Book - Manual of Human Embryology 16-4
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The Development of the Ear
The ear of mammals and of man consists of the actual senseorgan, the inner ear (labyrinth and cochlea), and the sound-conducting apparatus, the middle and outer ear. In addition to its function in hearing, which belongs only to the cochlea, the inner ear also serves for the perception of the condition of equilibrium. The epithelial lining of the inner ear comes from the ectoderm, that of the middle ear from the entoderm, and that of the outer ear again from the ectoderm.
The inner ear of the mammals and of man in its early stages of development resembles exactly the corresponding organs of invertebrates, yet it is not correct to derive it directly from these. The sound-conducting apparatus has been developed within the vertebrate stem, after an aquatic mode of life had been exchanged for one that was amphibious or terrestrial.
The Inner Ear
The anlage of the auditory organ has been frequently regarded as serially homologous with the anlage of the olfactory organ, the lens, and the epibranchial sense-organs or placodes (compare p. 129 and 182) ; but this question will not be considered here. The anlage of the auditory organ, like that of the olfactory organ and lens, first appears as a plate of thickened ectoderm, which may be termed the auditory plate, and it occurs in this condition in an embryo of about nine pairs of mesodermic somites (Normentafel of Keibel and Elze, Plate 4). In an embryo of thirteen or fourteen pairs of somites (Normentafel, Plate 6) these thickened areas of ectoderm become slightly depressed, as is shown in Fig. 203, and the auditory plate begins its transformation into the auditory pit. As may be seen from Fig. V r of the Normentafel, these depressed areas lie dorsal to the second branchial grooves. The auditory pits now deepen rather rapidly, so that in an embryo of 2.5 mm. which had twenty-three pairs of somites (Normentafel, Plate 7) they are almost closed. When the closure is complete the auditory vesicles still remain for a time in connection with the ectoderm by a short, solid cord of cells, and even when this has disappeared one may still recognize its point of attachment in the ectoderm and in the auditory vesicle, which has now become almost spherical (Normentafel, Plates 8-11 and 13-20). When thus formed the auditory vesicle is in relation medially to the medullary canal in the region of the fifth neuromere and laterally to the ectoderm (Fig. 204), from which it becomes separated by mesoderm almost immediately after its closure.
Fig. 203. — A., optic anlage; G.ac, ganglion acusticum; Hpl., auditory plate; N., neuromeres. X 25. (After Keibel and Elze, Normentafel.'Fig. 6a.)
Fig. 204. — G. ae., ganglion acusticum; G.tr., ganglion trigemini; Mh., mid-brain; 3.V. and SN., third and fifth neuromeres; >V. /., nervus facialis; Obi., auditory vesicle. X 30. (From Keibel and Elze, Normentafel, Fig. 9c. )J
Very early, in some cases even while it is still in connection with the ectoderm (Normentafel, 13), the recessus labyrinthi, which later becomes the ductus and sacccus endolymphaticus , is formed. Fig. 205 a-d shows fonr successive sections through the right auditory sac, and the anlage of the recessus labyrinthi can already be recognized in Fig. 205a. It arises in man, as in other mammals and in birds, in immediate relation with the point of closure of the auditory vesicle, and is therefore comparable to the ductus endolymphaticus of the Selachians.
Even although this homology is no longer clearly shown in the development of the reptilia, yet I can see no reason for doubting the equivalence of this structure throughout the entire vertebrate series. For a discussion of this question compare Keibel (1899), Krause (1901 *), Peter (1901), and Alexander (1901).
Fig. 205 a-d. — ^A^., 6 N., 6N„ fourth, fifth, and sixth neuromeres. X 30. (From Keibel and Elze, Normentafel, Figs. 11 b-e.) Later the point of origin of the recessus labyrinthi or ductus endolymphaticus becomes transferred from the dorsal to the medial wall of the auditory vesicle by a portion of the lateral wall of the vesicle growing upward between the ductus and the body wall. At a rather early period a portion of the extremity of the ductus becomes enlarged to form the saccus endolymphaticus, yet this structure does not represent the actual extremity of the recessus, for, as Tandler 25 has described and as I also have found, this is drawn out into a thread-like structure that later disappears. The further development of the auditory vesicle in man has been described by W. His, junior (1899), and by Streeter (1906-1907) ; for its development in mammals the works of Krause (1890), Alexander (1900), and Denis (1902) maybe consulted.
Fig. 206 shows the left auditory vesicle of an embryo of 6.9 mm. (age, according to His, about four weeks) seen from the outer surface. From the dorsal, broader portion of the vesicle the semicircular canals are differentiating and from the narrower ventral portion the anlage of the cochlea. The semicircular canals arise as pouches, the anterior and posterior canals from a pouch which forms from the dorsal border of the vesicle, and the external canal, somewhat later, from another pouch that is directed laterally. The dorsal pouch becomes divided by a slight notch into an anterior and a posterior portion, and then the borders of the three pouches now formed become thickened, while the two epithelial layers at the centre of each come together and fuse, the epithelial plates thus formed later degenerating and their places being taken by mesoderm. The broadened margins of the pouches, whose lumina are of course in communication with the cavity of the auditory vesicle, are the anlagen of the semicircular canals; of these the anterior is the first to be completed, then the posterior, and the external is the last. From the ventral end of the vesicle the cochlear anlage grows out.
- In the Normentafel of Keibel and Elze, Plates 55 and 65.
Fig. 207 a and b show the vesicle of an embryo of 11 mm.
Fig. 206. — Left auditory vesicle of a human embryo of 6.9 mm. nape length (age about 4 weeks;. Seen from the outer surface. X25. 1, vestibular portion; at o and b the vertical semicircular canals are indicated as low folds; S, cochlear portion; 3, recessus labyrinthi (aquseductus vestibuli, ductus endolymphaticus). (After His, Jun., Arch, fur Anat. und Physiol., Anat. Abth. Suppl., 1889, Plate 1, Fig. 4.)
