Paper - A further communication on the formation of the nasal cavities
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Frazer JE. A further communication on the formation of the nasal cavities. (1912) J Anat Physiol. Jul; 46(4): 416–433. PMID 17232937
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A Further Communication on the Formation of the Nasal Cavities
By J. Ernest Frazer, F.R.C.S., Lecturer on Anatomy in the Medical School of St Mary’s Hospital.
IN the course of investigations into the development of the nasal cavities, I was led to certain results which I stated in the shape of a preliminary communication in this Journal: I dealt then in particular with‘ the mode of formation of the septum and paraseptal structures, endeavouring to show how these came into their. ﬁnal position in the inner wall.
In this paper I propose to consider the general increase in size of the cavity and its consequent change of level, and then to deal with the development particularly as it affects the outer wall.
I. The General Growth of the Walls and Increase in Size of the Cavity
When looking at a series of models or drawings of the early mouth cavity, one is struck by the comparatively great changes of level of the posterior nasal openings. At the end of the ﬁrst month these are just inside the wide opening of the mouth, and by the end of the second or early in the third month their upper limit is on a level with the top of the pharynx.
The change of level takes place practically altogether in the second month, and is evidently associated with increase in height of the opening, increase in height of the cavities, and the appearance of a thick septum between them.
The explanation of these associated effects seems to lie in the conception of the direction in which the increasing vertical height of the walls acts and so brings about the increased depth of the cavity. If a straight line is drawn between two points, one above the other, and the line is increased in length, it must pass beyond one or other or both of the two points: if it passes above the upper one it might be described as increasing its length upwards, or if beyond the lower point as increasing downwards. If it extended beyond both points it would be increasing in both directions.
In the case of such a line the increase could only be made at one or both ends, and the position of the addition would determine the “direction” of the increase, but if a line is conceived capable of extending by interstitial growth it is evident that the “direction” in which it would make the elongation apparent would depend on which of the points offered least resistance : thus the situation in the line of the area of interstitial growth, throughout its whole length or only in one small spot, is of no consequence when compared with the resistances to be overcome by the increasing line.
This imaginary illustration will, I hope, be of use in helping towards an understanding of what I mean when I say that the increase in height of the walls of the cavities is in an upward direction: in other words, the growth of the walls of each cavity separates the roof and floor by raising the former and not by depressing the latter. _
My belief in the truth of this statement is based on the fact ‘that it seems to be the only view of the mode of growth that agrees with the observed facts, and it is borne out by an estimation of the angles formed at different stages between the corresponding parts of the cavities and the line of the basal axis of the skull.
To take the last point ﬁrst. I endeavoured to test the matter in the following way. A base line was drawn between the front margin of the foramen magnum and the point of entry of Rathke’s pouch into the skull from below. Lines were drawn from the front end of this to
(a) the highest part of the floor just behind the anterior opening, thus forming a-lower angle.
(b) to the front part of the upper limit of the cavity, thus forming an upper angle with the base line.
By comparing these with corresponding angles at different stages, it was hoped that some light might be thrown on the question.
One was faced at once by the diﬂiculty of procuring a sufﬁcient number of direct measurements. Sagittal section is almost a sine qua non for accuracy, and nearly all my sections are transverse: in very few cases were models sufﬁciently complete to serve the purpose. These diﬂiculties, of course, only apply to the earlier stages.
It thus comes about that I can only give one direct measurement in the fourth to ﬁfth week, with a lower angle of 112° and an upper of nearly 130°.
My next direct measurement came late in the third month, giving 130° for the lower and 160° (?) for the upper angle. After this the lower angle seems to remain practically constant at or about 130°, and the upper angle at 160° or more.
I have made four or ﬁve indirect or approximate measurements in the second and third months, and these give an ascending series in both angles, marked in the case of the upper angle, but more gradual in the lower one, leading up to the measurements given for the third month.
