. Adult cochleae required three to four weeks for complete decalcification, the fluid being renewed twice a week. Decalcification sufficient for microtome sections can be judged by testing with a needle, being careful to pierce the specimen in the region of the vestibule. The thinner walls of the bony labyrinth of the cochlea decalcifying first, it is always the thicker bony regions which give trouble with the sectioning knife.
An additional advantage of the bichromate-formalin-acetic mixture is that specimens fixed in it do not require a period of washing with water preliminary to dehydration for embedding. All the drawings given here were made intentionally from sections of cochleae fixed in this fluid.
Dehydration required especial care. Tectorial membranes fixed without distortion are often badlj^ disfigured by the treatment preparatory to embedding. Here no grade of alcohol of more than 10 per cent greater strength than the preceding was used. Further, it was found best to make up the desired grades of alcohol and let each stand before using long enough for all the fine bubbles of air resulting from the mixture with water to pass off. Otherwise these bubbles will collect upon and possibly form within the cochleae. The specimens were transferred direct from the fixing fluid to 20 per cent alcohol. Then, allowing them to remain in each grade from 3 to 12 hours according to convenience, they were subjected in succession to 30, 40, 50, 60, 70, 80, 90, 95 per cent and absolute alcohol. All clearing agents preparatory to embedding in paraffin, especially xylol, were found especially injurious except with the very youngest stages studied. The absolute alcohol was in all cases allowed to act at least 6 hours and then replaced by ether-alcohol and the specimens embedded in celloidin in the usual way.
The celloidin blocks were hardened with chloroform (not with water) and, to obtain transparency of the celloidin, the blocks were first placed in 95 per cent alcohol for an hour or so before the 80 or 70 per cent alcohol in which they were sectioned. Some of the blocks were transferred from the chloroform to cedar oil in which they remained long enough to become cleared, and then sectioned with the knife flooded with cedar oil. The latter procedure, though excellent for orientation, because of the transparency of the blocks, and for ease in sectioning, requires more time before and after sectioning than sectioning in alcohol and it did not seem to give any better results.
For transverse sections of the tectorial membrane, and remainder of the spiral organ, it is necessary that the edge of the knife passes parallel with the axis of the cochlea, and for transverse sections of the shortest or apical turn of the coil of the membrane, only those sections can be used which pass through the actual apex of the cochlea and the diameter of the cochlear nerve at its base. Celloidin sections varying from 10 to 20 micra were studied. The 3.5, 5.5, and 9 cm. stages were also embedded and sectioned in paraffin for thinner sections.
For staining, the sections were passed through the gradually decreasing grades of alcohol and the most generally satisfactory results were given by Delafield's hematoxylin 6 hours or overnight, washing in water, and both decolorizing and counterstaining with Van Giesen's picric acid-fuchsin mixture. Thence the sections were both washed and dehydrated with the increasing grades of alcohol, cleared in creosote and mounted in balsam.
All the drawings here given, except figures 10 and 11, were first outlined in detail under the new Edinger drawing and projection apparatus. By it, inequality of magnification of parts given by the camera lucida are avoided. The outlines of the nuclei and those of many of the cells could be traced. All except figures 10 and 11 were outlined with the same combination of lenses and the same adjustment of the apparatus and thus they are drawn to scale. The drawings were completed with the identical section projected for the outline placed, in each case, under the microscope and studied under high power for corrections and insertion of details.
The hog, ox and man belong to those species of mammals which possess the flat type of cochlea and the component structures in their cochlea are remarkably similar in form and character.
Wiedersheim ('93) gives the coil of the cochlea of the hog as having 4 turns, that of man as having nearly 3 turns and that of the ox, 3^ turns. Gray ('07) gives the hog as having 3^ turns and man, 2f turns. He did not examine the cochlea, of the ox, Wiedersheim must have taken into consideration the long basal end of the cochlea which does not take part in the coil. This end represents the direction of the first outgrowth of the cochlear pouch of the embryo. As shown by Streeter ('07) for the human, this first growth of the cochlear duct is straight, the coiling taking place in its further extension. In dissections of decalcified cochleae of the adult hog, it was found here that this basal end is of considerable length, comparable to nearly one-fourth of the basal, the longest turn of the cochlea, and that' it not only does not take part in the coil but that its tip curves slightly in the opposite direction as well as basal-ward. Including the uncoiled basal end, the pig's cochlea, were it all coiled, would comprise about 4 turns.
The cochlea of the ox is broader and slightly less flat than those of the hog and man. Measurements taken inside the bony labyrinths gave, as diameters of the basal turn in vertical sections passing through the apex and therefore including little or none of the uncoiled basal end, for the ox 8.4 mm., for the hog 5.8 mm. and for the human 6.7 mm. And, as heights of the cochleae, measurements from the apical side of the scala vestibuli in the apical turn to the basal side of the scala tympani in the basal turn gave for the ox 6.7 mm., for the hog 4.4 mm. and for the human 4.9 mm. In other words, these measurements indicate that the coil of the membranous labyrinth of the ox is approximately 8 mm. broad at the base by 7 mm. in height; that of the hog, 7 mm. broad by 4 mm. in height, and that of man, 7 mm. broad by 5 mm. in height. The figures given are averages computed from measurements under the compound microscope of sections of 5 different cochleae of the adult hog, 4 of the ox and of 2 human cochleae. The sections of the human were not so satisfactory for the purpose as were the others, owing to imperfect decalcification at the time they were cut. However, they were such that the indication that the human cochlea is relatively more flat than those of the hog and ox may be suggested as correct. The two scalae appear relatively larger in the ox than in the hog and relatively larger in the human than in either.
The cochlea of the pig at term appears to be slightly smaller than that of the adult hog. Averages of measurements taken in the same way of cochlea of fetuses at about full term gave 4.8 mm. as the width of the coil at the base by 3.4 mm. in height. Comparison indicates that the greater size of the adult is in large part at least due to an evident increase in size attained by the two scalae. Very probably the total length of the cochlea has increased but very little in the adult.
The length of the tectorial membrane, is somewhat less than that of the cochlear duct which carries it. It does not extend to touch the blind apical end (caecum cupulare) of the duct nor does it extend quite to the end of the caecum vestibulare, the basal end of the duct. And the duct is not*so long as the scalae on its either side, especially the scala tympani. It appears from the best that could be gathered from various sections and teased cochleae that the tectorial membrane is quite strictly co-extensive with the spiral organ. Its exact length not being the point in mind at the time the dissections of the adult hog were being made, only three membranes of the adult were teased out sufficiently intact to determine their lengths. Two of these were decided each to have a length of nearly 27 mm. and the outer about 26 mm. The teasing of these was done after fixation, which is better for measuring since the membrane does not stretch so readily. All three membranes were broken, but the pieces were all saved and could be arranged in order on the slide. Accurate measurement, of especially the apical or most curved turn of the coil, is difficult. It has to be done in segments. Much straightening and attempts to straighten the membrane usually increase the sources of error. The average length obtained from measurements of seven membranes of fetuses near full term and two from young suckling pigs, recorded in previous paper of the writer, was 25.5 mm. Some of those measurements were obtained by the use of a string laid upon outline drawings of the entire membrane, whole or in pieces, projected under known magnification. Allowing for error in measuring it can only be said that the tectorial membrane of the adult hog is probably a little longer than that of the fetus at term, but hardly more than 1 mm. longer.
The width of the tectorial membrane varies considerably. It is widest in the apical turn of its coil and tapers thence gradually and regularly to the basal end, the tip of which is its narrowest part. Its position is over the apical side of the spiral organ, its axial zone being attached upon the vestibular lip of the spiral limbus. Its widest portion spans over and projects beyond the spiral organ to an extent of more than one-fourth of its total width. Its narrowest part barelj^ spans the outer hair cells of the organ. Figures 1 to 4 are given to show its varying proportions and its position. These four figures are drawings to scale of the membrane and its environment as it appears sectioned in one side of a median vertical section of the cochlea of an adult pig. Thus the figures represent consecutively sections of the membrane across the 1st, 3rd, 5th and 7th half turns of its coil.
TABLE 1 - Giving in micra averages of the total width, the width of the attached axial zone and the u'idth of the outspanning zone of the tectorial membrane of the adult hog obtained from measurements, taken at the intervals of the coil specified, from jnedian vertical sections of five cochleae and of six teased out membranes
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TABLE 1 Giving in micra averages of the total width, the width of the attached axial zone and the u'idth of the outspanning zone of the tectorial membrane of the adult hog obtained from measurements, takenat the intervals of the coil specified, from jnedian vertical sections of five cochleae and of six teased out membranes 1st half
apical end
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Measurements of the width of the membrane taken at the above intervals of its coil from stained median vertical sections of five different cochleae of the adult hog and measurements at the same intervals of six membranes teased out from the adult are recorded in table 1. Short segments of the long basal turn of the coil of three of the latter membranes were lost in their removal, but in these cases the loss did not preclude the desired measurements, the basal ends being saved.
In explanation of table 1, it should be noted that, with the different vertical sections employed, the measurements of the width of the membrane could not be taken across the same identical regions of the coil. The celloidin blocks were not oriented on the microtome so that the median vertical sections always involved the same diameter of the cochleae and, therefore, the measurements of the ends of the membrane had to be taken in each case at vary in ji unknown distances from the actual tips of these ends. Owmo to the fact that the basal end of the membrane is not coiled, no median vertical section of the cochlea can pass through this end transversely. Manifestly oblique sections of either end could not be used for measurements of width, and transverse sections of the basal end must of necessity have passed considerable distances from its actual tip. With the teased out membranes coiled on the slide, the micrometer scale could be arranged as a radius involving the widest part of the apical end and crossing the other turns at the specified intervals. Afterward could be taken a separate measurement of the basal end, transversely across its narrowest part. The actual termination of each end of the membrane and of the apical end especially, is bluntly rounded. Measurements of the ends were taken near but not involving the rounded part.
Further, it seemed manifest in comparing the teased preparations with the sections that, in the sections the membrane had suffered some shrinkage. Practically all the shrinkage in width occurs in the thicker outspanning zone and especially toward the apical end where this zone is more voluminous. This explains some of the differences in the width of this zone shown in table 1 between the measurements from the sections and those of the teased out membranes. In all the sections of the adult cochleae, the first half turn of the membrane seemed to conform less in shape to what the teased preparations indicated it should be than in other regions of the coil. In the cochlea from which figures 1 to 4 were drawn, the outer edge of the outspanning zone in the first half turn is considered distorted and retracted axisward by shrinkage (compare OZ, fig. 1, with fig. 2). In the table as well as in the drawings, it appears less in width in this than in the third half turn. This explanation of the shape of the section of the apical end as due to shrinkage is supported by the study of the tectorial membranes of fetal pigs described in my previous paper. The depressed apical surface of the outer edge of this zone as shown in figures 3 and 4 is also probably the result of shrinkage.
