Paper - The early formations of the middle ear and eustachian tube - a criticism
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Frazer JE. The early formations of the middle ear and eustachian tube - a criticism. (1922) J Anat. 57(1): 18-30. PMID 17103958
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The Early Formations of the Middle Ear and Eustachian Tube - A Criticism
By J. Ernest Frazer, F.R.C.S. (ENG.),
St Mary’s Hospital, Professor in the University of London
Hǖschke, to whom we owe the description of the auditory vesicle as a structure derived from the surface layer, was also the ﬁrst to refer the origin of the tube and tympanum to the mandibular cleft or groove. This view, advanced in 1827 and subsequently, met with very general acceptance by anatomists of the time, and has been more or less prominent since then. It is true that certain doubters have arisen at intervals who denied its truth, altogether or in part: for example, Hunt (1876) working on the pig, came to the conclusion that the region arises from an “invagination ” of the pharyngeal mucous membrane, and Urbantschitch (1877) also held that the tube and tympanum were formed by lateral outgrowth which had nothing to do with the ﬁrst groove, while Kolliker and Moldenhauer (1877) do not appear to have looked on the parts as directly derived from the cleft by modiﬁcation, but rather from an outgrowth associated with it. Piersol (1888) and Siebenmann (1894) also, with a few others, had views which were not quite in line with the accepted one. However, in spite of defections, the formation of the tubotympanic cavity from the ﬁrst pharyngeal groove has generally retained its hold on anatomists as the orthodox description, and, since 1902, the detailed account on these lines given by Hammar (Archiv. f. mikr. Anat. V, 59) has been taken as the standard on which descriptions are based and to which references are made in the text-books and other works dealing with the matter.
In 1910 I first described these parts (B. M. J.) as made from the “tubotympanic recess” of the pharynx, a recess which has its ﬂoor formed by the ﬁrst two visceral arches, while the third arch makes its posterior boundary. A few years later I gave (J. A. P. vol. XLVIII) a fuller description, with special reconstructions, etc., amplifying the former more general one. As this description has been adopted in some of our principal text-books, and as it differs in certain fundamental conceptions from the more widely held views of Hammar, it seems necessary to point out‘ where, in my opinion, the earlier representation is open to criticism.
It will be well to conﬁne ourselves, in this matter, practically to the fundamental differences between the two views, and so to avoid a lengthy consideration. The view which I advocate is, shortly, that the tubo-tympanic recess is a part of the pharyngeal cavity,’ situated as already indicated, and that it is nearly completely separated from the main cavity by a forward growth derived from the third arch, which comes into contact ﬁnally with the ﬁrst arch in the front wall of the recess, so that only a narrow tubal passage is left above the place where they meet. This closing in of the recess from behind only affects its inner part, the outer portion still remaining as the tympanum, thus only communicating with the pharynx by the narrow unclosed front or tubal part of the recess; the tympanum, coming into existence in this way, has in its ﬂoor the outer end of the second arch and its groove, in front of this the ﬁrst groove and arch, while its inner wall is necessarily made by the forward growth from the third arch. Detailed descriptions and accounts of the models can be found in my earlier papers.
- 1 This was written in 1915. The paper was never ﬁnished, however, owing to war work. I have not finished it, as I intended, by some comparative observations, but have conﬁned myself simply to the difference of view in human development.
There is no essential difference between the two descriptions so far as concerns the separation of the tympanic region from the pharynx by a forward growth, but the origin of this forward growth is referred by Hammar to the second arch, and here comes in the fundamental difference. His account of necessity carries the second pouch backwards and inwards, and leaves it on the pharyngeal side of the forward growth, whereas mine leaves it undisturbed in the tympanum, because the forward growth arises on its Imp. aboral side. This brief account of the main questions at issue will serve to show the limits of the criticisms I desire to bring forward, and to indicate at the same time the inter-relations of the points with which we are dealing. I propose to show (a) that the conditions necessary for Hammar’s conception do not in fact exist, (b) that those requisite for the view I advocate are present, and (c) that the conditions actually found in sections are consonant
Fig-1. Ventral surface of left half of pharynx, 7 mm. Shows early stage of recess, with 2nd arch completely in floor. Ectoderm left, in oontact with 1st pouch on other pouches the extent of contact with ectoderm is shown by light areas.
tion I adduce, and in themselves disprove ; the other description,
(a) If Professor Hammar’s view is correct, and the tympanum is separated from the general cavity of the pharynx by tissues derived from the second arch growing forward between the two, it would seem evident that we ought to ﬁnd indications of that growth and of its effect on the neighbouring structures. The arch is placed below the floor of the recess (ﬁgs. 1 and 2) immediately in front of its posterior external angle, which is made by the dorsal portion of the 2nd lateral pouch.