The measurements are so few, especially in the second month, that in themselves they carry little weight, but their practical agreement is so striking that, in spite of theirsmall number, I venture to think that they may be taken as not only supporting the View advanced, but as completely going against the alternative of ‘a downward increase in height, and almost equally denying the existence of more general increase in both directions: that is to say, that the growth of the nasal cavities in size is not a growth par?) passu with that of the rest of the head, otherwise the difference between the lower and upper angles would remain unchanged, but is an opening out of this included angle, and the opening out is attained by the increase of the upper angle and not by the decrease of the lower angle.
By a careful study of the specimens I have been led to the belief that this ‘tpening upwards of the cavity probably reaches its end about the ninth week, and after this the growth in height is only in keeping with the general growth: the depression of the ﬂoor is probably prevented by the presence of the mandibular arch resting at ﬁrst on the pericardium, and in connexion with this it is interesting to observe that the ﬂoor is apparently depressed at first, before it comes into contact with the mandibular arch: the increase that takes place in the lower angle during the second month, though it indicates that the ﬂoor is raised, is probably only caused by the thickening of the ﬂoor in its growth, its lower aspect resting on the mandible. The effect of this mode of growth. on the level of the posterior opening can be seen in ﬁg. 1. One of the tracings shows the position of the cavity and opening at the beginning of the second month, with its appropriate angles: in the other we see the result of the raising of the roof, in that not only is the vertical height of the cavity increased, but the roof of the posterior opening, which follows the roof of the cavity with which it is continuous, is brought up to the level of the top part of the pharynx. Thus the roof of this opening, being raised, leads to an increase in the height of the opening: when the palate folds meet in the ninth week they close across the greater part of these elongated apertures} leaving only the small upper portions to open into the top of the pharynx as the posterior nares.
The appearance of the septum in situ also receives an explanation in accord with the observed facts by this conception of the mode of growth of the walls of the cavities.
The septum becomes apparent as these grow, and is usually described as “growing down” from the base of the skull: according to the View now propounded it would be more correctly described as uncovered or exposed or brought into existence by the pushing upwards of the roofs of the cavities and upper margins of the openings. From the early stages the condensation of the basis of the septum passes in a practically straight line from the base of the skull to the premaxillary region. The line and direction remain unchanged while the septum is “appearing,” the angle it forms with the base line perhaps even
1 There is, of course, a displacement forwards of the premaxillary region with the other nasal structures as one of the results of maxillary growth : this is understood to be a necessary consequence, and is not considered here, but it is evident that some of the elongation of the apertures is due to this, as can be appreciated in ﬁg. 1.
FIG. l. Scheme to show the eﬂ'ect of the increase in an upward direction of the walls of the cavity.
A, a, roofs, and B, b, ﬂoors of cavities at beginning of second and third months respectively. The mandibular arch, etc., in second month is also shown. The front part of roof A, making an angle of 130° with the base line, is brought up to ankle of 160° in a, with the result that the upper limit of the posterior opening is raised from P to p. The early floor is driven forwards and its upper surface raised as a result of thickening of the mass. The concentration of the basis of the septum extends from the skull base just in front of Rathke’s pouch to the premaxillary region, practically corresponding in level with the back part of B, b. Therefore in the early stage this basis is hidden in the greater part of its extent, but becomes evident in the later stage as the level A becomes the level a: the ﬁgure also shows how the angle formed between this septal structure
and the base line may slightly increase as a result of the pushing forward of the premaxilla.
slightly increasing. There is thus no indication of “ growing down ” even though the increase in the length of the visible edge is going on all the time. p Bearing in mind the persistence of the line of the septal basis from the time when there is practically no septum, there seem to be only two other possible explanations. The ﬁrst is to imagine a simultaneous backward and downward displacement of the premaxillary region, thus bringing the septum into prominence: not only is this impossible as the premaxilla manifestly grows forward, but it is at once negatived by the persistence or even slight increase of the angle just mentioned.