The attached axial zone (AZ, figs. 1 to 4) varies but slightly in width in the different turns of the adult cochlea. In the teased preparations, its width could be easily measured from the fact that the line of imprint of Buschke's teeth {Ht, fig. 2) upon the basal surface of the tectorial membrane could be seen. In these, its width was found to conform quite closely to the widths obtained by measurements from the sections (table 1). In figures 1 to 4, the width of this zone appears practically uniform throughout and table 1 shows the maximum variation to be but 16 micra, the zone being narrowest in the basal end just as is the outspanning zone. It will be seen below that the attached axial zone increases somewhat in general width between the fetus and the adult hog.
It must be remembered that the basal turn of the coil is much the longest of the turns, has the greatest radius of curvature, and that, therefore, there is a much greater distance or extent of the membrane between the sections of the 7th and 5th half turns than between the 5th and 3rd half turns, and especially greater than between the sections measured of the 3rd and 1st half turns, which latter involve the apical or shortest turn of the coil. Thus, though the proportions of the tectorial membrane may decrease uniformly from the apical to the basal end, one must expect greater differences between the sections and measurements which are taken transversely at the greater distances apart.
To sum up the studies made as to the width of the membrane, with computations from table 1, it may be advanced (1) that the membrane is widest at its apical end and decreases gradually in width toward its basal end, the tip of which is its narrowest part; (2) that the actual total width of its apical end in the teased out preparations is about 415 fx or 2.7 times greater than its basal end; (3) that its attached, axial zone varies ver}^ slightly in width, being but very little narrower in the basal end; and finally (4) that, as indicated in figures 1 to 4, its great variations in width occur almost wholly in the width of its thick outspanning zone. As computed from table 1, this zone in the teased out membrane may be about 390 ^ wider in the apical than in the basal end, or the apical 6.8 times the width of the basal end. The same computation applied to the measurements of the sections of the membrane give the outspanning zone in the 1st half turn 3.2 times the width, and in the 3rd half turn 3.7 times the width it has in the 7th half turn. That the differences in the width of this zone do not appear so great in the sections as in the teased out membranes, and that its 1st half turn appears less wide than its 3rd half turn, is explained as due to shrinkage of the apical end, especially in the first half turn, during the preparation of the material for sectioning, and to the differences in the regions of the membrane at which the measurements of its width had to be taken in the sections. The measurement of the 7th half turn especially was in the sections of necessity some distance from the tip of the basal end and therefore could not involve the narrowest part of the outspanning zone. Computed from the measurements of the teased out membrane, the width of the outspanning zone in the apical or shortest turn of the coil appears to decrease only about 1 per cent, while in the second whole turn the decrease is 41 per cent, and between the 5th half turn and the basal end, the longest interval between measurements, the decrease is 72 per cent.
This study of the proportions of the membrane suggests throughout that conclusions as to its actual functional shape based upon its appearance in even the best of sections of embedded cochleae are subject to considerable error.
The thickness of the tectorial membrane had to be measured wholly from the sections. Obviously, the teased out membranes could not be used for this. Measurements through thickest part of the outspanning zone at the different intervals are given in table 2. The thickest part usually appeared to be in the region of the small ridge on the basal surface known as 'Hensen's stripe' {HS, figs. 1 to 3).
TABLE 2 - Giving in micra averages of the thickness of the thickest part of the tectorial membrane of the adult hog obtained by measurements at the intervals specified in median vertical sections of five cochleae
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| TABLE 2 - Giving in micra averages of the thickness of the thickest part of the tectorial membrane of the adult hog obtained by measurements at the intervals specified in median vertical sections of five cochleae
1st half turn
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It may be seen from table 2 that, just as the outspanning zone is widest, so is it thickest in the apical end of the coil of the cochlea. After whatever shrinkage that may have resulted in the preparation of the sections, computations from the table indicate that in the apical end this zone may be 110 /i thicker or about 3 times as thick as it is at the 7th half turn. Between the 1st and 3rd half turns its thickness appears to decrease about 19 per cent,, between the 3rd and the 5th half turns about 18 per cent, and between 5th and 7th half turns, the longest interval between measurements, the decrease is about 48 per cent. The smaller amount of decrease between the 3rd and 5th half turns is probably due to irregularities of shrinkage, but this cannot be determined from the preparations used. The drawings, figures 1 to 4, made from a single cochlea show the decrease in thickness between the 1st and 3rd half turn to be least and that the decrease increases progressively between the 3rd and 5th and the 5th and 7th half turns. One would assume from studies of the teased out unshrunken membranes that this latter form of decrease in thickness is the normal one.
The volume of the tectorial membrane may be computed approximately from the areas of its transverse sections. Considered the chief vibratory mechanism in the auditory apparatus, variations in the volume of the membrane, or the 'load' it carries, in different regions are the most important of the proportions. Most all of its volume being carried in its outspanning zone, this zone must determine almost wholly the possibilities of vibration of the different regions of the membrane as well as of the membrane as a whole. The axial zone being attached upon the vestibular lip of the spiral limbus and varying very little throughout the length of the membrane, can hardly have much to do with the variations in natural vibration frequency" of which the membrane may be capable. In trying to compute the areas of the transverse sections of the outspanning zone, it was thought fairer to use for one dimension its width as found in the teased out and unshrunken membranes. Other dimensions had to be taken from the sections regardless of whatever shrinkage the zone had suffered in them. Using the widths obtained from the teased out membranes as the widths of parallelograms, the depths used were averages obtained from six measurements of the thickness of the zone taken upon the transverse sections of the different regions of the coil at about equal intervals between the outer edge of the zone and the edge of Huschke's teeth [HT, figs. 1 to 4). The first measurement was taken in each case near the outer edge and the last at the edge of Huschke's teeth. Of the several cochleae used for the purpose, these sections were used which were judged as passing nearest the apical end of the membrane. The areas of section of the outspanning zone in each region were thus obtained by multiplying its width in the teased out membrane by the average of its thickness in the sections.
Proportional volumes of the two ends and variations in the volume of the zone were most desired, and for such, realizing that by any further procedure the results could be only approximate at best, it was considered most feasible merely to multiply the area obtained for the section of the zone in each region by a given short length, say one millimeter of the length of the membrane. The volumes thus obtained vary as the areas of the sections. The areas obtained are as follows:
Transverse section of apical end 61,723.20 square micra Transverse section of 3rd half turn 43,293.45 square micra Transverse section of 5th half turn 18,442.14 square micra Transverse section of basal end 1,719.35 square micra.
The volumes of the different regions, computed in the above way, represented by these areas, may be found to show the following proportional relations: (1) A given short length of the apical end of the outspanning zone of the tectorial membrane may be 41.7 times the volume of the same length of the basal end of the zone. The volume of the same length of the 3rd half turn may be 25.2 times, and that of the same length of the 5th half turn may be 10.7 times the volume of that length of the zone in the basal end. (2) Taken from the basal end toward the apex, it may be found that between the basal end and the region of the 5th half turn, the longest interval between measurements, the volume of a given short length of the outspanning zone may increase 90.7 per cent; between the 5th and the 3rd half turns the volume of the same length may increase 57.4 per cent, and between the 3rd half turn and the apical end, the volume of the same short length of the zone may increase 29.8 per cent.
Just as does the width and thickness, so, as to be expected, does the volume of the zone appear to increase progressively from the apical toward the basal end. That the percentage of increase toward the basal end of the cochlea is progressive rather than regular must be due to the fact that, beginning with the basal end, each turn of the coil is longer than the turn apical to it and, therefore, there were greater lengths of the zone between the transverse lines at which the above measurements were made in each succeeding coil from the apical toward the basal. In studies made here of the teased out membranes of the adult hog and in the studies of the membranes of pig fetuses at term, published in the writer's previous paper, the tectorial membrane, and therefore its outspanning zone, viewed as a whole appeared to increase in width with uniform regularity from the basal into the apical end. If the increase is irregular at all, the impression may be obtained that it is less rapid in the basal than in the other turns of the coil. Drawings illustrating the appearance of the teased out membrane are given in the previous paper.
The above variations in the proportions of the tectorial membrane, assumed at least to approximate the normal adult, are thought to further support the suggestion that the tectorial is far more adapted as a vibrator^^ mechanism in the auditory apparatus than is the basilar membrane. Whether in accord with the Helmholtz theory, which involves sympathetic resonance, or with a modified telephone theory, the much greater variation of the tectorial membrane allows for it a much greater scale of activity than seems possible in the basilar membrane. Measurements of the assumed vibratory width of the basilar membrane, for example those by Kolmer ('07), show that its width (length of its supposedly existing vibrating fibers) at the apical end of the spiral organ is only about 1.8 times its width in the basal end. Averages of measurements here made in cochleae of adult hogs, taken from the line at which the auditory nerve fibers {AF, fig. 2) disappear into the spiral organ to the outer angle of the scala tympani, give the basilar membrane a width of 257.9 fi at the apical end and a width of 184.8 ^ . at the basal end (fig. 4), thus showing it to be about 1.4 wider at the apical than at the basal end. When the measurements were taken at the level of the floor of the spiral sulcus (about at the line AF, fig. 1), the width of the thus questionabh^ vibrating part of the spiral lamina was found to be only about 1.9 wider at the apex than at the base. It may be noted in the figures that this latter measurement includes in the basal coils some of the bony spiral lamina. As seen above, the width of the assumed vibratory part of the tectorial membrane, its outspanning zone, is 6.8 times greater at the apical than at the basal end. The basilar membrane does not consist of independent fibers and thus of fibers capable of resonant activity. Ayers ('91) described it as consisting of four layers of fibers, one of which runs at right angles to the other three. In my former paper, the main or radially arranged part of the basilar membrane was shown to be of the nature of a flat tendon, the tendon fasciculi (fibers of the earlier descriptions) being abundantly connected with each other by smaller collateral bundles. Vasticar ('12) said that it consists of six layers. He, however, included the layer of epithelioidal tissue, or syncytical mesenchyme, on the basal surface of the basilar membrane proper and the endothelium lining the scala tympani as layers of the membrane. The basilar membrane proper and the layer of epithelioidal sjaieytiimi upon its basal side vary very little in thickness in the different regions of the coil and the little variation is irregular.
Possible Vibration of the Spiral Lamina Variations of the Spiral Organ (of Corti)
In the adult hog the spiral organ increases in both width and thickness in passing from the basal toward the apical end of the coil of the cochlea. Near the basal end (fig. 4) it may be seen that the spiral organ proper is much thinner and narrower than in the apical region (figs. 1-2). Reading the sections of the organ consecutively in both sides of the median vertical sections of the cochleae, or even reading figures 1 to 4, which represent the organ in one side of a cochlea, it is suggested that the increase in the size of the organ occurs uniformly from base to apex. Measurements of the width and thickness of the spiral organ as it appears in section in one side of five median vertical sections gave the averages recorded in table 3.