Fig. 207 a and b. — Left anlage of the labyrinth of an 11 mm. human embryo seen from the lateral surface (a) and from behind (b). X25. (After Streeter, American Journ. of Anat., vol. 6, 1906.)
seen from the lateral surface (Fig. 207a) and from behind (Fig. 207b). In the anterior part of the upper pouch a perforation has already formed in the epithelial plate, and at the notch of the upper pouch the crus commune of the anterior and posterior canals is forming. In Fig. 208 the canals are formed, the anlage of the cochlea has commenced to coil and is quite distinctly separated from the saccular portion of the labyrinth above. The separation of the sacculus and utriculus is also recognizable, although it is not yet very pronounced ; it is formed as a fold which grows in from the lateral surface towards the point of origin of the ductus endolymphaticus. Fig. 209 shows a model of the membranous labyrinth of a 30 mm. fetus seen from the median surface. Except for the separation of the sacculus and utriculus, which is still far from complete, the definitive conditions are almost reached. The fold that separates the utriculus and sacculus grows deeply into the origin of the ductus endolymphaticus and divides it in such a manner that the ductus remains in communication with both structures. Some diagrams will render this process more readilyunderstood. The first of these (Fig. 210a) represents a frontal section through the head of an embryo, the anlage of the labyrinth being cut on either side of the brain in such a way that the ductus endolyinphaticus, the anterior part of the upper pouch, the lateral pouch, and the anlage of the cochlea have come into the plane of the section; that the cochlea should do so is actually not possible, since at a very early stage it no longer lies in a single plane.
Fig. 208.— Anlage of the left labyrinth of a 20 mm. human embryo seen from the lateral surface. X25 N.V., nervus vestibularis; N.C., nervus cochlearis. (From Streeter, The American Journ. of Anat., vol. 6, 1906.)
Fig. 209.— Anlage of the left labyrinth of a 30 mm. human embryo seen from the medial surface. X 25. A". C, nervus cochlearis; N.V., nervus vestibularis. (From Streeter, The American Journal of Anat., vol. 6, 1906.)
In Fig. 210b the marginal portions of the anterior and lateral pouches are broader, and the central portions have come closer together; the cochlea has begun to separate and the fold that will separate the sacculus and utriculus has commenced to form. In Fig. 210c the marginal portions of the anterior and external pouches persist as semicircular canals, the epithelial plates which originally united them with the utriculus being indicated by broken lines, the canalis reuniens is fully formed and the separation of sacculus and utriculus is complete, so that the ductus endolymphaticus communicates with each of these cavities only by a narrow canal.
The histological differentiation of the epithelium in the anlagen of the special sensory areas of the ear follows immediately upon the ingrowth of nerves into them, so that it would seem to be provoked by this ingrowth (E. Krause, 1901, in Hertwig's Handbuch, vol. 2, part 2, p. 108). The epithelium of the auditory vesicle is at first one-layered, and even later the arrangement of its nuclei in several strata is not necessarily evidence that it has become many-layered. The differentiation of the epithelium is most pronounced in the cochlea, proceeding in this organ from the basal coil to the apex. The originally cylindrical cavity of the cochlea becomes triangular, and on its basal wall two epithelial swelling appear, a larger one towards the axis of the organ and a smaller one situated more laterally. The cells of the larger swelling secrete the membrana tectoria, 26 Siebenmann observing that in 28 According to Rickenbacher (1901), the marginal zone is secondarily secreted from the smaller epithelial swelling in the guinea-pig. Discordant results have also been obtained in the guinea-pig by Czinner and Hammerschlag (1 : an embryo of 4.5 cm. its formation begins in the uppermost coil. Then the greater part of its cells undergo a diminution in height, so that they separate from the membrana tectoria and the sulcus spiralis internus is formed; from this swelling, however, as Van der Stricht (1908) has shown in a bat (Vesperugo noctula), the inner auditory and inner supporting cells are formed (Fig. 211).
Fig. 210 a-c. — Diagrams of the development of the labyrinth: a represents a frontal section through the head of an embryo, cutting both labyrinths; b and c represent the further transformations of one labyrinth. For further description see text. /. Bo., v. o. Bg., anlagen of the lateral and anterior semicircular canals; C , anlage of the cochlea; D. e., ductus endolyniphaticus (recessus labyrinthi, recessus vestibuli); 5c, sacculus; S. e., saccus endolymphaticus; I. T., v. o. T., lateral and anterior upper pouches (predecessors of the semicircular canals).
Fig. 211. — Radial section through the cochlear canal of a new-born child, from a preparation kindly supplied by Professor R. Krause, of Berlin. X150. Reissner's membrane was defective and is represented only diagrammatically where it was lacking. E., larger, e., smaller epithelial swelling; M. b., membrana basilaris; M. t., membrana tectoria; R.. Reissner's membrane; St. v., stria vascularis; V. sp., vasspirale.
While referring the reader to the original paper for the literature of the question and for details, it may nevertheless be noted that in Vesperugo noctula the membrana reticularis of the crista? and maculae acusticae is not actually a cutieular formation, but rather a system of protective ridges. As to the participation of the larger and smaller epithelial swellings in the formation of the elements of the organ of Corti, there are formed 1 1. From the larger swelling: J. A row of inner auditory cells.
1. A row of inner supporting cells. II. From the smaller swelling: 1. A row of inner rod cells.
2. A row of outer rod cells.
3. Three rows of outer auditory cells.
4. Three rows of Deiters's cells.
5. Hensen's cells.
Later there occurs a pushing of the cells from the periphery toward the axis. The inner rod cells form from the beginning a cell row uninterrupted by any other cell, while the outer ones, on the contrary, are commingled with the outer sensory ceils; this condition explains the numerical difference between the outer and inner rod cells in the adult.