The second explanation would suppose an extension downwards and backwards of the margin I of the septal basis by its extension backwards along the skull base: as its continuity with the skull is at exactly the same place in the early and late stages, there can be no ground for this assumption.
It is not necessary to go further in ‘the matter. It is sufﬁcient here to point out that, on the view advanced in this paper, the appearance of the septum between the openings as their upper parts retreat upwards is_‘__inevitable, without supposing any septal growth at all other than the general interstitial increase that keeps it in the line of its earliest. stages. This can be seen in a schematic fashion by reference to ﬁg. 1. The direction of the hinder edge of the adult septum is a later acquisition and does not mark its primitive direction.
II. The Development of the Outer Wall
The main parts of the complete cavity are all present in their early state and essential relations at the end of the ﬁrst month, but, before proceeding with an examination of the cavity at this stage, it may be as well to give a short account of the processes that have led to the making of such a cavity. L
The olfactory ﬁeld is at ﬁrst an area of thickened ectoderm, practically ﬂat, situated in front of the eye region and below its level, and facing more in a lateral direction than downward} In a 4'5 mm. embryo the maxillary process, if present at all, is in its earliest stage and is behind and below the eye—which has only just reached the surface where the lens thickening is commencing~——and is not in relation with the olfactory ﬁeld.
The olfactory ﬁeld, as is well known, is converted into an olfactory pit by the growth of its margins, but before this takes place the maxillary process comes into relation with the ﬁeld. I do not possess human material at this stage, but the accompanying drawing of the under aspect of the head of a mouse embryo (ﬁg. 2) shows the maxillary process as a rounded mass coming up against the posterior margin of the thick “ ﬁeld,” which exhibits the early longitudinal groove that marks the line of the future “ pit.”
Even at this stage it can be seen that the margins of the ﬁeld that are
1 The terms of position used in adult description are employed in this paper, as they are more convenient when dealing with successive stages of such a rapidly changing region. A Further Communication on the Formation of the Nasal Cavities 421
about to form the walls of the pit are those bounding it at the sides and in front; behind, the groove is open.
It is also evident that the groove leads on to the lower surface of the maxillary process, and not to its inner side; in other words, the maxillary process, which has been strictly deﬁned according to the well-marked limits of its condensation, comes equally into contact with the back of the inner and outer parts of the ﬁeld, or inner and outer future walls of the pit.
FIG. 2. Mouse embryo. Model from below and right side.
I., II., ﬁrst and second arches ;' ‘maxillary process a rounded mass projecting forward from I. and reaching in front to the back of the oval olfactory area, which shows a shallow, illdefined ve; X., dotted line showing level of condensation of maxillary process; 0., lens vaginating: the rounded swelling behind this, above the maxillary process, is apparently only due to the presence of the Gasserian ganglion. '
The succeeding stages in the development can be best shown schematically as in ﬁg. 3. - .
It can be seen from this that the growth of the maxillary mass at ﬁrst keeps pace with that of the wallsof the olfactory pit, so that this depression has its lateral and front walls formed by the growth of the margins of the ﬁeld, but its back wall is a secondary formation by the maxillary process. The maxillary process now has a groove leading to the pit, and the outer side of the groove is prominent, forming the “ maxillary process ” of the ordinary description, so that the groove can be said to run back from the open pit to the buccal side of the maxillary process. But the groove is ﬂoored by the inward extension of the process, which, further in, comes up against the back of the inner wall of the pit; this is the part that I termed the “ septal ingrowth ” in my former paper. ~
Later, the growth of the maxillary process outstrips—in its outer part ——that of the pit walls, and grows below the outer wall, ultimately spreading across the open mouth to reach the inner wall, and in this way closing the central portion of the open mouth of the pit and leaving only an anterior and a posterior opening. About the same time, or perhaps
FIG. 3. The mandibular arch is cut away.