TABLE 3 - Giving averages in micra of the widlh and thickness of the spiral organ, as measured in one side of vertical median sections of cochleae of five adult hogs
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| TABLE 3 - Giving averages in micra of the widlh and thickness of the spiral organ, as measured in one side of vertical median sections of cochleae of five adult hogs.
Width Thickness
1st half tcrn 223.3 113.1
3rd half turn
221.1 104.4
5th half turn 208.7 85.2
/TH HALF TURN
130.0 55.4
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The measurements of width of the spiral organ, averages of which are given in table 3, were taken from the level of the surface of the epithelium lining the internal spiral sulcus {ISS, fig. 2) on the axial side of the organ to the level of the surface of the cells of Claudius (CC, fig. 2). The measurements of thickness of the organ were taken from the basilar membrane proper through the middle of the outer hair cells to the vestibular surface of the organ. As shown in figures 1 to 4, the vestibular surface of the organ inclines axisward appreciably in the apical turns, the outer sustentacular cells being higher and forming a thicker ridge than in the basal end. Thus, measurement through the middle of the outer hair cells will give an approximate average of the thickness in each case. The degree of the axial incline of the surface of the organ decreases toward the basal end, at which it is almost absent. Attention is further called to the fact that in the basal end of the coil of the adult cochlea (f g. 4), both the epithelium lining the internal spiral sulcus and that known as the cells of Claudius are considerably thicker than in the more apical regions. The axisward incline of the outer rods of the organ increases slightly in passing from the basal toward the apical end. This is not so evident in figures 1 to 4 as it was in other sections of adult cochleae. Gray ('00), in suggesting a modification of the Helmholtz theory of hearing, noted that the rods of the spiral organ and the hair cells became smaller in passing from the apex to the base of the cochlea.
The resonance theory, elaborated by Helmholtz, was based upon erroneous anatomical descriptions of the basilar membrane by others. Requiring that the membrane be composed of separate fibers of varying length and free to exercise sympathetic vibrations in response to sound waves of varying length imparted to the endolymph, the theory must be abandoned. The basilar membrane not only does not consist of independent fibers, but it is blanketed on both its sides by thick continuous layers of other tissue, the thickest of which is the cells of the spiral organ itself. The telephone theory of hearing, suggested by Rinne in 1865 and Voltolini in 1885, elaborated by Rutherford in 1886 and further by Waller in 1891 and Meyer in 1898, was likewise applied to the basilar membrane. Denying the possibilit}^ of the selective or sympathetic resonance required by the Helmholtz theory, it -assumes that the vibrations producing sound act upon the basilar membrane as a whole; that the vibration frequencies induced in the tympanic membrane by given sound waves are repeated by such extents of the basilar membrane, beginning at the basal end, as the resistance offered by the components of the apparatus and the inertia of the membrane will allow. At one time in its development, the telephone theory was appUed to the cochlea as a whole instead of being confined to the basilar membrane.
Any application of the telephone theory, either to the entire cochlea or to the basilar membrane requires, of course, agitation of the spiral organ and in such a way that the hairs of the hair cells are involved with the basal surface of the tectorial membrane. In other words, the theory must include the possible vibration of the -entire membranous portion of the spiral lamina.
I do not desire to dispute the possibility that energy imparted to the cochlea by sound waves may throw into vibration the membranous spiral lamina. On the contrary, the possibility seems to me supported in my experiments with the model referred to above, and by studies of the character of the basilar membrane, and especially by the above observation that the cellular part of the spiral organ increases in bulk in passing from its basal to its apical end. Since the base of the stapes imparts the wave motion to the cochlea at its basal end, since the higher pitches are known to be mediated by the basal end of the cochlea, and since sound waves of greater vibration frequency (higher pitch) are known to be 'damped out' more readily in overcoming resistance than those of lesser vibration frequency, it seems indeed suggestive that the membranous spiral lamina not only gradually increases in width or bulk but also that its chief load, the cellular spiral organ, itself increases from the basal toward the apical end. But, on the other hand, I beg to emphasize the fact, very evident to me, that the tectorial membrane is far more adapted for the vibratory activities required. It is infinitely more flexible than the basilar membrane, or membranous spiral lamina, especially when both are in their position. In my previous study I had opportunity to compare the two when both are teased out and found that even then the tectorial membrane as compared with the lamina is as a strip of thin paper compared with a board. The arrangement of the fibei-s in the structure of the tectorial membrane, giving it sufficient elasticity against stress applied longitudinally to enable it to retain its position close upon the spiral organ but giving it practically no resistance to stress applied transversely, renders it peculiarly adapted for undulatory motion. The vibrations are imparted by the base of the stapes by way of the fenestra vestibuli (ovalis), first to the fluid in the scala vestibuli, which scala is on the apical side of the spiral organ. The tectorial membrane projects over the apical side of the spiral organ and therefore is in the logical position for being most readily disturbed by the motion imparted. Finally, among other advantages, the tectorial membrane varies far more in its proportions than does the membranous spiral lamina and especially more than the basilar membrane. In width, its vibratory or outspanning zone is about 7 times wider in the apical than in the basal end while the width of the apical end of the supposedly vibrating part of the spiral lamina is only about 1 .4 times its width in the basal end. The differences })etween the volume of the two ends of the tectorial membrane very evidently exceed those of the lamina to a much greater extent than do the differences in width, and variations in volume are the most important for the functions ascribed to the membranes in the telephone theory.
It has been computed that the actual force of the vibrations of the tympanic membrane, produced by sound waves, is increased about 30 times as repeated in the vibrations of the base of the stapes, and that the amplitude of certain waves as imparted to the tympanic membrane may be reduced as much as 76 times in their transference to the base of the stapes. Both force and amplitude are decreased in overcoming resistance in the cochlea. It is quite possible that very strong sound stimuli may throw into vibrations both the tectorial membrane and the membranous spiral lamina while the less strong and more ordinary stimuli affect the more adapted tectorial membrane alone. If the two were equally affected by the given strong stimuli, their resultant movements would be parallel and the required stimulation of the hair cells in the way usually supposed would not occur. If, however, the position and greater flexibility of the tectorial membrane should result in greater excursion or amplitude of vibration in it than in the lamina, then the hair cells could be stimulated in the way supposed and in accord with the vibration frequency of the wave motion applied. It might be advanced that vibration of both the tectorial membrane and the membranous spiral lamina in case of very strong sound stimuli would be an economical arrangment in the structure of the cochlea. By such, the tectorial membrane, whose motion alone actually stimulates the hair cells, may in these cases be kept within working distance of .the protruding hairs, or by such may be obviated injuriously forcible impingement of the tectorial membrane upon the hairs, which might result from extraordinarily great excursions produced in it by the strong stimuli, were the lamina to remain undisturbed. In the more ordinary and less strong sound stimuli, amplitudes and vibration frequencies must occur which are incapable of throwing the lamina into vibration at all but which may affect the tectorial membrane, its excursions decreasing, with the strength of the stimuli, to the functional limit of the auditory apparatus. I hope to discuss more fully the probable action of the tectorial membrane with a description of experiments with the model referred to above.
Structure of the Tectorial Membrane
In the study of the tectorial membrane of the adult hog, I have found little reason to modify the conclusions drawn in my previous paper as to structure from its study in cochleae of fetal pigs. It consists of fibrils or filaments of very varying lengths imbedded in a gelatin-hke, probably keratin ous, matrix. The directions in which its fibrils are arranged result from the gradual changes in position and the increase and decrease in number of the individual cells which produce its fibrils. The greater epithelial ridge, which produces the membrane, grows thicker, wider and longer for a considerable period after it has begun production, and then recedes in thickness and width and finally disappears as a producing structure. The axial side of the ridge ceases to grow and its cells cease to produce the membrane earlier than the outer side of the ridge. Thus the fibrils of the axial side of the membrane are the shorter and merely curve, from the basal side axisward, due to the growth in width of the ridge, and apexward from the cells producing them. The fibrils of the outer side of the membrane are longer and curve outward, apexward and axisward from the basal side, claimed as due to first the great increase in width and then the decrease in width of the greater epithelial ridge, both of which changes in width occur after the beginning of the membrane.
From the study of the adult, I see no reason for describing the tectorial membrane in more than two zones, the attached axial zone and the outspanning zone. To let Hensen's stripe mark the boundary of a third zone is, I think, unnecessary other than that the stripe forms the axial boundary of that width of the membrane which projects beyond the interlocked phalanges of the pillars of the spiral organ. A 'border plexus' or a thin outermost zone does not exist in the adult nor in the older fetuses. The outer edge of the membrane is bluntly rounded and, in contour, slightly scalloped. The fibrils forming the edge curve from the basal surface outward, around the edge and then apexward and axisward. The appearance described by me as an "accessory tectorial membrane" in the fetus is present on the membranes of the adult hog. In one specimen of the teased out membrane it appeared partly lifted away from the basal surface in places but in none of the mounts from the adult was it so completely lifted away from the main body of the tectorial membrane and so separately visible as in the specimen from the fetus described and illustrated in the previous paper.
Held ('09) described the thickness of the tectorial membranes of the rabbit as composed of three layers: A 'Decknetz' on the apical surface; a middle fibrillar layer on the main body of the membrane composed of fibrils embedded in a matrix (Zwischensubstanz), and a thin homogeneous layer bounding the basal surface. All three layers come together in the outer edge of the membrane which he calls 'the selvage' and which he describes as blunt and irregularly fibrous. By irregularly fi.brous is meant that bundles of fibrils curve around the selvage giving it the lobed or scalloped appearance in contour mentioned above. His descriptions of the course of the fibrils and their abundance conforms in the main with mine.
Upon looking at Held's figures of his Decknetz I felt sure at fxrst that he was describing the structure I referred to as an accessory tectorial membrane with its regularly arranged fibrils washed together and cohering in irregular bundles, making his net, and that either he had mistaken it to be on the apical surface or I had been mistaken in thinking it on the basal surface of the tectorial membrane. In his description of it and in his figures, the outer border of his Decknetz is shown to be thicker and its meshes finer than in other parts. The 'accessory membrane' might appear thus if its fibers were disarranged by the treatment. He thinks the 'bars' of his net are assembled from finer fibrils and states that the direction oi the bars in places conforms with the direction of the fibrils in the fibrillar IsLjer or main body of the membrane below. However, he describes his Decknetz as extending on the apical surface of the membrane from and usually involving its outer edge, over the entire outer zone and upon the axial or inner zone which is attached upon the vestibular lip of the spiral limbus. While one of the two systems of fibers in my accessory membrane appeared to conform with the direction of the fibers in the main body of the tectorial membrane, the latter being crossed at very acute angles by the other system, only in the basal turn did the outer edge of the accessory membrane seem to extend to the outer edge of the main body, and only in the basal turn did its very delicate axial edge seem to me to extend further axisward than the line of Hensen's stripe. As seen by Held, his Decknetz is considerably wider. He states that he could not follow it completely over the attached axial zone because of the density of this region.