Fig. 2. Pharynx, 16 mm. A, dorsal aspect of recess; I and II, 1st and 2nd pouches; B, view from outside, below, and behind, showing area of manubrial invasion, uninva/ded remnant of 2nd arch, 2nd pouch, and, internal to these, the impression of the forward growth.
If the arch thicken, its area as represented in the ﬂoor must increase, for if it is going to affect the cavity by enlargement, this enlargement must be in close relation to the cavity. But it does not enlarge: on the contrary, its area becomes smaller, and no condensation is visible within it. Its skeletal bar is close to the lining membrane of the ﬂoor of the recess when it ﬁrst becomes recognisable, and it retains this situation throughout, emcept when and where the mass of the forward growth comes in between it and the ﬂoor. It might be said that here we have the enlargement for which we are looking, the evidence of the existence of the postulated growth from the 2nd arch. Certainly it is evidence of growth, but not necessarily from the 2nd arch, and certainly not capable of effecting the results claimed for it: leaving the question of its apparent origin out of the argument for the moment, to be dealt with later, it will be found not to fulﬁl the conditions that are required if it is to be accepted as the desired effective growth from the 2nd arch. For one of these conditions is that it must be effective from the outer and front side of the pouch, in the neighbourhood of the outer margin of the recess, not from its posterior margin. But this growth, separating Reichert’s bar in part from the ﬂoor of the recess, only does so under the inner part of that ﬂoor, altogether internal to the region of the pouch, and up to the time of completion of the “ separation stage ” it does not extend appreciably under the outer part of the ﬂoor (see ﬁg. 5). Moreover, there is never at any time any sign of growth in the outer or juxta-marginal part of the 2nd arch,» or antero-extemal to the pouch: the bar remains in contact with the ﬂoor here as in the earlier stages, and in fact is there still in the adult as the tympano—hyal. There cannot, therefore, be any question of growth in this part of the arch, unless Reichert’s bar takes part in it, on its crest, so to speak. It is hardly necessary to say I have given drawings in previous papers illustrating the stages of the growth, and I need only now refer to ﬁg. 2 as showing its deﬁnite position. The ﬁrst view is of the recess in a 16 mm. embryo, seen from above: B shows the recess laterally from below and slightly from behind. The triangular area labelled as 2nd arch is the part related to Reichert’s bar, but the bar is separated from the recess, further in, by the forward growth, which therefore shows in the model as a hollow between the arch area and the pharynx. This impression shows the growth to be wedge-like, with its base behind. Plainly, it does not affect the outer part of the 2nd arch, and cannot displace the pouch towards the pharynx, and these characteristics remain. In ﬁg. 3, they are as apparent in the 8th week, though by this time the alleged shifting inwards of the pouch ought to have been accomplished and done with long before. We can say, then, that on the one hand there is no growth capable of causing inward displacement of the pouch, and on the other the growth that is present is not only unable to bring about this movement, but, owing to its situation, could only move the pouch outwards—if it were near enough to affect it at all.
Fig. 3. Pharynx during 8th week from below. To show how the forward growth progresses internal to the 2nd arch area and pouch, next the pharyngeal wall, and is evidently pushing forward from behind, that the bar has nothing to do with the growth, save that this comes in between it and the more internal part of the recess.
When we come to consider the effects of the postulated growth from the 2nd arch we,ﬁnd a similar absence of evidence that effects which would of necessity result from such growth have any existence whatever in fact. A growth of this sort, if it pushes the second pouch medially must do so by pushing it in front of the structures of the third arch, and must then occupy the place, so to speak, of the displaced pouch, and lie between the structures of the 3rd arch and the lining membrane of the tympanic cavity. These seem to be absolutely necessary consequences of such a process, if it occurs, and afford objective tests to which observation can be directed in the embryo.
In this connection the 9th nerve gives us an excellent standard of position in the various stages. The nerve is a particularly useful 3rd arch structure, for it is placed, from the earliest stages of the recess, immediately aboral to the 2nd pouch, being in fact separated by this pouch from the nerve and bar of the 2nd arch. I have illustrated these relations frequently, and they are indicated in some of the outlines given in fig. 4.