On one side the olfactory pit is formed, its ﬂoor being at a deeper level than that of the maxillary mass which forms its back wall. The maxillary process is deeper in its outer part M., so that a groove, X., runs forward on its inner side towards the back end of the pit opening. On the opposite side the effect of further maxillary growth is shown covering in part of the outer wall of the pit and bulging across its slitlike opening, so forming ., P., anterior and posterior primitive nares. .At N. the two walls of the pit have also fused over a small area, deep to the maxillary growth. S., septal ingrowth of maxillary process.
before this takes place, the two walls of the pit have come into contact at the mouth of the pit, and fused over a very small area; this is shown in the scheme.
Thus two primitive nwres are substituted for the long original single opening, and the olfactory pit is converted into the olfactory fossa, such as is found at the end of the ﬁrst month.
We can now change the terms that were appropriate when talking of the early stages; the “ﬂoor” of the pit becomes the “ roof” of the early fossa, its open mouth is closed by a “ ﬂoor” formed of two distinct junctions, while the composition of the lateral and front and back walls remains the same.
It is possible now to comprehend the values of the parts seen in the fossa at the beginning of the second month,.as shown in ﬁg. 4; in this and in the succeeding ﬁgures the outer wall of the left cavity is represented.
Nearly the whole of the wall is formed by a rounded prominence which is the inner aspect of the outer wall of the original pit, the “lateral nasal process,” as it is usually termed. In the model the whole process can be
FIG. 4. Outer wall of left nasal fossa. From a model.
Represents the fossa opened by removal of the mesial nasal process: this exposes the lateral nasal process forming the outer wall of the fossa, and shows the small (cut) area where the two processes were fused. Behind this the ﬂoorot the fossa is formed by the maxillary process, out where it meets the mesial wall, but visible as a prominent mass lateral to this and lying below the back part of the lateral wall. The osition of the various structures is easily seen by referring to the schematic
formation of thele t fossa in fig. 3. F., forehrain ; P., pericardium; M., cut man bular arch.
taken between the ﬁngers, andthe nature of this part of the wall is very evident. The prominence is continuous bya curved surface with the front
wall, but is separated by an angular sulcus from the projection of the depressed roof} T
The sulcus opens behind into a deep fossa, lying between the back parts of the roof and lateral swellings, and below this fossa the lateral swelling
1 This roof, or “ ﬂoor” of the olfactory, pit was shown in the previous paper to form part of the septum. N 0 further reference to it is made in the ‘present paper, but frequent
useuwas made of it in estimating the position of_ various points in the ﬁna roof and outer wa . ‘ 424 Mr J. Ernest Frazer
is continued by a narrow bar into the posterior part of the wall. This posterior part is evidently maxillary in nature, and forms a deﬁnite back Wall to the fossa. Below, the ﬂoor is made up largely of the maxillary process growing in to meet the mesial nasal process, but above the front part of this is seen the area of fusion between the two nasal processes; as this area of fusion, so far as it affects the side wall, will be constantly referred to, it will be convenient for the purpose of this paper to use the term “ ﬂoor process” to describe that part of the lateral nasal process that turns in to form part of the ﬂoor by meeting the mesial process, so
distinguishing it from the remaining maxillary ﬂoor.
FIG. 5.-—-Sixth week. Model.
LN., Mx., lateral nasal (primary) and maxillary (secondary) arts of outer wall; F.p., ﬂoor process Pal., palate ridge; N., extent.of inclusion of nasal nerve n condensation of nasal processes.
In front of the ﬂoor is the comparatively large anterior opening of the cavity, While the small posterior opening behind it is partly blocked by the bucco-nasal membrane.
I have not considered the formation of this membrane, for it does not, in my opinion, affect in any way the morphological values of the various parts that enter «into the formation of the cavity, although it is hardly necessary to say that it is of great importance when studying the phylogenetic or general value of the cavity as ‘a Whole; I hope to be able in the near future to deal with it and many other minor points that are necessarily left out in this account.