Held's homogeneous layer bounding the basal surface of the tectorial membrane is, I think, nothing more than the finely granular and faintly fibrous pale staining layer found by Rickenbacher ('01) for the guinea-pig after decalcification with nitric' acid and staining with eosin. After the procedure used by me, both in the previous and in the present studies, to obtain sections of the membrane, the stain I used shows a thin more densely staining layer bounding both the apical and basal surfaces. I described this as a peripheral condensation of the substance of the membrane {PC, figs. 1 to 3) and explained it as representing, on the apical surface, the product of the first activities of the then young producing cells, and on the basal surface as the last product of the then declining activities of the producing cells. That it appears condensed as compared with main body of the membrane, I thought due to the different actions of the reagents upon these superficial and less completely formed products of the beginning and waning functions of the cells. As noted by Held for his homogeneous layer, the peripheral condensation is not so noticeable along the axial side of the basal surface. Here the maturely active cells are probabl}^ torn from their product more suddenly than from the thicker part of the outspanning zone. Also, as noted b}^ Held, the very thin extreme axial edge of the attached axial zone appears dense in the preparations. This edge corresponds to only the earlier product of the activities of the cells, for here production of the membrane ceases in the early fetus and the cells sink back into inactive form while the remaining are still forming the 'peripheral condensations.' I have called attention to sections of the apical surface cut parallel to it, showing a coarse reticular arrangement and have suggested that the reticulum in these sections represented the first formed ends of the fibers of the membrane, not so adequately embedded in matrix as in the main body, washed together and cohering in anastomosing bundles. In the transverse sections, these form part of the peripheral condensation. If this suggestion is possible, it could explain Held's Decknetz of the apical surface.
Held did little more than look over the figures given in jny paper. He kindly gives a 'Zusatz' at the end of his paper in which he states that he did not know of my paper till after his manuscript had been closed, and that he thought some of my figures defective, two of them 'artificial monstrosities' of the tectorial membrane resulting from swelling. Held had never studied the tectorial membrane of the hog, which I think is similar to the human. All the drawings of the present paper are intentionally made from projections of sections of cochleae fixed and prepared for sectioning according to the identical procedure used by Held for his sections of the cochleae of the rabbit. In so far as possible, I judge my preparations were no more shrunken than were his. Judged from measurements and the appearance of the teased out membrane, I am quite sure they were not swollen. I have not studied the tectorial membrane of the rabbit. He frequently referred to the tectorial membrane of the adult rabbit but, unfortunately, does not figure it. His figure 17, six days after birth, is the oldest stage he exhibits in the rabbit. The rabbit being one of the rodents in which parturition is comparatively premature, his six day old rabbit was scarcely more developed in than a pig fetus about term.
On the Development and Attachment of the Tectorial Membrane
The essential steps in the development of the spiral organ and its tectorial membrane were worked out by Corti himself in 1851 and by Kolliker in 1861. Many other papers have dealt with phases of the process, repeating and adding to the findings of Corti and Kolliker, the most recent being those of Held and Prentiss. Permission is asked here merely to review the process as to its bearing upon two points: the position which the tectorial membrane acquires in its maturel}^ functioning condition and the attachment it retains. As indicated in my previous paper, both these points have been touched upon repeatedly without agreement in results. The most recent papers fail to agree upon them. I have here tried to contribute quantitatively something as to the growth changes in the width of the membrane, and as to the growth changes resulting in its adult position with reference to the cells of the spiral organ and also resulting in the one attachment it retains in the hog.
In the hog, the length of the fetus is but an approximate criterion of the stage of the development of its cochlea. Fetuses obtained in California, for example, I have found average longer at a given stage of the spiral organ than do fetuses from hogs raised in Louisiana. The hog brought to the slaughter-house averages larger in San Francisco than in New Orleans, due no doubt to differences in breed and feeding, for very probably the Louisiana hogs average older and smaller at killing than those in California. All the fetuses here used were obtained in New Orleans.
In fetuses of about 5 cm. (fig. 5), the coiUng outgrowth of the cochlear pouch has already acquired nearly three turns. Only the first indications are then appearing of the liquefaction of the mesenchyme, the progress of which gives the scalae on the two sides of the cochlear duct. Along the basal side of the cochlear duct the epithelium has already become much thicker than that of the apical side, but as yet there is no differentiation of the epithelium into the greater and lesser epithehal ridges of the later stages. Upon the axio-basal surface of the epithelium of the cochlear duct appears the ) .^ginning of the tectorial membrane (TM, fig. 5), being produced by the thicker epithelium. In the sections of embedded material, this product appears densely but finely fibrillar. The fibrils may be seen continuous with the apical ends of the long cylindrical cells, but in the whole product they appear very much tangled and matted together as though an interfibrillar substance were insufficient in either amount or quality to individually support them. When compared with the body of the membrane of the later stages, one receives the impression that the fibrils here, which represent the apical ends of the fibrils of the later stages, are smaller than in the later stages. As may be noted by comparison with the figures following, the extreme axial edge of the young membrane {AZ, fig. 5), though very thin, is almost as thick as it is in the later stages. The cells producing this edge never become higher, never become so actively productive of the membrane as those of the remainder of the thick epithelium, and they grade into the non-productive cells of the duct. The outer edge of the young membrane is likewise thin at this stage, the cells producing it grading likewise from the less productive into the non-productive cells. However, the outer edge of the thicker epithelium grows in width and activity along with the rest of the thickening. The entire thickness of the tectorial membrane so far produced in figure 5, only represents a part of the width of what I have called the peripheral condensation on the apical surface of the sections of the further developed membrane, or it perhaps represents a part of Held's Decknetz.
In the outer side of the thick epithelium, there usually appears at this stage a lighter area (L, fig. 5) resulting from the nuclei here being placed farther away from the apical ends of their cells. Comparison with later stages (figs. 6 and 7) suggests that this area represents the outer margin of the greater epithelial ridge.
In fetuses of about 8 cm., the increase in number of the cells in the epithelium of the basal side of the cochlear duct has rendered the thicker epithelium there still thicker and wider, and has also resulted in a second but lesser thickening along the outer edge of the first. At this stage the cells of both the greater and the lesser epithelial thickenings, or ridges, are engaged in the production of the finely fibrillar, tangled structure characteristic of the first and seemingly imperfectly formed part of the tectorial membrane. The cells of the lesser ridge, which later become differentiated into the elements of the spiral organ, seem to retain but a very short time this tendency to produce fibrils similar to that of their adjoining neighbors of the greater ridge, along with whom they have developed. While producing fibrils, they are never as actively productive as even the cells of the extreme axial edge of the greater ridge, and their product is more sparse and less mature than even the first product of the greater ridge. They represent the edge of the productive grading into the non-productive cells, and, as they differentiate into the cell elements of the spiral organ, they cease production altogether.
In figure 6, from a pig fetus of 9 cm., the first evidence in my series of the differentiation of the elements of the spiral organ is shown. The first to be distinguished is the series of inner hair cells {I AC). While in some sections of this stage and earlier, there may appear shrinkage cracks between the cells of either of the ridges, the beginning of the spiral tunnel has not appeared at this stage, though Prentiss represents the tunnel in his figure of the second turn of a pig of 5.5 cm. Figure 6, as does each of the figures following, represents a section through the 3rd half turn (the first half of the 2nd turn) of the coil. Development and differentiation begins in the basal end and precedes toward the apex. Therefore a section through the basal turn would show a more advanced stage of development.
The tectorial membrane in figure 6 has thickened so that a main body and an axial edge may be easily distinguished. The beginning of the invasion of the axial edge of the greater ridge by the mesenchymal tissue to produce the vestibular lip of the spiral limbus may be seen. Soon the cells of this axial edge, involved in mesenchyme, will cease to produce tectorial membrane, the cessation at the extreme edge having already begun. The outer edge of the body of the membrane is bluntly rounded and coincides with the outer edge of the greater ridge producing it. Attached upon this outer edge of the membrane are the few fibrils produced by the cells of the lesser ridge.
The amount of the fibrils or fibers produced by the lesser ridge reaches its maximum in pigs between 8 and 14 cm., depending wholly upon the extent to which the elements of the spiral organ have differentiated. The change of character of any of the cells of the ridge into that of any of the various elements of the spiral organ once established, those cells cease to produce the fibrils, become devoted to a different function. From the first they are never more than the outer border of less active cells grading into the non-productive type of the rest of the duct. These fibrils are always more sparse and appear to be supported by a far less amount of the interfibrillar matrix than those of the membrane proper. Further, the amount of the fibrils varies considerably in different individuals in the same stage of development. Figures 6 and 7 (LF) show an amount maybe a little greater than an average in my preparations from pig fetuses. Prentiss must have found a greater amount in the pig than any of my preparations show, especially in his figure 6 (pig of 13 cm.), which show^s the product extending well over upon the cells of Claudius and relatively thick, considering that it shows the effect of considerable shrinkage and agglutination. His argument stronglj' urges that a large outer zone of the tectorial membrane is produced by the cells of the spiral organ (and the cells of Claudius?), which zone he claims remains permanently attached in the adult to the cells producing it.
Held describes and figures for the rabbit fetus these tangled fibrils arising from the lesser epithelial ridge and later attached to the cells of the spiral organ, and calls them 'Haftfasern.' He thinks that a part of his Decknetz and a narrow outermost zone of the membrane is formed from them. In none of his figures from the rabbit do his Haftfasern show an abundance and arrangement similar to that of the membrane. On the contrary, his figures for his fetuses up to near term, show what I consider the outer edge of the tectorial membrane proper to terminate bluntly at the axial side of the series of inner hair cells. He states that his narrow outermost zone (formed from the fibrils in question) is not distinguishable in the older stages and he thinks it a retrogressive structure, for later, when the whole outspamiing zone of the membrane is becoming free, he finds only a thin relatively coarse net upon the outer edge (selvage) of the membrane.