Now it can be said at once that nothing, during the formation of the tympanum out of the recess, passes in front of this nerve or separates it from the lining membrane of the cavity, nor does the second pouch change its relation to it, but continues to project in all stages between it and the facial nerve and bar. The later stages shown in ﬁgs. 3 and 4 exemplify this, and these conditions and relations between the parts remain until modiﬁed in ‘certain minor respects by developments associated with the later growth of the auditory capsule—but this is long after the separation of the tympanum has been effected and completed; The forward growth begins internal to the 9th nerve, that is, on its aboral side, and is plainly evident a considerable time before the period when the 2nd pouch is displaced according to Hammar’s description; it can be seen, already deﬁnite, in the 16 mm. embryo shown in ﬁg. 2. At this time, then, the growth is altogether internal to the position of the 2nd pouch, situated between it and the pharyngeal wall, and it increases in size in this situation: yet, if we accept the description given by Hammar, the growth which can be seen to be wedging itself in, from behind, between the pharyngeal wall and the pouch, must suddenly and rapidly displace this pouch inwards, that is, it must begin to press on it from its outer side, and push it inwards in despite of the wedge-like ma.ss which has been laboriously forming itself internal to it. Not only is such a reading of the facts almost impossible in itself, but is absolutely negatived by the observations which can be made on any human embryos of the proper stages: the inner wedge is a veriﬁable reality, the positions of the nerves and the pouch are evident, and evidently constant, but there is not the slightest hint of any lateral or antero-lateral pressure on the pouch, or of any shifting of it in front of the 9th nerve, or of any separation of this nerve from its relation to the tympanic cavity by a lateral or antero-lateral growth or in any way whatever. I have given ﬁgures showing their relations and conditions in previous papers, where they can be seen if desired, but ﬁg. 4 gives a series of small outlines of some of the models I have made of this region, and in those in which the nerves are indicated it can be seen that their essential positions with regard to the cavity and second pouch remain unchanged, as does the pouch itself, placed between them.
Fig. 4. Dorsal views of stages of separation of recess from pharynx from behind. Embryo lengths are given above each outline. R, Reichert’s bar; X, posterior limit of manubrial invasion; ep. epithelial lamina below tubal part.
It is to be regretted that Professor Hammar did not include among his ﬁgures illustrations of the ventral aspect of the various stages which he modelled. I confess that I ﬁnd his accounts of these aspects difficult to follow, but I suppose that his models were very much the same as mine of similar stages, and for this reason regret the omission of ﬁgures, for the ventral aspect is a very interesting and suggestive one where the position and extension of the forward growth are concerned. The ﬂoor of the recess, seen from within, such as I have described and ﬁgured in this journal (1913), gives a demonstration of these matters perhaps even more striking, at any rate to the observer who is not accustomed to dealing with models of cavities.
We come now to what is, if I am not mistaken, the only direct evidence brought forward in support of Professor Hammar’s contention that the 2nd pouch is displaced inwards to the pharyngeal wall. It is the description given by him of the conditions in an embryo of 18-5 mm. (N.S.) length, with an illustration of the model: the drawing is reproduced in Keibel and Mall (text-book) where it can be seen by those who cannot consult the original monograph.
The measurements given by Hammar cannot be compared directly with those given in the references to my specimens, for he uses N.S. length, whereas mine are given in greatest total length. I imagine that his 18 mm. stage would correspond more or less with a “greatest length” of about 22 mm.
I may confess at once that the ﬁgure is to me the illustration of some condition quite abnormal: one may make guesses about the nature of the abnormality, but I have no doubt in my own mind as to its existence. I have examined every embryo which I could ﬁnd sectioned, between 18 and 30 mm., and have found nothing whatever to suggest a remote resemblance to the curious appearance presented by this specimen. This merely negative evidence would not, of course, be of very much weight in itself, and I would have had very great hesitation in stating my belief——however strongly I might hold it —that the specimen is not normal, if that belief did not receive such strong indirect support from these negative observations. For all my specimens exhibit conditions pointing to an even progress of events, such as is illustrated in the series in ﬁg. 4, and among these specimens are embryos of 18, 22, and 25 mm. (not to mention one or two others apparently lying between these limits but of doubtful official measurement). Now these three embryos are all in the line of even progress, as are the 26 mm. and 28 mm., and none show any sign whatever of growth lateral to the 2nd pouch, or of displacement of this pouch: yet somewhere between these stages there is said to occur one in which these things take place, one which therefore starts de novo, runs its course, produces its effect, and leaves no sign of its occurrence behind‘ it, and all this while the embryo is increasing in length some 2 or 3 mm. The mere statement of the necessities that follow on the negative observations to which I have referred is enough to indicate the support they render to the opinion formed on the ﬁrst view of the ﬁgure of the model, and, when the existence is remembered of the solid forward growth internal to the pouch, the suggested occurrence would seem impossible. I do not wish to lay stress on the appearance of the “pouch” in this ﬁgure, though it is strangely incompatible with the idea of a structure being possibly displaced by solid growth, and I am content to leave the matter here, pending an explanation of the curious condition which will enable it to occupy its proper place.