In the sixth week the conditions are very interesting (ﬁg. 5), because they show the transition form between the early stage and what is practically the ultimate condition in its essentials. The prominence of the lateral nasal process is easily recognised, as are also the ﬂoorprocess, the connexion with the posterior part, the deep fossa above this, and the continuity with the front wall. But the interest centres round the posterior part of the cavity; here the back Wall seems to have turned on to the outer wall, and to be forming its upper and back part.
It is plain that the rapidly growing maxillary process, in opening up the cavity, has enlarged the posterior opening and has turned down the back wall in to the outer wall, and as a consequence has added a maxillary portion to the back of the original lateral nasal prominence.
We thus get a condition in which a secondary maxillary area on the outer wall is being added to the primary original area, and the inferior turbinal mass at this stage shows a large anterior primary end continuous below with the ﬂoor process, and behind with a posterior maxillary or secondary part. The inferior turbinal shows a slight groove which seems to indicate the junction of its two parts; the groove not only corresponds in position and relations with the hinder end of the early lateral prominence but also with a change, seen under the microscope, from the thicker and more deeply stained condensation of the nasal process to that of the maxillary tissue.
At this stage we have, then, an outer wall which consists of a larger primary part in front and a smaller secondary part behind and above; a curved line drawn up from behind the ﬂoor process to the lower end of the deep fossa, and from thence through the fossa to the roof would, for practical purposes, divide the two areas.
In the eighth week the secondary part of the inferior turbinal has grown in length with the increased height of the posterior opening and cavity as a whole, owing to continued growth of the maxillary area, and in the ninth, week the length of the turbinal is almost proportionately completed.
The stages of formation of the wall after the sixth week are perhaps better followed by observation of the growth of the other turbinals. The formation of these is also, like that of the greater part of the lowest turbinal, an example of the exuberance of growth in the maxillary process.
In the sixth week the small maxillary ﬁeld above and behind the pyramidal-shaped fossa represents probably only the middle turbinal, and the area that will form the upper mass has not yet come into existence.
In the eighth week (ﬁg. 6) the recently added area is beginning to project as a superior turbinal, while the older lower and front part forms a well-marked middle protrusion; this is pushed into the fossa from above and behind, and effects a decided change in its appearance. In the earlier stage the recess is comparatively large and broad, and roughly pyramidal with its apex below and behind, but in the eighth week it has become more of a curved groove as a result of the downward and forward “push ” p of the maxillary growth. The concavity of the curve, looking upwards and backwards, receives the middle protrusion, which also very slightly overlaps the groove, to the extent indicated by the dotted line.
The upper turbinal at this stage is merely an eminence separated from the middle mass by a shallow sulcus which it does not overlap.
But this is not the only result to be noticed; the fossa or groove has been carried down from its original position, and its upper end is separated from the roof-level by tissues that are apparently growing forward between them. In the seventh week the groove is found only just reaching the top level of the wall, and in the ninth week the process has gone on further than in the eighth week, so that it may be looked on as progressing with the growth of the maxillary mass.
FIG. 6.—Eighth week. Linear reconstruction and model.
F.p., ﬂoor process; Mx., secondary area; LN., primary.
I am rather doubtful about the nature of this change; possibly it may be looked on simply as a raising up of the ﬂoor by increased mesenchymal formation, the original fossa being simply an expression of the thinner marginal layers at the boundaries of the primary and secondary areas in this region.
Whatever the ultimate conception of its nature may be, the forward growth takes place as shown in the ﬁgures, and the groove is brought down to a lower level. Later, in the ninth week (ﬁg. 7) the upper forward growth is overlapping the upper end of the groove, and is continuous below and behind with the middle turbinal, which is now deﬁnitely overhanging the curved sulcus. In this way the hiatus semilunaris is brought into existence, and the early condition of the operculum of the middle turbinal
FIG. 7. Ninth week. Linear reconstruction (with model).