Rickenbacher, for the fetal guinea-pig, shows a mass upon the lesser epithelial ridge, after the hair cells have differentiated, considerably greater than I have seen in the pig. He described it as of the nature of his granular, pale staining layer in which fibrils appear, stating that in guinea-pigs of 5.5 cm., the tectorial membrane proper ceases at the immediate axial side of the inner hair cells and that the pale staining mass becomes a fibrous 'Deckschicht' upon and produced by the spiral organ, and that it becomes a process of the tectorial membrane proper (p. 395). In another place he states that it adds a small outer zone to the membrane. It is later detached from the spiral organ, he agreeing that the outspanning zone of the developed tectorial membrane of the guinea-pig is free.
Prejitiss charges that I misrepresented Rickenbacher in using his name as one who agreed that "there is no good reason to assume that the cells giving rise to the organ of Corti ever have anything to do with" the development of the tectorial membrane. In again going over Rickenbacher's paper, I find Prentiss' charge a true one, and I cannot conceive of my reason at that time for using his name as I did in the sentence quoted by Prentiss. It is barely possible that his name was used instead of that of another author, Hensen probably, for while only in the passage of his paper incorporated by Prentiss does Rickenbacher state at all definitely that he considers a part of the tectorial membrane proper developed from the lesser epithelial ridge, yet in several places one may infer that he does o consider it. In the passage incorporated by Prentiss, the pa t in question is referred to as follows: "Die schmale Randzone ist eine sekundare Bildung, welche an dem kleinen Epithelial wulst ahgemndert wird." (Italics are mine). In my paper, as stated, no attempt was made fo enter fully into the processes of development of the membrane. Sections from relatively few early fetuses were examined. The purpose was to learn something of the nature of the membrane already developed. In the few sections I had representing the stages of the differentiation of -the spiral organ, the fibrils over the lesser ridge must have been exceptionally sparse, for I considered them as artefact and omitted to show them in the one figure offered dealing with development. Granular masses and filaments are usually seen in the sections adhering upon other parts of the epithelium lining the cochlear duct, especially in its angles, and I described the fibrils I saw as a thin, frayed reticulum, sticking upon the lesser ridge and continuous with the tectorial membrane, composed of coagulum filaments resulting from the action of the fixing fluid upon the endolymph. I considered Rickenbacher's pale staining substance over the spiral organ as artefact, most especially his detailed presentation of it in his figure 15, for in this he showed it as adhering to the young spiral organ by five stout processes, four of which each blended upon and surrounded only the hairs of the inner and outer hair cells. Between these few attaching processes were shown what were evidently shrinkage spaces, though he called them 'intercommunicating spaces in which the endolymph circulates.' He called the processes 'fiber bundles,' which was perhaps true, but the predominant structure of his mass appeared to be granular and I considered it artefact, as it was, especially the arrangement of his fiber bundles. In the present study, I cannot agree for the pig with any of his interpretations of his Deckschicht. I agree with him that the tectorial membrane becomes free from the spiral organ.
Up to pigs of about 16 cm., the greater epithelial ridge grows thicker and wider and the activity with which the membrane is produced by it increases. The outer third of the greater ridge becomes thicker than the axial two-thirds, the position of its nuclei giving the impression that its long cylindrical cells have been forced apexward by growth pressure. Figure 7, from the 3rd half turn of a pig of 14 cm., shows this but to a less extent than may occur at IG cm. The rapidly growing, outspanning zone of the tectorial membrane coincides with the surface of the greater ridge throughout. Its outer third cups around or clasps the more elevated outer third of the greater ridge, and its rounded edge is curved basalward to terminate at the inner hair cells of the young spiral organ (figs. 7 and 8). The cells comprising the outer margin of the greater ridge are directed outward, diverging toward a direction parallel with the surface of the spiral organ and thus maintaining the position approximately vertical to the basal surface of the enclasping outer edge of the membrane. The bluntly rounded outer edge of the membrane is frequently torn loose in the preparation of the sections. Then it may be straightened outward over the spiral organ, either projecting free from it or often pressed down upon it when distorted. Quite often, when torn free or distorted by pressure, the edge appears shrunken to a flattened, dark staining projection. Even when undisturbed and undistorted, the outer edge may project over the inner hair cell, as in figure 7. Then the fibrils supplied to this edge by the reclining cells of the outermost part of the greater ridge may be traced to curve into and around the edge, contributing to its rounded contour.
The fibrils produced by the lesser ridge and attached to the now thickened outer edge of the tectorial membrane, appear drawn more straight by the growth of the membrane and appear continuous with the 'peripheral condensation' of the apical surface of the outer edge. These fibrils and the apical peripheral condensations are considered above as homologous in that both represent the product of the first activities of the cells, a product less completely formed or organized than the body of the tectorial membrane later produced by the cells of the greater ridge. The fibrils, early formed by the lesser ridge, must contribute but a very small part of Held's Decknetz, for it is hardly possible, considering the physical character of the tectorial membrane and the very evident delicacy of the fibrils, that they can be pushed around and upon the apical surface of the membrane. In figure 8, which represents a section of the 3rd half turn of the cochlea of a pig of 19.5 cm., is indicated, I think, the process of rupture and disintegration of the few and always loosely associated fibrils produced by the lesser epithelial ridge.
The further increased thickness of the outer edge of the tectorial membrane, added to it from the basal side, has begun to draw into bundles and tear apart the sparse fibrils. From the preparations of stages below and above that represented by figure 8, I think that the cells of the lesser ridge, now the more advanced spiral organ, ceased to produce fibrils in pigs of about 12 to 14 cm., and never actively produced them. Being at the beginning involved with those produced by the greater ridge, the fibrils on the lesser ridge and later spiral organ remain continuous with the outer edge of the tectorial membrane proper but they contribute practically nothing to it. When produced in unusually large amount, they may produce the appearance of the loose plexus attached to the freed outer edge of the membrane observed by Held in the late fetus, or they may produce the "border plexus of Lowenberg" described in late fetuses by others. But, they partiallj^ if not entirely disintegrate. There is no evidence of them upon the clean, rounded, outer margin of tectorial membrane of the adult pig or even of the pig at full term : they do not show on the fully developed spiral organ, unless they may possibly contribute something to its lamina reticularis. In figure 8, the beginning of the cessation of production and the recession of the cells of the greater ridge is well advanced. The attached axial zone of the membrane ceases earliest to grow, of course. The invasion and growth of the mesenchymal tissue to form the vestibular lip of the spiral limbus soon involves the epithelial cells, which never become so high in the region, pocketing them in an inactive condition. Cessation of activitj^ of the cells of the greater ridge proper begins under Huschke's teeth and progresses outward. The activity seems to wane, the last product being less complete than earlier, then the cells begin to recede, their fibrils torn asunder as the space between them and the basal surface of the membrane increases. The receding cells decrease in number. The cells of the outer and thickest part of the greater ridge remain active longest, produce the thickest part of the membrane and its outer edge. With the recession and reduction of the outer part of the ridge, the membrane has attained its adult proportions. The greater ridge grows progressively wider and thicker toward the apical end of the cochlea and remains active longer at the apical end, thus producing the progressively broader and thicker apical end of the membrane. Figure 9, drawn from a section of the 3rd half turn of the cochlea of a pig of 22 cm., suggests the persistent activity of the outermost part of the greater ridge. In it, the final retrogression of the producing cells (GR) is represented. This retrogression seems to occur after the cells of the axial two-thirds of the ridge have completed their retrogression and decrease in number to form the cells lining the spiral sulcus. The indentation in the basal surface of the membrane over the disintegrating cells is thought to be artefact, for tracing backward through the stages suggests that ver}^ little if any of this region of the membrane is contributed by the outer part of the greater ridge. The process by which the spiral organ assumes its position under the outspanning zone of the membrane is about completed and, in the process, the cells which produced the outer regions of the membrane have been carried axisward from their product. Between the membrane and the organ and disintegrating cells, there is, in some of the sections of this stage, a suggestion of delicate fibrils, some of which seem to be drawn axisward. So, if that portion of the membrane which now lies over the disintegrating cells is contributed to by the cells at all, it is most probably only a few delicate fibrils drawn axisward, parallel against and contributing to the peripheral condensation of the basal surface.
Attention is called to the fact that the spiral organ has increased in size between the stage of 19 cm. (fig. 8) and 22 cm. (fig. 9). Though its growth is not quite completed, it has approached the thickness possessed by the adult in this turn of the cochlea. The growth increase in the thickness of the organ is indicated in table 4 in which averages of the thickness are given in micra. The measurements were taken from the surface of the organ, through the middle of the outer hair cells, to the basilar membrane.
Cochleae from one litter of pigs averaging about 21 cm. showed the differentiation of the spiral organ completed in the 3rd half turn. The cochlea from which figure 9 was made came from a pig of 22 cm. and the two of this stage measured for the results recorded in table 4 were chosen from this lot of pigs. The spiral organ was some thicker in one of the two than in the* other. The variations in the thickness of the 7th half turn, shown in table 4, are due largely to the varying distances from the actual basal end of the organ at which the plane of section passed, for at 19 cm., in my specimens, the differentiation of the organ in the basal region is about completed. Examination of the various stages, and the measurements, show that with the beginning of the differentiation of the spiral organ from the lesser ridge, the organ begins to increase in thickness throughout the cochlea, and that, though the increase takes place most rapidly in the stages before full term, it seems to continue after birth. The increase is greatest in the apical end.
TABLE 4 - Giving in micra averages of the thickness of the spiral organ (of Corti) as found for the pig in the various stages of development and the regions of the coil of the cochleae specified
Table 4. - to be formatted
| Table 4. |
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| TABLE 4 - Giving in micra averages of the thickness of the spiral organ (of Corti) as found for the pig in the various stages of development and the regions of the coil of the cochleae specified
SPECIMENS jg.^ jj^jp g^P jj^^j.
TURN
85.2
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The growth increase in the width of the organ could not be accurately measured in the 1st and 3rd half turns till above pigs of 22 cm., because, as shown in figure 9, its differentiation was not complete. It is usually completed throughout in pigs of full term. The varying width of the organ in the adult is given in table 3.
As suggested in the figures, the cells of the greater epithelial ridge finally all sink till they are represented only by the relatively few, broad, fattened cells lining the internal spiral sulcus. The outermost five to eight of the cells of the greater ridge retain a portion of their high cylindrical form and become the inner supporting cells of the spiral organ. As such, they retain to an extent their outwardly reclining direction. In the recession and disintegration of the cells, the decrease in number is such that the cells (nuclei) in the greater epithelial ridge of the apical turns at 15 to 16 cm. are twenty to twenty-five times the number of the cells which line the internal spiral sulcus and comprise the inner fourth (the smaller supporting part) of the mature spiral organ. Held and others before him have shown for other mammals that the retrogression and disintegration of the cells of the greater ridge begins at the axial edge of the ridge and proceeds outward, thus freeing from attachment first the axial border of the outspanning zone of the tectorial membrane; and Prentiss notes that the "inner half of the spiral organ" (inner supporting cells) is derived from the greater ridge.