The development of the stria vascularis is as yet uncertain; Retzius supposed that its epithelium became vascularized, while others contend that the elements lying between the blood-vessels are connective-tissue cells and others believe them to be of mixed origin." The epithelial sense-organ becomes enclosed within a membranous and a bony capsule. The mesenchymatous tissue in its neighborhood condenses and becomes converted first of all into cartilage, which, however, does not extend quite to the epithelium, being separated from it by a layer of connective tissue, which, while thin on the outer surfaces of the semicircular canals and cochlea, is elsewhere well developed. Around the semicircular canals, the utriculus, and sacculus this tissue soon separates into three layers, — a perichondria] layer, a dense membranous layer resting directly upon the epithelium, and an intermediate loose mesenchyme, which is usually described as mucous tissue. In the cochlea special conditions obtain. Its modiolus and lamina are •not preformed in cartilage, the dense connective tissue which appears in the regions of these structures at a relatively late period ossifying directly; the tissue, however, which encloses the cochlear canal behaves like that which surrounds the semicircular canals. The transverse section of the cochlear canal from being circular becomes triangular, the anlage of the lamina spiralis being attached to its inner angle. Upon its outer side there lies only a relatively thin sheet of dense connective tissue, while on the other two sides, on the upper one, which becomes the epithelium of Reissner's membrane, and on the lower, from which the organ of Corti is formed, and also on the modiolus and the anlage of the lamina spiralis, the mesenchyme becomes divided into three layers as described above. The spaces occupied by the loose mesenchyme are converted by the disappearance of that tissue — Bottcher (1869) speaks of a fatty degeneration — into the perilymphatic spaces, in the case of the cochlea into the scala vestibuli and the scala tympani. This transformation begins in the vestibular region, where the space first formed is termed the cisterna vestibuli. Some strands of the tissue persist in connection with the semicircular canals, and probably serve to maintain them in their proper position. In the cochlea the formation of the scala? proceeds from the base towards the tip.
It may be noted that, according to Gaupp (1907, p. SSI), the oar capsule of the mammals and of man cannot be regarded as exactly homologous with that of the amphibia and fishes. Gaupp says : "Several peculiarities in the configuration of the labyrinth region in the amniotes, especially in mammals, become intelligible in the supposition that the ductus eoehlearis during its development penetrates into the lateral part of the base of the chordal portion of the skull and transforms this original solid portion of the skeleton into a part of the ' ear capsule.' ' The ear capsule of the mammals is accordingly equivalent to the ear capsule of the amphibia plus an additional part derived from what was originally a portion of the base of the skull.
- 27 Compare Merkel (1893), who cites the older literature, Leimgruber (190.3), and Shambaugh (1906).
For accounts of the ossification of the ear capsule one may consult Vrolik (1873) and Siebenniann (1897) ; for the postembryonic growth of the labyrinth Alexander (1902) and Sato (1903).
The development of the n. acusticus and its ganglia is closely related to that of the labyrinth, and in the description of it given here I shall follow the account given by Streeter (1906-1907), which differs from that of His, junior (1889), in showing a closer relation of the ramus sacculi and of the ramus ampullar posterioris to the utricular and ampullary branches and a greater independence of the n. eoehlearis. The acustico-facialis ganglion is originally a single cell mass lying in front of the anlage of the auditory vesicle, but later it forces its way between that structure and the lateral wall of the anlage of the brain. At a rather early period the geniculate ganglion of the facialis separates from the outer side of the cell mass, nerve-fibres from which have already penetrated the central nervous system. After the separation of this ganglion one can distinguish in the acoustic ganglion a pars superior and a pars inferior, and from each of these (in an embryo t)f 7 mm.) a nerve passes towards the auditory vesicle (Fig. 212). In an embryo of 9 mm. the ganglion cochleae becomes distinct at the lower border of the ganglion complex and one sees fibres growing from it towards the brain (Fig. 213 a and b), and a second twig, the ramus sacculi, has arisen from the pars inferior. Figs. 214 a and b and Figs. 215 a and b, after Streeter (1906-1907), show the further separation of the ganglion cochleae in embryos of 20 and 30 mm. greatest length.
The development of the middle ear in man has been made clear by the thorough investigations of Hammar (1902). 28 Three periods may be recognized in the development: 1, the period of the primary tympanic cavity (formation period, Hammar), beginning in the first month (embryo of 3 mm.) and lasting into the seventh week (embryo of 18.5 mm.) ; 2, the period of the tubotympanic canal ( Hammar 's separation period), ending in the beginning of the third month (embryo of 24 mm.) ; and 3, the transformation period, in which the tubotympanic canal is transformed into the definitive tympanic cavity and the tuba auditiva ; this last period is not completed at birth, but is continued into postfetal life. A dorsal prolongation of the first pharyngeal groove forms at an " s Of older works on mammals that of Piersol (18S8), who studied the rabbit, may be mentioned.
early period, and takes the form of a flat pouch whose tip and outer wall are at first in contact with the ectoderm of the first branchial groove, although in the fifth week they become separated from it by mesenchyme growing in between in the dorsoventral direction (Fig. 216, Dors. I).
Fig. 212-215b. — The differentiation of the left ganglion acusticum into the ganglion vestibulare and the g. cochleare and the development of its branches; the g. vestibulare is finely stippled and the g. cochleare coarsely. Figs. 213a, 214a, and 215a are views from the medial surface and Figs. 212, 213b, 214b, and 215b from the lateral. (After Streeter, 1906-1907.)