References as before.
is seen, continued above and in front into the primary area. A cartilaginous buttress is raised on the primary part of the capsule that supports this front attachment of the operculum, which may be looked on probably as a part of the primary area, strengthened by its own cartilaginous support; the cartilage joins that supporting the growing middle turbinal, and in this way a kind of hood is thrown over the upper part of the groove.
It thus comes about that in the ninth week the groove is deﬁnitely overhung by a middle turbinal, which covers in its top part completely and is continued into the primary area. In the same way the upper turbinal is forming a hooded covering for its groove, and passing in front into the middle turbinal, and the appearances on cutting away the whole overhanging mass are shown in ﬁg. 8.
Corresponding views in the tenth week and fourth month are given in ﬁgs. 9 and 10, and the general mode of extension and growth of these two turbinals up to this time can be appreciated from these ﬁgures without further description. The interrupted line in the last two ﬁgures, connected with the semilunar sulcus, marks on the surface the position of the growing antrum, which is thus seen to commence its formation rather earlier than is usually described.
FIG. 8. Ninth week, showing result of cutting away the projecting parts of the upper and ~ middle turbinals.
It is apparent, from what has been said, that the lower and front margin of the semilunar sulcus is provided by the primary area of the outer wall, and in association with this part one of the so-called turbinals that appear late in foetal life is developed. These later formations do not come- within the province of the present paper.
The lower end of the primary area corresponds with the ﬂoor process, and this is easily recognised at any stage of development or in the adult, being the ridge that runs from the anterior extremity of the turbinal to n the ﬂoor behind the meatus.
The upper limit of the primary area would correspond with the spot at which the end of the sulcus reaches the topmost level of the outer Wall, but as this region is altered by the hooded growthof the end of the middle turbinal, it is necessary to ﬁnd another indicating medium.
FIG. 9. Tenth week. Linear reconstruction (with model) ; projecting upper two masses cut away. Ant. is outline of antrum.
FIG. l0. Fourth month. Linear reconstruction.
In the earlier stages this is easily obtained. In my previous paper on the nasal cavities, Iﬂpointed out that the original ﬂoor of the pit is turned down on the septum in the fossa, and forms its upper and front part supported by a thick bar in the central basis of the septum: it is only necessary to trace up the posterior edge of this thick part of‘ the septum to reach the point required on the roof.
In the third month and afterwards, When the thickness of the front portion of the septum is lost, its former posterior border can be put with sufficient accuracy in a line drawn down from the back part of the crista galli, which, in its cartilaginous stage, is a continuation of the thick anterior bar of the septum.
Thus, by making use of the various points mentioned, we can divide the outer wall with approximate accuracy into an anterior primary area and a posterior secondary area. The anterior ﬁeld includes the whole of the atrium, and region of the agger nasi, the front part of the lower turbinal, the lower and front boundary of the hiatus semilunaris, and the area between this and the front of the lower turbinal, the anterior end of the middle turbinal and the area above it lying in front of the line continued up from the sulcus to the roof below the back part of the crista galli: it reaches the ﬂoor by means of the “ﬂoor process,” and in front of this extends down as far as the limen, or just below this.
The concavity of the atrium receives the thick anterior part of the septum, and may owe its form to this: the indeﬁnite ridges that lie below and in front of it, diverging from the front end of the lower turbinal, are probably remains of the curved connexion between the outer and front walls of the pit, spread out by the downgrowth of the latter: the limen nasi is the inner border of the free edge of the lateral pit wall, and the vestibule below it is in part the lower surface of this free edge and in part probably an added area from the outside of this. The formation of the vestibule belongs rather to the later stages of development.