The inner supporting cells, during their differentiation from the cells of the greater ridge recede slightly. Then they increase in both height and size in the increase in the size of the spiral organ so evident in the apical coils of the adult hog. However, as shown in the figures and as will be noted below, the outer supporting cells (cells of Hensen) increase in height and size considerably more than the inner during the growth of the apical regions of the organ. The cells of Claudius also decrease slightly in height and increase in width between the earlj^ fetus and the adult form of the spiral organ. The decrease in height seems to result from loss in their distal ends, for the nuclei are situated earlier in the middle and proximal (or basal) ends of the high columnar cells and later in the distal ends of the low columnar and cubic cells. In the adult, the cells of Claudius appear to increase considerably in height in passing from the apical to the basal end of the cochlear duct. It may be suggested that both the spiral organ and the cells continuous with it, though their differentiation is completed earliest in the basal end, do not undergo so extensive differentiation in the basal as in the apical end; (compare figures 1 to 4). Further, from study of the different stages of development, it seems probable that at least four cells of the lesser ridge take' part in forming the boundary of the tunnel of the spiral organ in a given section of it: (1) Each of the pillars of the organ appears to be produced b,y a separate cell which is entirelj^ used up in the process and its nucleus disappears; (2) the 'foot cells/ one bracing against the inner side of the basal end of each pillar, seem to be derived separately, and though they may aid in nourishing the pillars, they probably have little or nothing to do with their production.
Figures 10 and 11 are given to illustrate respectively the actual and the more usually apparent relation of the basal surface of the growing tectorial membrane to the cells of the greater epithelial ridge producing it. Each of the figures represents a few cells of the outer side of the middle third of the greater ridge, figure 10 from the 5th half turn of a pig of 15 cm. and figure 11 from the 3rd half turn of one of 14 cm. The letter /, indicating the hne of junction between the membrane and cells, is placed on the axial side of each drawing. Figure 10 shows an appearance seldom found in the sections of embedded material, but the appearance is, I think, the correct one. By close observation of the specimen from which it was made one may note from 3 to 8 fibrils given off from the apical end of each cell. The fibrils pass almost vertically from the surface of the cells and then curve axisward in the part of the membrane previously formed. Each fibril shows a slight, elongated enlargement in its immediate junction with its cell. The number of fibrils observed from each cell probably depends upon the planes in which the cells are split by the section more than upon variation in the sizes of the cells. The cells average about 7 m in diameter. Held computed for the rabbit from 33 to 38 fibrils per each 100 m of the surface line of section of the greater ridge. If 5 fibrils be considered as the average per cell, then each 100 ^ of surface of the ridge in the pig may involve about 70 fibrils. Taking into consideration the third dimension of the cell, as many as 25 fibrils may be given off by each cell.
That, begmning in the apical surface of the membrane, the fibrils course from the axial side outward and then basalward to their cells of origin is due to the fact that the greater ridge increases greatly* in width outwardly while the fibrils are being produced. Later, toward completion of the membrane, as the cells begin to recede along the axial side of the ridge and the ridge begins to decrease in width, the last produced ends of the fibrils are drawn axisward in the basal surface of the membrane. This drawing axisward is done only by the cells of the thicker and more persistent outer part of the greater ridge, the drawing, I think, extending in the basal surface only about as far axisward as Hensen's stripe, which it helps to form as well as helping to form the basal peripheral condensation as seen in the sections. The fibrils thus drawn axisward must form the arrangement on the basal surface of the outer part of the teased out membrane described by me as an "accessory tectorial membrane."
The increase in the number of cells in the greater ridge during its increase in width results, of course, in an increase in the number of fibrils and thus in the width of the membrane as compared with the earher stages. If the claim made by Prentiss that the tectorial membrane is a honey-comb or chambered structure, each chamber coinciding with and produced by a cell, were true, then in the sections would be found occasionally strips as wide as the cells, walls of the chambers parallel with which the plane of section had passed. Such strips have never been seen. On the contrary, the fibrillar character is one of the most evident possessed by the membrane.
In sections in which the condition similar to that shown in figure 10 could be seen, the region of junction (/) appeared fighter than the earlier formed part of the membrane. This is interpreted as indicating that the interfibrillar matrix here has not, as yet, been completely formed. Figure 11 represents the condition, some degree of which is the usual appearance in all sections of dehydrated cochleae while the membrane is being formed. It is interpreted as the result of the action of the reagents, used in the preparation for sectioning, upon the last and not yet perfectly formed part of the membrane. The matrix in this region must be in a stage more readily shrunken by dehydration and clearing than in the earlier formed part of the membrane. The fibrils appear agglutinated into kregular bundles with spaces between them more or liess free. In the area drawn the bundles were formed for the most part, one on the end of each cell, similar to the way cilia often appear in brushes in sections of the uterine tube and the ducts of the testis, for example. When shrinkage and agglutination are less evident than in figure 11, the bundles are smaller and may be formed indiiferentlj^ upon and between the ends of the cells. When more shrinkage has occurred the bundles may be larger and the shrinkage spaces may extend over several cells. The spaces are similar to those Rickenbacher pictured and considered as designed for the circulation of endolymph. Figures 10 and 11 are not camera drawings.
The attachment and functional position of the tectorial membrane are interrelated. The outspanning zone, after it is produced, becomes entirely free, first from its slight attachment to the young spu-al organ (lesser epitheUal ridge) and later from its parent cells, and the process by which it becomes free is the chief of the processes by which it later attains its functional position over the spiral organ. Held ('09) quite fully reviewed the ideas advanced by the different authors as to how the auditory hair cells, developed in the lesser ridge at the outer edge of the growing membrane, acquire their much more axial position well under the membrane, and I cited most of them in my former paper. I think all the processes possible have been suggested. When the younger stages are compared with the stages after birth (compare figures 7 and 8 with figure 2) it is seen that the spiral organ may become so shifted in its relative position that its hair cells come to stand under the middle of the basal surface of the outspanning zone of the membrane, and that the outer edge of the membrane may project in the 3rd half turn (and no doubt more in the 1st) well beyond the organ and even over the first 10 or 12 of the cells of Claudius. Under the narrow basal end of the membrane (fig. 4), the shifting of position is barely sufficient to bring the outer edge over the outer hair cells of the organ. In this region the greater epithelial ridge completes the membrane earlier, is active during a shorter period, and increases in width during production much less than in the apical turns. It may be noted further that in the more apical turns the outer supporting cells increase in height so much more than the inner that the apical surface of the spiral organ becomes markedly inclined axisward.
The ideas advanced as to how the spiral organ attains its relative position under the membrane include: (1) that the organ becomes shifted axisward bodily along the basilar membrane; (2) that growth in width and thickness of the vestibular lip of the spiral limbus serves to tip and project the membrane, attached upon it, over the spiral organ; (3) that the increase in the size and height of the outer supporting cells (cells of Hensen and Deiters) presses the hair cells axisward, causes the apical ends of the pillars to lean axisward and actually slightly displaces axisward the feet of the pillars; (4) that the membrane is produced in its functioning position with reference to the organ instead of there occurring a shifting of position of either the organ or the membrane.
The latter idea, that the tectorial membrane is produced in situ without changes in its position relative to the hair cells, was naturally one of the first to be advanced. Early discarded, it has been revived by Held and adopted by Prentiss. It requires, of necessity, that in the apical turns, the thicker, outer two-thirds of the outspanning zone be produced by the spiral organ, largely after its elements have been differentiated, and by the cells of Claudius (compare figure 7 with figure 2). A stage of development has never yet been described and, I think, never been seen, certainly not in the pig, in which this thicker (and, in the apical turns, by far the greater) part of the membrane was being produced by the cells of the differentiating spiral organ and the cells of Claudius. In the pig, and in the published figures from other mammals, the few sparse, loosely arranged and unsupported fibrils described as produced by the cells of the lesser ridge, attain their maximum amount long before the differentiation of the elements of the spiral organ is completed. They are produced, I think, most rapidly in the pig before the differentiation is well under way. The cells of the lesser ridge have never been seen in a relation to the growing membrane similar to that of the cells of the greater ridge seen in all specimens, and further, what is most evidently to be the whole out spanning zone of the membrane is found during its production, in all trustworthy preparations of the pig, overlying the cells of the greater ridge only and associated with them only. The idea is very simple but I am convinced the actual anatomy in development does not support it. From my study of the specimens I, as yet, find no evidence that the few fibrils seen coming from the lesser ridge and continuous w^ith the outer edge of the membrane proper can contribute appreciably, if at all, to the bulk of the membrane. I think they disintegrate for the most part at least, leaving the outer edge of the functioning membrane clean and bluntly rounded, its fibrils curving from the apical surface around the edge and axiswaVd in the basal surface. Both Rickenbacher's and Held's figures, from other animals, show what I consider the outer edge of the later tectorial membrane extending only to the inner hair cells.
Comparison of the different stages suggests that each of the other three ideas is tenable in part, two of them especially; that the relative position of the organ to the basal surface of the membrane does become shifted and that three factors may enter:
(1) The growth increase in thickness of the vestibular lip of the spiral Umbus can have nothing to do with inducing the membrane to span the spiral organ. • Averages obtained from measurements in several cochleae of each of six stages from 13 cm. to the adult show that in all the turns of the cochlea, the lip acquires its adult thickness in the fetus of 14 to 15 cm. At this stage the membrane proper is still being produced in all the turns of the cochlea and does not anywhere project over the spiral organ. The thickness of the lip in the different half turns in the pig of 14 cm. and the older stages, including the adult, were found to be strikingly similar. Further, the vestibular lip attains less thickness throughout in the apical turn than in the basal, while the membrane projects over and beyond the organ more in the apical turns. So it is hardly probable that the membrane is tipped outward by increase in thickness of the vestibular lip of the spiral limbus. The width of the vestibular lip, while difficult to measure definitely, likewise seems to increase very little between 14 cm. and the adult. The small increase in width is mostly attained in that part of the lip upon which the axial zone of the tectorial membrane is attached, that is, from the insertion of the vestibular (Reissner's) membrane to the edge of Huschke's teeth, and the edge of Buschke's teeth projects outward farther in the full term fetus and adult than in the earlier stages (compare figures 2 to 7) . The average widths of this part of the hp, which is most of it, are given in table 5. The greater variations in the averages for the width at the 1st half turn are due to the varying distances from the tip end of the lip at which the knife passed, for the lip narrows suddenly in terminating in the hammulus. The slight increase in width, shown in the table, between 13 cm. and 22 cm. can have little to do with other than the growth of the attached axial zone of the membrane. At 13 and 14 cm. the part of the lip in the 1st half turn is not sufficiently outlined to measure. Most of the increase seems to occur after 22 cm., that is, between this and the fetus at term and the adult, when the membrane is becoming and has become detached from its parent cells and from the spiral organ. This latter increase in the width of the Up may contribute a sUght projection of the tectorial membrane outward and over the spiral organ. The width of this part of the lip is about the same as the width of the attached axial zone of the membrane.