The tip of the dorsal prolongation is the anlage of the anterior recess of the tympanic membrane; from it the tnbotympanic groove (tt. R.) extends orally to the pharynx, and aborally is the groove for the tensor tympani (T.R.), this latter groove extending medially to the root of the second visceral arch into the posterior tympanic groove and through this into a dorsal prolongation of the second pharyngeal groove (Dors. II). Between the tnbotympanic and the tensor grooves the roof of the pharynx is depressed by the auditory vesicle (impressio cochlearis, Impr. cochl.).
Fig. 216. — Pharynx of an 8 mm. (nape-length) human embryo seen from the dorsal surface, the chorda, aorta, and branchial arch arteries having been removed. The anlage of the tympanic cavity and tuba is represented by a yellowish color. X 28. The following explanations of the lettering serve also for Figs. 217, 218, and 219 a and b. Amb., incus; Cochl., cochlea; D. nl., ductus nasolacrimal ; Dors. I, //, 777, dorsal prolongations of the 1st, 2d, and 3d pharyngeal grooves; F. conch., fossa concha?; Ggl. VII, IX, X. ganglion of the facialis, glossopharyngeus, and vagus; Ggpl., plate of the external auditory meatus; Ham., malleus; Hgr., manubrium mallei; ht. R., posterior tympanic groove; Hyp., hypophysis; Hyp. St., stalk of hypophysis; Impr. cochl., impressio cochlearis; Meek., Meckel's cartilage; N., nasal cavity; Org. I, II, III, 1st, 2d, and 3d branchial cleft organs; Par., parotid gland; Pfh., tympanic eminence; pr. bv., processus brevis mallei; pr. Gg. primary auditory meatus; pr. P., primary tympanic cavity; pr. Zl., primary dental ridge; R., Reichert's cartilage; Rec. ant., recessus membrana tympani anterior; Rfc. post., recessus membrane tympani posterior; T. R., groove for the tensor tympani; tt. R., tubotympanic groove; tub. R., tubal groove; vi. R., anterior tympanic groove. (After Hammar, Arch, fur mikr. Anat., vol. 59, 1902.)
The dorsal prolongation of the first pharyngeal groove (together with its various parts, such as the tubotympanic groove, the tensor tympani groove, and the anterior tympanic pouch), the posterior tympanic groove, and the portion of the impressio cochlearis which at first lies medial to these structures, form the primary tympanic cavity; its area is represented in Fig. 216 by a yellowish color. The ventral part of the first pharyngeal groove, into which the tympanic cavity is at first prolonged, soon atrophies completely. When the first pharyngeal and branchial grooves become separated, the primary tympanic cavity is forced by the thickening basis cranii from its upright wing-like position into a horizontal one (Fig. 217). The tubotympanic and posterior tympanic grooves increase in height and thereby the tubotympanic groove becomes divided by a knee-shaped bend into a very short tubal and a much longer anterior tympanic groove, while the posterior recess of the tympanic membrane (Rec. post.) becomes formed at the oral end of the posterior tympanic groove. Between the anterior and posterior recesses of the membrana tympani lies the blastema of the tendon of the tensor tympani, producing a notch, the incisura tensoris tympani (T.E.). Behind the incisura the lateral wall of the tympanic cavity is pushed inwards by the anlage of the manubrium mallei, forming the impressio manubrii. " This impression lies aboral to the point of contact of the first branchial groove and the first pharyngeal pouch; the manubrium mallei consequently projects," according to Hammar, "into the tissue of what will later be the second arch. ' ; It must, however, grow into this secondarily, since, as will be seen later, it belongs genetically to the first branchial arch.
Fig. 217. — Pharynx and the neighboring parts of a human embryo of 18.5 mm. (nape-length) seen from the dorsal surface. X21. The lettering as in Fig. 216. (After Hammar, Arch, fur mikr. Anat., vol. 59, 1902.)
The tubotympanic canal is formed from the primary tympanic cavity by a constriction which takes place in an aboral-oral direction, and it is on this account that Hammar speaks of a period of separation. The beginning and end of this period are shown in Fig. 218 and Fig. 219 a and b. The constriction begins at the boundary between the aboral end of the posterior tympanic groove and the dorsal prolongation of the second pharyngeal groove, and is produced by a proliferation of the tissue of the second branchial arch. By it the elongated, slit-like pharyngeal opening of the primary tympanic cavity is gradually shortened, and, finally, the tubotympanic canal forms a triangular, prismatic tube, slightly enlarged towards its posterior blind end and directed laterodorsally and aborally from its entrance into the pharynx. Its anterior tubal portion, which the tubal groove has helped to form, is at first quite short. During the transformation period, which now succeeds, the tubotympanic canal assumes a flattened, slightly spiral form; it stands at first with its walls almost horizontal; but becomes directed along the outer surface of cartilaginous ear capsule, which is increasing in length, so that from the third to the fifth month it has an almost vertical position. In the sixth month the os petrosum rotates around its long axis towards the outer side, the cupola of the cochlea becoming depressed, and a temporary depression of the tympanic cavity is thereby produced, so that its walls again assume an almost horizontal position. In the seventh month the cavity gradually returns to the half-upright position, which it still retains at birth.