The argument, then, against the acceptance of the explanation given of this 18-5 mm. (N .S.) specimen is not founded on its extraordinary appearance, but on the stability of relations between the pouch and the structures on each side of it, and the wall of the recess close to them, and on the impossibility of conceiving the necessary growth appearing and acting and disappearing in the very short intervals existing between the specimens examined. The argument, to my mind, is an effective one, and disposes of any attempt to shift the pouch across to the pharynx, whatever may be the opinion held about the specimen concerned.
This, of course, does not necessarily affect the fact that a remnant of the 2nd pouch is found on the pharyngeal wall. It is more convenient to deal with this later: for the present it is enough to point out that the remnant is there long before the postulated “shifting” is said to occur, so that the theoretical necessity for supposing such a movement as that which we have just been considering does not exist.
(b) Turning to the consideration of the view which I advocate, it is evident that a good deal of what has been said already is applicable positively in its favour. Thus, the forward growth takes place from behind, on the inner or aboral side of the 2nd pouch, i.e. from the area of the 3rd arch: the 2nd pouch remains unaffected, altogether external to, and on the oral side of, the growth: the growing mass pushes the limits of the recess-cavity before it, hence must pass forward over the 2nd arch structures, and so we ﬁnd it separating the skeletal bar from the recess: but the outer part of the bar still remains in relation with the outer part of the ﬂoor of the recess, which becomes a postero-external part of the ﬂoor (outer wall) of the tympanum as the separation proceeds. It is unnecessary to dilate further on these matters, for their bearing on the question is quite clear, and I do not think there is any fact whatever, either of direct observation, or of any “ necessary effect,” which is not in absolute accordance with the view which I have brought forward concerning the nature of the forward growth. As for the particulars, they can be considered with the following section.
(c) The conditions in the human embryo. I have previously given full accounts, with ﬁgures from models, etc., of the various stages of evolution of the tube and tympanum from the tubo-tympanic recess, so that it is only necessary here to run over the main facts of the stages. In ﬁg. 4 are given miniature reproductions of the outlines of the region concerned as found in the models made from various stages: the series is not complete, but it is unnecessary to put others between those shown. The most important difference between the early and late members of this series is plainly that due to the increasing concavity of the posterior margin of the projection of the tubotympanic recess: this margin proceeds at first with an irregular line inwards and rather backwards from the 2nd pouch to the pharyngeal wall. There quickly comes a forward bowing of the line, growing into a marked and deep forward convexity, particularly noticeable towards the inner part of the margin, and this progresses until only a constricted tubal portion is left connecting the pharynx and the tympanum. This concavity is produced by the forward growth which we have been discussing. The area of the second arch is seen on the lower aspect at 7 mm. in ﬁg. 1. It is invaded later from in front by a manubrial extension from the 1st arch, and only a small portion of its territory remains uninvaded, namely the posterior part, covering Reichert’s bar and just. in front of the 2nd pouch. The 2nd pouch can be seen from the dorsal side as an angular projection in all the stages shown in fig. 4. The manubrial invasion, the position of which within the area of the 2nd arch is described by Hammar, is marked behind by the little secondary point X in the outlines, so that the remaining unchanged part of the 2nd arch is easily placed between this and the blunt angle of the 2nd pouch.