Table 5. - to be formatted
| Table 5. |
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| TABLE 5 - Giving in micra averages uf the width of the vestibular lip of the spiral iimbus from the insertion of the vestibular membrane to the edge of Huschke's teeth in the different sizes of the pig and the different turns of the cochlea specified
SPECIMENS
14 cm. 15 cm. 16 cm. 19.5 cm. 22 cm. near term adult
152.0 190.0 171.0 68.4 174.8 182.4 133.0 186.2 182.4 161.7 209.0 197.6 179.4 216.6 214.5 193.8 206.7 206.7 1 1
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TABLE 5 - Giving in micra averages uf the width of the vestibular lip of the spiral iimbus from the insertion of the vestibular membrane to the edge of Huschke's teeth in the different sizes of the pig and the different turns of the cochlea specified
(2) As seen in the drawings, the spiral organ grows in thickness between the time the tectorial membrane begins to span over it and the adult stage. The greatest increase in thickness occurs in its outer side by the increase in the length of the outer sustentacular cells, and the apical ends of these curve axisward. As a result the apical surface of the organ is made to incline axisward. The outer pillar of the organ also increases in length more than the inner pillar, and as a result of this, and perhaps of the increase in the length of the outer supporting cells also, the apical ends of the two pillars are forced axisward. At their differentiation, the apical ends of the pillars incline strongly outward (see fig. 8, 19 cm.) in conformity with the pressure of the cells of the outer part of the greater epithehal ridge. It seems evident, therefore, that the apical surface of the organ is forced axisward during growth and therefore under the tectorial membrane to a shght extent. The normal spaces between the elements of the spiral organ, including the large NueFs space, no doubt result in part from this movement of the organ axisward. Nuel's space, however, is present in considerable size before the upgrowth of the outer supporting cells has started (compare figures 8 and 2). This space increases in passing from the basal to the apical end. Any movement of the surface possibly produced in this way is, however, not enough to account for the change in relative position of the organ evident in the apical turns. In the basal end, the great increase in the height of the outer supporting cells does not occur, thus the apical surface of the organ is not inclined axisward, though, the pillars do not lean outward as might be expected. The change in the relative position of the organ with reference to the basal surface of the tectorial membrane is less in the basal end and progressively increases in passing toward the apical end.
(3) To accomplish the very marked shift in the position of the spiral organ with reference to the basal surface of the tectorial membrane, especially in the apical turns, some process is necessary by which the entire organ is moved axisward. During differentiation of its elements, the spiral organ is situated along the outer edge of the membrane and is even attached to this edge by the sparse fibrils formed by its cells before their differentiation is completed (fig. 7). In the adult the hair cells in the apical turns come to stand under the middle of the wide outspanning zone (fig. 2).
Pressure displacement of tissue in the direction of the least resistance is common in organogenesis and, in the adult, such displacement is a common observation of the surgeon. Comparison of the stages of the pig in the different turns of the cochlea shows that the feet of the pillars separate during the growth of the organ. The separation is greatest toward the apical end. Hensen ('71 and '73, cited from Held) found by measurement in the ox that the feet of both pillars are moved axisward. In the apical turn he found that the foot of the outer pillar shifted about 37 At and that of the inner pillar about 95 fi, thus not only indicating actual movement but also accounting for the separation of the feet during the enlargement of the organ. I do not know between what stages of ox fetuses the measurements were made. If two elements of the spiral organ move axisward at all, it is possible that all the elements may move. If the elements move at all, it is possible that they may move sufficiently to account for the change in the relative position of the organ. During the rapid increase in the width of the greater epithelial ridge, the differentiating spiral organ is moved outward.
The cells of the greater ridge, when it is widest (in pigs from 14 to 16 cm.), are about twenty-five times greater in number than the cells, derived from the ridge, which later line the internal spiral sulcus. The cells lining the sulcus in the adult are broader than the cells of the greater ridge, but they average certainly not more than three times as broad. As shown above, at from 13 to 16 cm., the outer third of the greater ridge is thickened into a rounded elevation, its cells and nuclei being displaced apexward and its outermost cells being forced in the outward direction (figs. 7 and 8), indicating considerable growth pressure. In the later recession and disintegration of the cells of the greater ridge, as the production of the tectorial membrane is completed, the pressure must not only be relieved, but, from the marked decrease in number of cells, there may result an almost negative pressure.
Measurements show that the space occupied by the width of the greater epithehal ridge increases throughout the coils of the cochlea up to pigs of 15 to 16 cm. (when the ridge is widest) and thereafter it begins to decrease very perceptibly. The measurements were taken from the membrana propria of the epithelium of the greater ridge, at its most axial extension under Huschke's teeth (S, figs. 1 and 7), to the apical end of the inner hair cell of the spiral organ. The averages of the measurements are recorded in table 6. As in the other tables, some of the variations in width of the greater ridge, and internal spiral sulcus, evident in the 1st and 7th half turns are due to the varying distances from the apical and basal ends of the cochlear duct at which the knife passed. In the specimens from pigs of 9 to 11.4 cm., the 1st half turn was not completed, and in those from 13 to 14 cm., the differentiation of the vestibular lip of the spiral hmbus in this turn was not sufficient for the measurement. In the 3rd half turn of the 9 cm. pig (see fig. 6) this differentiation had not taken place, and thus this measurement is only approximate. Individual pigs of a given length vary in the degree of development of the cochlea, especially if from different litters.
TABLE 6 - Giving in micra averages of the width of the developing and developed internal spiral sulcus, measured from the membrana propria of the epithelium under Huschke's teeth to the apical ends of the inner hair cells of the spiral organ, in the different pigs and the different regions of the cochlea specified
Table 6. - to be formatted
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| TABLE 6 - Giving in micra averages of the width of the developing and developed internal spiral sulcus, measured from the membrana propria of the epithelium under Huschke's teeth to the apical ends of the inner hair cells of the spiral organ, in the different pigs and the different regions of the cochlea specified
SPECIMENS
TURN
TCRX
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It may be seen from table 6 that between pig fetuses of 9 and 15 cm. the width of the greater epithehal ridge increases appreciably, and between 15 cm. and the adult the width of the space occupied by it decreases. The increase and the decrease is evidently greatest in the apical part of the cochlea. The decrease in the 1st and 3rd half turns may be as much as one-third of the width of the greater ridge when at its maximum size and activity. During the development of the spiral organ from the lesser epithehal ridge, the apical ends of the inner hair cells are in Une along the outer edge of the growing tectorial membrane. In the 3rd (and no doubt the 1st) half turn of the adult organ, these ends of the hair cells are situated along about the middle of the width of the outspanning zone of the membrane (fig. 2). In other words, it is suggested that in the apical coil of the cochlea, after the tectorial membrane is about completely produced and while the spiral organ is enlarging, the inner hair cells, and therefore the organ, may be moved axisward a distance of about half the maximum width of the greater epithelial ridge, the maximum width of the ridge representing approximately the width of ,the outspanning zone of the membrane produced upon it.
It is suggested that a movement axisward of the organ of about one-third the width of the outspanning zone may be induced by the pressure of the growing cells of Claudius and the enlarging supporting cells on the outer side of the organ, and be allowed by the retrogression and decrease in number of the cells of the greater ridge on its axial side. The remaining onesixth of the distance estimated for the apical coil may be accomplished by the other but less effective processes mentioned, namely, the inclination axisward of the apical surface of the organ. Table 6 shows that the movement is not so much in the basal part of the cochlea and figures 3 and 4, the 5th and 7th half turn of the adult, show that the distance of the inner hair cell from the outer edge of the membrane is less in the basal coils than in the apical turn. At the basal end, the greater ridge remains narrower and produces the narrow end of the membrane. Where the greater ridge is widest, there the membrane is produced widest, the reduction in the cells of the ridge is greatest, the enlargement of the outer supporting cells is greatest, and there, as a result, the change axisward of the spiral organ is greatest.
Prentiss states that measurement "shows no important change in the position of the spiral organ from the 13 cm. to the 18.5 cm. stage, nor later in the newborn" pig. He allows the inference that he measured "the distance between the inner angle of the cochlea and pillar cells," but gives no records of measurements. His averages for the 13 and 18.5 cm. stages and those for the newborn pig would have been interesting for comparison with like measurements that could be made in the specimens used here. On page 434 he describes the greater ridge as in,creasing in width by growth and multiplication of its cells, "carrying the spiral organ outwards," while he denies in other passages the possibiUty of the spiral organ being carried axisward.
With the completion of the retrogression and disintegration of the cells of the greater ridge and the simultaneous movement axisward of both the entire spiral organ and its apical surface, the thick, outspanning zone of the tectorial membrane, of necessity, becomes free. The few loosely arranged fibrils, produced by the lesser ridge before its completed differentiation into the spiral organ, may persist and remain attached to the outer edge of the membrane till torn asunder in the freeing process. There is no evidence of them on the membrane of the adult. In the fetus at term, when the process of freeing is being completed, it is possible that remnants of them may at times adhere as a delicate plexus to the outer edge of the membrane.
It is possible that the outer margin of the tectorial membrane, which, during its production, cups around the outer part of the greater ridge, may straighten outward slightly upon being freed. If so, such behavior would contribute sUghtly to the projection of the membrane beyond the spiral organ.
The thin axial zone remains attached upon the vestibular lip of the spiral Umbus. The cells producing this are never thick, do not undergo the growth disturbances undergone by the greater ridge proper, and the fibrous mesenchymal tissue added about them in the formation of the hp aids them in retaining the attachment. This, I am convinced, is, in the pig and probably all mammals, the only attachment of the tectorial membrane after acquiring its functioning form.
The freeing of the outer zone may not be an absolute necessity for the mediation of auditory impulses. In the birds the sensory cells, corresponding to the hair cells of the mammal, are dispersed in a simple epithelial ridge producing both them and the tectorial membrane, and the ridge not receding and disintegrating, the membrane remains attached to its parent cells. In the mammal, the otolith membranes of the maculae are homologous to the tectorial membrane, though simpler, and these remain attached in part to their parent epithelium, situated in which are the special sensory cells. Experiments with the model I have tried to construct, imitating the tectorial membrane and its environment, suggest that, were the outspanning zone attached, it could be disturbed by vibratory disturbances in the endolymph, though not so readily nor so definitely as when the zone is free. However, in the actual anatomy of the mammalian cochlea it is evident that this zone is free. Held ('09) cites Pre3^er for the observation that auditory reflexes do not begin in the guinea-pig till one-half hour after birth, and both Held and Rickenbacher found that the outspanning zone of the membrane is freed at about the time of birth. More suggestive are the observations of Kreidl and Yamase ('07) upon rats. As is known, the young of these rodents, unlike the guinea-pig, are born in a comparatively fetal stage of development. They found that auditory reflexes do not begin in them till 12 to 14 days after birth. Held found in rabbits, which, like the rat, are born in a fetal condition, that the outer zone of the tectorial membrane is not free at 6 days after birth, though it is entirely free in the adult stage. He does not describe cochleae of rabbits between 6 days after birth and his stages in which the zone is free.