Fig. 218. — Pharynx of a human embryo of 20.5 mm. (nape-length) seen from the dorsal surface. X 21. * (as in Figs. 219 a and b) at the knee-shaped bend where the tubal part of the tubo-tympanic canal meets the much longer tympanic portion, which is directed laterally and aborally; **, boundary between the primary tympanic cavity and the rest of the second pharyngeal pouch (Dors. II). For explanation of the lettering see Fig. 216. (After Hammar, Arch, fur mikr. Anat., vol. 59, 1902. )
The tuba grows rapidly in length, and on the formation of its cartilage (in the fourth month, according to Siebenmann, 1897) its lumen becomes slit-like. In the third month (fetus of 50 mm.) the lumen of the tympanic cavity disappears for the most part by wards the end of fetal life the posterior (more rarely the anterior) recess of the tympanic membrane gives off a process, which extends upwards from the processus Torevis of the malleus on the lateral wall of the tympanic cavity; this forms Prussak's space. The enlargement and further development of the tympanic cavity is associated with the presence of a submucous, peritympanic areolar tissue, which makes its appearance in the third or fourth month and is fully formed in the sixth or seventh. It forms two masses, the tympanic and the epitympanic areolar tissues. The tympanic tissue appears first on the inner wall of the cavity and later on the lower and posterior walls, but is lacking over the summit of the promontory. The epitympanic tissue lies above the cavity; in the seventh month it surrounds only the aditus and does not correspond in its dimensions with the later extent of the tympanic cavity, but a growth of it takes place commensurate with this extension. Furthermore the extension of the tympanic cavity in the later fetal months does not take place gradually and continuously, but to a certain extent discontinuously, a phenomenon that finds its explanation in the fact that a breaking down of the areolar tissue to form cavities filled with fluid takes place at intervals and with the absorption of the fluid the tympanic cavity seems suddenly to extend into the region thus prepared for it.
Fig. 219a. — Pharynx and right external auditory meatus of a human embryo of 24 mm. (nape-length) seen from the dorsal surface. X21. For explanation of the lettering see Fig. 216. (After Hammar, Arch, fur mikr. Anat., vol. 59, 1902.)] its epithelial surfaces coming into contact, but it is again completely restored at the beginning of the fourth month (fetus of 90 mm.) and persists from that time onward, although it becomes F. conch. Amb. Rec. ant. Ham.
Fig. 219b. — The same with the cartilaginous labyrinth, the malleus and incus, and Meckel's and Reichert's cartilages seen from the oral surface. X21. For explanation of the lettering see Fig. 216. (After Hammar, Arch, fur mikr. Anat., vol. 59, 1902.) very narrow in the sixth month, owing to the depression of the cavity which then takes place. Concerning the further development of the tympanic cavity the following may be said. 29 To
- 28 1 shall use the terras lateral, medial, anterior, posterior, upper, and lower in the sense in which they are ordinarily used in connection with the fully developed tympanic cavity.
Imbedded in this areolar tissue are originally the auditory ossicles, the chorda tympani, muscle tendons, and ligamentous connective tissue, and, as the tympanic cavity enlarges, all these structures become inclosed in folds of its mucous membrane. Hammar (1902) has followed the development of each of these folds, but I can only refer to his investigations here. In addition to the typical and constant folds, whose origin has just been explained, others also occur which are simple reduplications of the mucous membrane and as such are very variable and frequently quite transitory. According to the statement in the majority of text-books (Kolliker, 1879, Minot, 1894, 0. Schultze, 1894, Kallmann, 1898, Tourneux, 1st ed. 1898, Bryce, 1908), the areolar tissue disappears only after birth, the tympanic cavity at that time almost or completely lacking a lumen. Frequently, too, the disappearance of the tissue has been brought into direct causal relationship with the occurrence of respiration and this idea has gained some forensic importance. Wendt (1893) says: "1. When in a mature or nearly mature fetus or in a new-born child the thickening of the mucous membrane of the tympanic cavity is found to be still completely existent, an energetic respiration, intra-uterine or post partum, has not yet occurred. " "2. When the mucous membrane of the tympanic cavity is found degenerated or without macroscopic enlargement in a fetus or a new-born child, a strong respiration, intra-uterine or post partum, has taken place. ' ' Consequently he holds that the investigation of the tympanic cavity (the ear test) is capable "of replacing the lung test within certain limits." Among more recent authors Aschoff (1897, p. 295) and Siebenmann (1897) have pronounced against this view and the conditions have been more fullv elucidated by Hammar (1902).
Pneumatic cells are to be found at the close of fetal life in process of formation, especially from the upper squamosal portion of the cavity and, to a lesser extent, from the petrous portion and from the posterior and lower parts of the actual tympanic cavity.
The development of the auditory ossicles seemed to have been finallv settled, after long discussion, 30 by the observations of Baumgarten (1892), Dreyfuss (1893), Zondek (1895), Hegetschweiler (1898), and especially of Broman (1898, 1899), when Fuchs (1905) again unsettled the whole matter.
I shall give here, in the first place, Broman 's account. According to him, the dense mesodermal blastema masses of the first and second visceral arches are each divided in their proximal parts into a lateral and a medial portion by the trigeminus and facialis respectively. From the first arch are formed the malleus, incus, and Meckel's cartilage, the incus from the proximal part of the lateral blastema, and the malleus and Meckel's cartilage from the distal part of the medial blastema; the distal part of the lateral blastema is largely used in the formation of the outer ear.
From the second arch are formed the stapes and Eeichert's cartilage, both coming from the medial blastema and being originally connected by a bridge, which Broman terms the interhyal. From the proximal part of the lateral blastema of the second arch is formed the later ohyal (Broman) — the intercalare of Dreyfuss — which later fuses with the capsule of the labyrinth ; the distal part of this blastema takes part in the formation of the outer ear. Broman believes that he has proved that skeletal parts of different origin have also their own prechondral nuclei ; in the region where two such nuclei come into relation with one another there will be at least transitorily a disk of blastema ("intermediate disk"). These conditions are found in the anlagen of the auditory ossicles. Considering first of all the incus, its anlage unites with the labyrinth capsule while still in the blastemic stage and again becomes distinctly separated from it only as it passes into the prechondral stage. The previously established connection with the anlage of the stapes becomes the crus longum and from the intermediate disk the articulation develops; the posterior part of the blastema forms the crus breve. In general the incus acquires its definitive form while still in the prechondral stage, only the process lenticularis forms later, after the beginning of ossification. This begins at a single spot in the upper part of the cms longum and extends finally into the processus lenticularis, which, since it possesses no ossification centre of its own, cannot be regarded as an epiphysis.