If we come now to consider the genesis of the forward growth which determines the general arrangement of the regions, it would seem clear that it can only be one of two things. The shape of a cavity, such as that of the pharynx or the tubo-tympanic recess, depends on the structures that form its walls, and we cannot imagine that the cavity itself, or its lining layer, possesses the power to alter its form: power of growth exists, of course, in certain parts, as in the lateral pouches, but it cannot be claimed for the layer in general, and even in the case of the pouches the direction in which the growth is effective seems to be determined by the surrounding structures. The only structures in relation with the ﬂoor of the recess, and determining its form, are the mesodermal pharyngeal arches, and the cavity lies compressed between these and the otic capsule, etc., in relation with its roof. The forward growth begins in the ﬂoor region, as can be seen in ﬁg. 2, and hence has an “ arch value ” in origin, and, as it evidently appears near the hinder part of the recess, the 1st arch can be excluded. The floor growth, as can be seen in ﬁgs. 2 and 3, and especially in reconstructions of the recess seen from within, such as I have shown in vol. XLVIII of this journal, is evidently pushing forward from the region of the back wall of the recess, and as it increases in size it begins to push this back wall forward and thus to cause the concavity here as seen in “ cast” models, so that the ﬂoor growth is a little in advance of the concavity. But the hinder wall of the recess is from the beginning made by the 3rd arch, so that the forward growth must be looked on as derived from this arch, and all its relations and details seem to be consonant with this origin. Some of these have been brought forward earlier in this paper, and others have been dealt with at greater length in previous papers: I need not recapitulate these nor I think lay any further stress on the matter, for it seems to me almost self-evident, and apparent to any unprejudiced eye looking at the reconstructions.
The outer part of the 2nd arch remains undisturbed, with its pouch angle, though the whole region is pivoted round a little so as to face somewhat differently, but the rest of the arch is separated from the ﬂoor of the cavity by the increasing thickness of the forward growth. This can be well seen and appreciated in the reconstructions of the ﬂoor (1913) but are shown here (ﬁg. 5) in another way: the drawings are from linear reconstructions made along the line of the hyoid bar, showing its relations with the ﬂoor, and exhibit the increasing thickness and results of the new growth passing between the bar and the ﬂoor of the recess. Different enlargements are used, to make the sections more or less of the same size. The line of section is not transverse, but would meet its fellow of the opposite side at a wide angle, being directed rather forward, so that the breaking of continuity between tympanum and pharynx from behind does not appear in the sections till relatively late. The ﬁrst sections show the increasing depth of the growth, and sinking of the hyoid bar, correlated with increasing height of the pharyngeal cavity (of which the growth is seen to form the outer wall here) while the general level of the recess and of its pharyngeal opening is kept at about its original position with relation to the roof of the pharynx. This relation of the growth from the 3rd arch to the pharyngeal wall gives it the opportunity of sending a process forward to join the (maxillary) palate fold, which process then becomes apparent on the pharyngeal wall as the pharyngeal extension of the palate. I described this process ﬁrst in 1910; subsequently Michio Inouyé showed the pharyngeal extension to be a secondary part, joining after its appearance, in moles and mice.
Fig. 5. Reconstruction sections across pharynx in the line of Reichert’s bar. To show increasing distance between bar and inner part of recess due to forward growth. Outer part of bar remains in relation with recess. '
Reconstructions in this line of section through the second month exhibit little additional (save that the palate extension begins to show) and are not reproduced, but at 35 mm. the growth has deﬁnitely pushed the hinder border of the recess over the line of the bar: here the survival of the relation between bar and recess is seen in the outer part, while the hinder part of palate fold, which has elongated, comes into the section and illustrates its association with the growth.
The 2nd pouch, the angle seen between the 9th nerve and the nerve and bar of the 2nd arch, remains throughout the series. Hammar, believing that this pouch—which he recognises as such in his earlier specimens—undergoes shifting in his 18-5 mm. (N.S.) stage, naturally does not recognise it in the recess after this: but his reconstructions of later stages show it nevertheless, although it has "not been digniﬁed by labelling. I need not repeat what I have said already about the constancy of its relations, but I desire to insist on these as matters of primary importance, for the pouch cannot move when the structures in front of it and behind it are undisturbed, and when nothing comes between them and the lining of the cavity in relation with them. Whether the angle is really the dorsal part of the pouch or merely the stump of this part is only of academic interest, but the constancy of relations is practically the important matter, and is enough to ensurethat the angle in the later stages is the same thing as that in the earlier ones, with all the consequences which I have endeavoured to show depending on this.