Summary
- The bony labyrinth of the adult pig, as well as that of the pig fetus, is not fused with nor enveloped by the petrous part of the temporal bone and thus may be readily removed.
- A fixing fluid which also slowly but sufficientlj^ decalcifies is best used for normal appearances of the cochlear structures, and, especially with cochleae of adult pigs, it is necessary to make a small hole in the bony labyrinth to relieve pressure of gas formed within during decalcification.
- Measured within the outer wall of the bony part of the coil, the diameter of the basal coil of the cochlea of the adult ox is 8.4 mm.; that of the adult pig, 5.8 mm.; and that of man, 6.7 mm. The height of the cochlea, from the apical side of the scala vestibuH in the apical turn to the basal side of the scala tympani in the basal turn, was found to be, for the adult ox, 6.7 mm. ; for the adult pig, 4.4 mm., and for the human, 4.9 mm. The human cochlea seems to be slightly more flat than those of the ox and pig. The cochlea of the pig at term, exclusive of the outer bony wall, is slightly less in size than that of the adult pig. The shght difference is apparently due to an increase in the size of the two scalae in the adult.
- The length of the tectorial membrane of the adult pig is estimated from measurements to be about 26 mm. or less than 1 mm. longer than in the fetus at term.
- The attached, axial zone of the tectorial membrane of the adult pig is of practically the same width throughout the entire length of the membrane.
- As to the proportions of the outspanning zone of the tectorial membrane of the adult, indicating its adaptedness to serve as the chief vibratory structure of the auditory apparatus: '(a) it is widest and thickest at its apical end, tapering gradually and evenly to its basal end, which is its narrowest part ; (b) the width of the apical end of the outspanning zone appears from averages of measurements to be 6.8 times its width at the basal end; (c) in thickness, the apical end of the outspanning zone averages in sections of it 3 times its thickness at the 7th half turn of the coil; (d) in volume, a given short length of the apical end of the outspanning zone may be more than 41.7 times the volume of the same length of the basal end of the zone. Between the basal end and the 5th half turn, the longest interval between measurements in section, the volume of the outspanning zone may increase 90.7 per cent; between the 5th and 3rd half turns, the increase may be 57.4 per cent, and between the 3rd and the 1st half turns, it may be 29.8 per cent. These variations in the proportions are urged in support of the suggestion that the tectorial membrane is far more adapted to serve as the vibratory structure than is the basilar membrane.
- In the adult pig, the spiral organ (of Corti) increases appreciably in both thickness and width in passing from the basal to the apical end of the cochlea.
- That the membranous spiral lamina (basilar membrane) may be thrown into vibration by certain strong stimuli is not denied, and that the chief load carried by it, namely, the spiral organ, increases in passing from the basal to the apical end of the cochlea is considered as suggestive of its possible vibratory behavoir. The superior advantages of the tectorial membrane are enumerated and some of its possible activities compared.
- Pig fetuses of a given length vary very much in the stages of development of the cochlea, especially if from different litters. For a given stage of development, it seems that fetuses obtained in Louisiana average smaller than fetuses obtained in California.
- The cells of the lesser epithelial ridge, which differentiate into the elements of the spiral organ, at an early stage produce a delicate film of loosely arranged, imperfectly embedded fibrils. These have been erroneously considered by some as increasing to form an outer zone of the tectorial membrane; by others as forming a permanent attachment of the membrane to the spiral organ. It is urged that the cells of the lesser ridge, at first grading from the outer edge of the greater ridge, never actively engage in the production of fibrils, and, as they differentiate into the elements of the spiral organ, they cease the production altogether. They contribute little, if anything, to the formation of the adult tectorial membrane, though their few fibers may remain attached to its outer edge till torn asunder in the later processes. The membrane is produced by the greater ridge.
- With the differentiation of the spiral organ from the lesser epithelial ridge, the organ begins to increase in thickness and, though most of this increase occurs in the stages before full term, some of it seems to occur after birth. Growth changes of the organ occur least in the basal end of the cochlea. It is suggested that at least four cells of the lesser ridge take part in a given section in forming the elements comprising the walls of the spiral tunnel.
- The outermost part of the greater epithehal ridge becomes thicker by growth pressure than the remainder, is active for a longer period, produces the thicker part and outer edge of the tectorial membrane, and its outermost cells, in the process of recession and disintegration of the ridge, seem to differentiate into the inner supporting cells of the spiral organ.
- In the production of the tectorial membrane, each cell of the greater epithehal ridge may contribute an average of 25 fibrils to the membrane. Each fibril seems to show a slightly elongated enlargement at its junction with its cell. In the region of the immediate surface of the ridge, that of the product of the most recent activity of the cells, the interfibrillar matrix does not appear as abundant, or so completely produced, as in the older body of the membrane. This less completely formed part of the membrane shows shrinkage effects of the reagents in all sections of the cochlea.
- With the growth in width of the greater epithelial ridge in the early stages, the differentiating spiral organ (lesser ridge) situated along the outer edge of the growing tectorial membrane, is carried outward. The developed spiral organ acquires its position well under the basal surface of the tectorial membrane almost entirely by being carried axisward during the completion of the membrane. As the cells of the greater ridge recede from the membrane they disintegrate to about one-twentyfifth of their greatest number, thus more than relieving all growth pressure on the axial side of the spiral organ. During this process, the outer supporting cells of the spiral organ increase in size and height, producing growth pressure on the outer side of the membrane and producing an inclination axisward of the apical surface of the organ. In the apical turn, where these changes are greatest, the hair cells of the organ may be carried axisward a distance nearly half the width of the membrane. The upgrowth of the outer supporting cells also forces axisward the apical ends of the elements of the spiral organ and in this way contributes a small part to the shift in the relative position of the hair cells. A slight increase in width of the vestibular lip of the spiral limbus may contribute a still smaller part by extending the membrane outward.
- The outspanning zone of the tectorial membrane is, and of necessity becomes, free with the completion of the recession and disintegration of the cells which produce it, and by the movement axisward under it of the spiral organ. Such of the few fibrils produced earlier by the lesser ridge as may persist are of necessity torn away in the changes in position and all of them probably disintegrate.
Literature Cited
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1910 Sur la structure de la tectoria. Compt. Rend. Acad. Sc, T. 150.
1911 Les sangles des cellules de soutenements de I'organe de Corti. Jour, de I'Anat. et de la Physiol., An. 47.
1912 Sur la structure de la lame spirale membraneuse du limacon. Compt. Rend. Acad. Sc, T. 154.
Wiedersheim, R. 1893 Grundriss der vergleichenden Anatomie der Wirbelthiere. Jena.
Plates
Reference Letters
AF, lamina of peripheral auditory nerve fibers
AAZ, attached axial zone of tectorial membrane
BM' basilar membrane
CC, cells of Claudius
DC, supporting cells of Deiters
E, endothelium lining scala tympani
ES, epithelioidal syncytium
P\ auditory nerve fibers to outer hair cells
GR, greater epithelial ridge or thickening
HC, outer supporting cells (of Hensen)
HS, Hensen's stripe
HT, Huschke's (auditory) teeth
ISC, inner supporting cells of spiral organ
ISS, internal spiral sulcus
I AC, inner auditory hair cells
./, region of junction between fibrils and producing cells
L, lighter outer margin of young greater ridge
LF, fibrils produced by cells of lesser epithelial ridge
LM, liquefying mesenchymal tissue
LR, lesser epithelial ridge or thickening
NS, Nuel's space
OZ, outspanning zone
PC, peripheral condensation of incomplete structure
SG, spiral ganglion
SL, spiral ligament
SP, spiral prominence, stria vascularis
ST, scala tympani
SV, scala vestibuli
TM, tectorial membrane
VL, vestibular lip of spiral limbus
VM, vestibular (Reissner's) membrane
Plate 1
EXPLANATION OF FIGURES
Figures 1 to 9 are reproduced to scale
1 Section across the 1st half turn of the spiral organ and its tectorial membrane, cochlea of adult hog.
2 Section across 3rd half turn of spiral organ of adult hog; same cochlea as figure 1.
Plate 2
EXPLANATION OF FIGURES
3 Section across 5th half turn of spiral organ of adult hog; same cochlea as figures 1 and 2.
4 Section across 7th half turn of .spiral organ of adult hog; same cochlea as figures 1 and 2, and 3.
Plate 3
EXPLANATION OF FIGURES
5 Section of 3rd half turn of cochlear duct of pig of 5.5 cm., showing thickening of epithelium of axio-basal side and beginning of tectorial membrane.
6 Section of 3rd half turn of cochlear duct of pig of 9 cm., showing lesser epithelial ridge and the fibrils produced by it, and advancement in production of the tectorial membrane.
7 Section of 3rd half turn of developing tectorial membrane and differentiating spiral organ of pig of 14 cm.
Plate 4
EXPLANATION OF FIGURES
8 Section across 3rd half turn of developing tectorial membrane and differentiating spiral organ of pig of 19.5 cm. The recession of the cells of the axial side of the greater epithelial ridge is well under way.
9 Section across 3rd half turn of spiral organ and its tectorial membrane of pig of 22 cm. The recession and disintegration of the cells of the outer part of greater epithelial ridge is ncaring its completion, leaving the inner supporting cells of the spiral organ.
Plate 5
EXPLANATION OF FIGURES
10 Drawing from the outer part of the m'ddle third of the greater epithelial ridge in the 5th half turn of the cochlea of a pig of 15 cm., showing, at J. detail of the relation of the cells of the ridge to the fibrils and tectorial membrane they produce.
11 Drawing from the outer part of the middle third of the greater epithelial ridge in the 3rd half turn of the cochlea of a pig of 14 cm., showing, at /, a usual appearance in sections of the relation shown in figure 10, interpreted as due to shrinkage produced by the reagents used in preparing material for sectioning.
Cite this page: Hill, M.A. (2024, April 16) Embryology Paper - On the proportions, development and attachment of the tectorial membrane (1915). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_On_the_proportions,_development_and_attachment_of_the_tectorial_membrane_(1915)
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