- 30 For the important morphological questions connected with this problem and for the historical development of the discussion see E. Ganpp (1S99).
The malleus shares a prechondral nucleus with Meckel 's cartilage; it is separated from the incus by an intermediate disk, in whose place a joint will form at a somewhat late date, 31 Schmidt (1903) finding the first indication of a joint cavity in a fetus of 9.6 cm. vertex-breech length. In the cartilage stage it is still united with Meckel's cartilage and only becomes separated from it at the commencement of ossification. This takes place from a single centre, situated in the neck. The so-called processus anterior (Folii) is not formed by it; it is a membrane bone that unites with the rest of the malleus at a late period (in the sixth fetal month, according to Dreyfuss, 1893, p. 652).
The blastema of the stapes is perforated by the stapedial artery, and in the earlier stages of its development it is known, on account of its form, as the annulus stapedialis. Both it and Reichert's cartilage have their own prechondral cartilage; the interhyal, situated between the stapes and Reichert's cartilage, never reaches the prechondral stage, but the laterohyal has its own prechondral nucleus. Up to the second half of the third month of embryonic life the stapes remains ring-shaped, but at that time it begins to assume its definitive form, and at the end of the third month the stapedial artery also, as a rule, disappears. The capsule of the labyrinth takes no part in the formation of the footplate of the stapes. The precartilage in the vestibular fenestra of the human embryo does not, it is true, become directly transformed into connective tissue, as it does in the rabbit and guinea-pig, but becomes transitorily true embryonic cartilage (Dreyfuss, 1893), and finally becomes converted into a thin layer of connective tissue, which does not differ from the perichondrium of the fossa of the fenestra. The precartilage of the cochlear fenestra is transformed directly into connective tissue in man also. The centre of ossification for the stapes is situated usually in its base.
The connection of Reichert's cartilage with the capsule of the labyrinth is formed by the laterohyal. The malleus and incus consequently belong to the first visceral arch and the stapes to the second ; the labyrinth capsule does not take part in the formation of the stapes, as it does in that of the functionally corresponding skeletal structure of the amphibia.
Quite different are the results obtained by Fuchs (1905) from a study of the rabbit. He denies any primary ontogenetic relation between the anlage of the stapes and that of the hyoid cartilage; the entire stapes is formed from the capsule of the labyrinth. The connection of the malleus-incus anlage with the cartilage of the first arch is also secondary. — Since fundamental differences in the developmental processes in man and the rabbit are excluded, there is here an irreconcilable contradiction. It may be remarked that, according to my experience, there is great difficulty in tracing back a skeletal structure to the early prechondral stages, 32 and it carries with it the dangers of subjective interpretations. In my opinion, however, one may for the present accept the old view of Reichert that the malleus and incus are formed from the first visceral arch and the stapes from the second, for the topographical evidence upon which Fuchs bases his conclusions does not seem to be free from objection; Siebenmann (1894), who regards the discussion over the origin of the ossicles from the first or second visceral arch as quite superfluous, and contends that each of these structures, as well as Meckel's and Reichert 's cartilages, is a quite independent element, disregards too much the importance of comparative anatomy for his position to be seriously discussed.
- Concerning the mode of articulation and the manner in which the articulations develop, very discrepant statements exist. Compare Schmidt (1903).
It may finally be remarked that at birth the auditory ossicles have already reached their definitive size (Urbantschitsch, 1876). Nevertheless they later increase in weight, with the exception of the stapes, which has already attained its definite weight in the eight months fetus (Eitelberg, 1884, Bistrzycki and von Kostanecki, 1891).
Concerning the muscles of the middle ear only a few remarks are necessary. The tensor tympani has made its appearance at the end of the second month (Broman, 1899), and is connected at its distal end with the tensor veli palatini, a connection which is dissolved at the end of the third month, although it may persist throughout life (Schwalbe, 1887, p. 506). 33 The stapedius arises at the middle of the third month. According to C. Rabl (1887). it seems to form a genetic group with the m. stylohyoideus and the posterior belly of the digastric. 34 The muscle is surrounded by a membrane of connective tissue, which later ossifies (Broman, 1899, Dreyfuss, 1893) to form the eminentia pyramidalis.
The cartilage of the tuba auditiva is formed in the fourth month (Krause, 1901 2 ) ; Zuckerkandl (1906) found no trace of it in a human fetus of 5.1 cm. It has no relations to the visceral skeleton.
- 32 This naturally holds true also in the estimate of Broman's statements concerning these stages.
- 33 According to Killian (1890), the tensor tympani is derived from the m. pterygoideus and with this ultimately from the m. adductor mandibular of the Selachians. A thorough description of its development in the pig has been given by Esehweiler (1904).
- Killian (1S90) derives the stapedius from the m. depressor maxilla? inferioris of the Selachians.
A classic account of the development of the human auricle or pinna was given by His (1885), and to it additions have been made by Gradenigo (1888) and especially by G. Schwalbe (1889 1 ' 2 * 3 , 1895, 1897). Mention should also be made of the recent investigations by Henneberg (1908), for, although they did not include human embryos in their scope, yet they yielded such uniform results in forms as widely separated as the rat and rabbit on the one hand and the pig on the other, that a far-reaching importance
Fig. 220. — Cranial end of a human embryo at the beginning of the second month, with the auricular hillocks. X 12. c, Schwalbe's free auricular fold. (After Schwalbe, from Bardeleben's Handbuch, vol. 5, 2, p. 127, Fig. 12.) may be credited to them. In this connection, finally, the observations of Baurn and Dobers (1905) on the pig and sheep may be mentioned.