Two more points remain for consideration. The ﬁrst has to do with the existence of a derivative of the 2nd pouch on the pharyngeal wall, claimed by Hammar to be the displaced structure originally found on the recess. I have never wished to deny the pouch-value of this, for I believe it to be a part of the 2nd pouch, but only to represent its ventral portion: the dorsal portion remains as the angle so frequently mentioned, and the forward growth passes forward between the two. The elongated pouch of a 7 mm. embryo is seen in ﬁg. 1. As the head grows in breadth the outer and dorsal part is drawn out still further, and contact with ectoderm (precervical sinus) becomes conﬁned to the inner part. An embryo of 135 mm. shows the drawn out pouch extending along the lower aspect of the short hinder margin of the recess, and from its inner end, deﬁnitely on the pharyngeal wall, there is the remnant of the external pharyngeal duct leading down towards the former site of the precervical sinus. A 15 mm. embryo shows the complete pouch, though much less clearly, and even at 16 mm. (ﬁg. 2) there appear to be remnants of it in the irregularities on the posterior margin of the recess: but although the forward growth is breaking up this intermediate part of the pouch, its outer and inner (dorsal and ventral) portions remain unaffected, on the recess and side wall of the pharynx respectively. This short account will show that I differ from Hammar in that I recognise this pouch-remnant as being there ab initio, and it can be found there by anyone who takes the trouble to look for it.
The second point does not directly concern the main object of this paper, but I call attention to it because of its possible bearing on the explanation of certain cysts found immediately below the petrous region—cysts which are put into that heterogeneous collection which clinicians lump together as “branchial,” but which certainly are not branchial if that term were used in its proper sense. The point referred to is the occurrence in the normal development, in the last phases of the “separation stage,” of an epithelial (double) lamina below the short tubal connection between tympanum and pharynx.
Fig. 6. (a) Section, 35 mm. below level of patent tube. Lam. epithelial lamina connecting pharynx (Ph.) with tympanum (T); M. Meckel’s cartilage; R. Reichert’s cartilage. v (b) Schemes to show formation of lamina; explanation in text.
Its extent is seen in the 35 mm. reconstruction in ﬁg. 4a and its appearance on section is shown, with diagrams representing its formation, in ﬁg. 6. The ﬁrst of these diagrams is a scheme of a section across the inner part of the short recess of an early stage, parallel with the wall of the pharynx: the 3rd arch (horizontal lines) forms the hinder part of the ﬂoor and limit of the recess, the 1st arch (M) forms its front part, and the 2nd arch (dotted area) is between these two in the ﬂoor, separated from them by the 2nd and 1st grooves respectively. In the succeeding schemes the 3rd arch is seen to extend forward under the lining membrane, over the second, obliterating the 2nd groove in doing so, and gradually pushing in the hinder boundary of this part of the recess. The 2nd arch thus drops out of relation with the recess here.
The forward growth of the 3rd arch goes on, pushing in the back wall still further, but along the ﬂoor it is limited in front by the first groove. The result is that the growth applies itself to the surface of the 1st arch; the lower portions of the epithelium, thus pressed together, adhere, and so the epithelial lamina is formed. The tracing of a section from a 35 mm. embryo exhibits this lamina connected on the one hand with the lining membrane of the lower part of the tympanum, and on the other with the lower part of the pharyngeal opening of the tubal part, through both of which parts the section passes.
It is clearly evident that the original lst groove is represented by the free edge of this lamina (see schemes) and has nothing to do with the formation of the cavity of the tube, which is really a remnant of the old recess cavity, persisting in its most anterior part, immediately below the roof. The tube is more or less parallel with the groove, although this is not quite a true description, but the common statement that it is this groove is wrong and misleading in many ways, and ought not to be made by anyone acquainted with the details of its ontogenétic development.
The epithelial lamina, made in this way, does not seem to last very long. It is well marked in my specimens of 35 and 37 mm., it is apparently commencing in a third of ? 32 mm., and there is no sign of it in one of 40 to 42 mm. It seems to be at its best about the 35 or 36 mm. stage. I have not, so far, found any apparent remnants of it in still older specimens, but the possibility of such remnants existing is undeniable and for this reason—as well as from a desire to foster a truer conception of the formation of the tube—I call attention once more to its presence.
This paper has been almost entirely controversial, and hence difficult to summarise. I have endeavoured in it to show that the conditions which must be present, if Hammar’s description is correct, do not in fact exist; that those which must be present if my description is accurate are in fact there, and the description is founded on them; and that the stages found in the human embryo are only consonant with the explanation I have given of them. This means that the separation of the tympanum from the pharynx is effected by a forward growth from the third arch, and not from the second, as Hammar maintains, and that the outer part of the 2nd arch, with the dorsal part of its pouch, remains in the tympanum. The ventral part of this pouch is on the pharyngeal wall.
I have also recalled the existence of an epithelial lamina below the developing tube, and have pointed out that the tube can not be described correctly as a persisting ﬁrst pharyngeal groove.
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