The region surrounding the first branchial groove develops in such a way that three elevations or hillocks (auditory hillocks, auricular hillocks, colliculi branchiales) are formed on the mandibular arch and three more on the hyoid arch. They are numbered in Fig. 220 from the ventral toward the dorsal extremity of the mandibular arch and in the contrary direction on the hyoid.
Behind the three hyoid hillocks the free auricular fold is formed, according to Schwalbe quite independently, as a fold of the integument similar to what occurs in the formation of the eyelids ; it is identical with the structure which His termed the cauda helicis and Gradenigo the helix hyoidalis. Later a slight swelling appears over the dorsal end of the first branchial groove; it unites caudally with the free auricular fold ; apically it fuses with the third hillock and then extends ventrally to in front of the beginning of the second. The investigation of the further history of these structures is unusually difficult, and I give herewith, after
1. Mandibular hillocks: Hillock No. 1..
Tragus. . .
Hillock No. 2..
Helix j Hillock No. 3..
Helix 2. Hyoidal hillocks: Hillock No. 4 . .
Hillock No. 5..
Antitragus Hillock No. 6..
Lobulus auriculae 3. Helix hyoidalis (Gradenigo), free auricular fold (Schwalbe ), cauda helicis (His) Helix . . . .
4> Helix mandi
Gradenigo (man and mammals).
Depressed beneath the surface
(Proc. infer, hel. Crus helicis. mand.) crus helicis
Baum and Dobers (pig and sheep).
Henneberg (rat, rabbit, and pig).
Crus helicis, anthelix
(Proc. sup. hel. Part of helix as- Helix ascendens . mand.) degener- cendens (anates terior)
Anthelix.. (Proc. sup. hel. Crus inf. anthe- Cranial plica long hyoid.) crus inf. licis anthelicis in part I
Crus helicis, part of helix
Part of scapha, crus anthel. sup.
hyoid.) degener- cis inf. ? plica longitudi- crus anthel. inf.
ates nalis, tip of auri cle
Depressed beneath the surface
Antitragus Caudal plica long., Part of scapha, anti which later dis- tragus, plica anti appears; anti- tragica = crista tragus? anthel. inf.
Helix and anti- , Helix posterior The remaining tragus (posterior he- parts of the lix fold), lobu- auricle; antitra lus auriculas gus
He'ix and tragus.
Anterior helix fold (part of the helix ascendens)
Schwalbe and Henneberg, a synoptical table of the results obtained hj different investigators.
I have not been able to get a clear picture from the investigation of the human embryos at my disposal, but it seems certain that the tragus is developed from the first auricular hillock and the antitragus from the sixth, and that the auricular lobe is a later formation that has nothing to do with the hillocks. According to Schwalbe, the point of union of the secondary swelling with the free auricular fold marks the point of the satyr tubercle; Darwin's tubercle is formed almost at the middle point of the border of the free auricular fold. A folding over of the posterior border of the auricle, which is so pronounced in mammals and leads to temporary epithelial adhesions, is to be observed in the human ear at the beginning of the third month; the unfolding takes place in the course of the third month, and the partially covered anthelix again becomes visible. Three angles then become evident on the posterior border, which is not yet curved in upon itself : an upper one, the apical angle which corresponds to the satyr tubercle, a posterior one, corresponding to Darwin's tubercle, and a lower posterior one (Fig. 221 a and b).
In the fourth month there is formed on the lateral surface of the free auricular fold between the helix ascendens and the posterior edge of the auricles a system of folds, first described by Schwalbe (1897) and consisting of five swellings separated by shallow grooves (Fig. 222). The swellings correspond to the longitudinal ridges of many mammalia, but they have already dis
Fig. 221 a and b. — Left auricle of a sixth month's human fetus, ac, base of the auricle; c, tip of the auricle; 6, apical tubercle; cl, lower posterior angle, incisura auris posterior; a f g e, hillock region; a b c d g /, free auricular fold. (After Schwalbe, from Bardeleben's Handbuch, vol. V2, p. 130. Fig. 13 a and b.)
Fig. 222. — Left auricle of a human fetus of the fourth month. X 2. The five transitory transverse folds are developed; they correspond to the longitudinal ridges of the ears of long-eared animals. (After Schwalbe, from Bardeleben's Handbuch, vol. V 2 , p. 130, Fig. 14.)
appeared in the human ear in the fifth month. In the fifth and sixth months the auricle has the shapes that Schwalbe has characterized as the Macacus and Cercopithecus forms. As to the growth relations of the auricle one may consult Schwalbe in Bardeleben's Handbuch (1897).
The auricular cartilage in man, according to Munch (1897), is an independent formation and remains independent; it shows no relations to the hyoid cartilage during its development. The auricular muscles are derived from the platysma (Ruge, 1887 ). 35 Turning now to the development of the external meatus, we must return to a stage in which the first branchial groove is at the height of its development, the auricular hillocks having just appeared. In such a stage one may, with Kastschenko (1887 1 ' 2 ), recognize in man also an upper, middle, and lower auricular groove; the upper one in man is, however, merely indicated and corresponds to the point at which the facialis organ (the first visceral cleft organ of Ilammar) has formed, and only at this point does the entoderm of the first pharyngeal pouch come into contact with the ectoderm of the branchial groove. This contact is soon lost and the upper auricular groove disappears without leaving a trace. The middle and lower auricular grooves become cut off ventrally by the union of the first and sixth auricular hillocks, between which the incisura intertragica persists, and form the fossa angularis (His). The transformation of this fossa angularis into the cavity of the auricle is associated with the transformation of the auricular hillocks and the formation of the auricle, concerning which, as has been already pointed out, the results are not quite concordant. From the ventral part of the fossa, dorsal to the incisura intertragica, the primary meatus, according to Hammar, grows inwards in the latter half of the second month as a slight, funnel-shaped canal. In opposition to Moldenhauer (1877) and Urbantschitsch (1877), Hammar expressly states that it is not formed and elongated by a thickening of the tissue surrounding it. In the fourth and fifth month the lumen of the primary meatus is temporarily occluded in its inner portion by the thickening of its epitrichial layer. This closure, however, disappears with the occurrence of the cornification of the epidermis, and the meatus again becomes provided throughout its entire length with a lumen, which persists until birth, and only towards the end of fetal life is more or less narrowed by the vernix caseosa. In the beginning of the third month the meatal plate (lamina epithelialis meatus), which is essentially a prolongation of the lower wall of the primary meatus, grows inwards and soon reaches the outer end of the tympanic canal (Fig. 219 a and b), and then pushes its way inwards and downwards along the lower wall of the tympanic cavity. The portion of the plate that is in relation with the lower outer portion of the wall of the tympanic cavity becomes rounded and is to be distinguished as the tympanic portion from the part that is more lateral in position. In the seventh month a splitting of the plate occurs and the lumen so formed becomes continuous laterally with that of the primary external meatus, the definitive or secondary meatus being thus formed. The lumen of the tympanic portion of the meatal plate forms the recessus meatus. In general the region of the primary meatus corresponds to the cartilaginous portion of the adult passage, but in the roof it extends also into the region of the osseous meatus. The structural differences of the integument in different portions of the definitive meatus stand in relation to its manner of development. The development of hairs and glands is limited to the region of the primary meatus, while the part derived from meatal plate (with the exception of the tympanic membrane) has the lower surface of the epidermis ribbed and possesses no hairs or glands. At birth the recessus meatus has almost acquired its definitive size, the postfetal growth effecting for the most part the remaining parts of the meatus.
- B Dobers (1903-1904) holds that in the pig and sheep the muscles of the posterior portion of the auricle are not derived from the platj^sma.
Concerning the development of the tympanic membrane Hammar (1902) states that from the beginning the inner end of the primary meatus is pushed downwards and outwards by a rounded elevation, the tuberculum membranse tympani. As the primary meatus grows inwards this tubercle is also carried inwards, and when the primary meatus and the tympanic cavity have come to lie opposite each other it becomes the primary tympanic memInane, into which the manubrium and processus brevis of the malleus have now grown. Later, by a thinning and modification of its connective tissue, it becomes the definitive tympanic membrane. In a fetus of 4.3 cm. vertex-breech length Dreyfuss (1893) was already able to distinguish three layers in it, one corresponding to the subcutaneous tissue, a second, the membrana propria, which stands in intimate relations to the annulus tympanicus, and a third, the submucous tissue of the tympanic cavity. That these three layers are present from the beginning and that the middle one is to be regarded merely as an unossified portion of the annulus tympanicus cannot, assuredly, be accepted. The membrane obtains a free outer surface only after the splitting of the meatal plate has taken place. 36 The pars flaccida is formed in the last month of fetal life. In the fifth month a short ridge is formed at the anterior part of the boundary between the tympanic and nontympanic portions of the meatal plate, above and in front of the processus brevis of the malleus. On the splitting of the meatal plate in the seventh month, this ridge becomes converted into a groove open to the meatus, the terminal groove, and against the floor of this groove Prussak's space, formed from the tympanic cavity, applies itself in the tenth month and so forms the pars flaccida. Concerning the growth relations of the meatus one may consult Schwalbe (1897, p. 171) and Symington (1885, 1889).
Much has been written concerning differences in the form of the external ear, on account of attention having been directed to them from the psychiatric and criminalistic stand-points. Their importance in these respects has certainly been greatly overestimated. Occasionally fetal ear forms are to be observed in adults, and these may be regarded as inhibitions of development (compare, in addition to Schwalbe, 1895, 1897, Schaffer, 1892).
38 Dreyfuss (1893) gives an account of the manner in which the special relations of the manubrium mallei and tympanic membrane are brought about. The work of Draispul (1890) is rather inconclusive.
A cleft which traverses the lower portion of the pinna between the tragus and antitragus has had a certain importance assigned to it in connection with the question of the inheritance of traumatisms. It was supposed that in such ears the ear lobe was divided because the mother's ear lobe had been divided by an ear-ring. Careful studies showed (His, 1889, Israel, 1890, Rohrer, 1894, von Swiecicki, 1890) that the cleft did not really lie in the region of the ear lobe, but between the anlagen of the tragus and antitragus and that at all events the phenomenon was to be regarded as an inhibition of development, the first and sixth auricular hillocks remaining more or less widely separated.
Fistula? auris congenita seem to me to be referable to inhibitions of development in the region of the dorsal part of the first branchial groove (upper auricular groove, facialis organ). Gradenigo (1893) explains them as an insufficient closure of the furrow between the crus prsetragicum — corresponding to the spina helicis (scutulum) — and tragus, and His (1889) refers them to an insufficient fusion of the furrow between the crus helicis and supratragicum (prsetragicum of Grradenigo). Schwalbe (1889 2 , 1889 3 ) believes many of the anterior auricular appendages to be abnormally developed spina? helicinae (sen tula) separated from the main cartilage, and the same view has been expressed by Gradenigo (1893).
For accounts of malformations of the human auricle one may consult, in addition, Moldenhauer (1892), Piel (1904), and Alexander (1904).
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|Manual of Human Embryology II: Nervous System | Chromaffin Organs and Suprarenal Bodies | Sense-Organs | Digestive Tract and Respiration | Vascular System | Urinogenital Organs | Figures 2 | Manual of Human Embryology 1 | Figures 1 | Manual of Human Embryology 2 | Figures 2 | Franz Keibel | Franklin Mall | Embryology History|