If we now turn back to the septum we ﬁnd the corresponding primary surface there. In the ﬁgures in my previous paper the posterior area C is maxillary. A is primary, being the “ﬂoor” of the original pit, and B can now be recognised as that part of the inner wall that meets the ﬂoor process of the outer wall. It follows necessarily that the remaining surface in front—with the probable exception of the vestibular part - corresponds with the mesial wall of the pit: this surface was not considered in the previous paper, but its value is now apparent, and it can be all placed under B as primary inner wall.
While, therefore, we recognise two main areas, primary and secondary, on the outer wall, we ﬁnd similar primary and secondary surfaces on the inner wall, but in this case the primary area can be subdivided into a part formed from the wall, and a part from the floor, of the original olfactory pit.
The line between the primary and secondary areas on the septum can be drawn roughly from the roof below the back of the crista galli to the oriﬁce of Jacobson’s organ, and from thence to the ﬂoor just behind the “ﬂoor process.”
The cavity enclosed between the two primary parts on the walls would thus correspond with the original pit-cavity, whereas the remaining posterior part has no existence at this stage, but is brought into being as a result of the growth of the lateral and mcsial portions of the maxillary process.
It was my intention to include an account of the cartilaginous capsule, but a fairly full description of this structure would add greatly to this already sufficiently long paper. I will, therefore, merely state that the capsule shows corresponding primary‘ and secondary parts, that these support the areas of the walls and show even a short “ ﬂoor process” in the case of the outer wall, that the paraseptal cartilages are conﬁned to the maxillary districts, and that the spot where the nasal nerve enters the capsule marks the upper or posterior limit of the primary part, whereas its point of exit from the cm°tz'l<¢L(/iazaous capsule seems to correspond with the highest part of the front wall. This point of exit is not the same as the point of emergence between the nasal bone and lateral cartilage, but is under cover of the bone.
It remains to say a few words about the meatuses of the nose, as these are formed in the ﬁrst few months of development.
To deal first with the middle meatus. It is plain from the foregoing account that the hiatus semilunaris is simply a modiﬁcation of an early fossa, and that it lies between the primary atrium and the secondary bulla ethmoidalis. It is equally evident that the overlapping of this groove by the middle turbinal is brought about by its growth, and the extension of the protuberance backwards is presumably due to a similar cause; there seems to be no reason for supposing that there is any ingrowth of the meatus, but only an outgrowth of the turbinal.
The superior meatus appears to be formed also by the growth of the turbinal mass: at least, this View agrees with the position and relations of the various surfaces seen in the models at different stages, and there is no sign of ingrowth of the meatus, nor does this explanation of its appearance agree with the observed facts.
The formation of the inferior meatus appears to be of a different nature, and it seems to be preceded by a solid plate of cells even at the six-weeks stage: these may be an ingrowth from the wall, but their appearance suggests a dif‘l'erentiation in s2'.z‘7_I.. in the maxillary tissue. I cannot speak with certainty 011 the subject, but in my opinion the increase in depth of the meatus and of the turbinal mass hanging over it, is due to the growth of the maxillary process pushing up the "primary area from below in the case of the former, and in the latter increasing the projection formed from the ﬁrst. There is some reason for thinking that the front end of the meatus is formed separately, either as an ingrowth or as a result of overlapping by the enlarged extremity of the turbinal,'separating this from the floorprocess. My ground for this statement rests on the observation of the conditions in one or two embryos, in which such an assumption appeared to be necessary, but I have not paid much attention to the point, and simply mention it as a possibility. T
- The olfactory pit has its front and side walls formed by the nasal processes, and its back wall made by the maxillary process, on to the under surface of which the “groove” leads back from the pit. The maxillary process comes up against the ends of both mesial and lateral nasal processes from behind.
- The outer part of the maxillary process grows forward and inward, superﬁcial to the lateral nasal process, to meet the mesial one, thus closing in the middle of the cleft-like opening of the pit, and leaving an anterior and posterior opening; deep to (above) this, the two nasal processes also fuse to a slight extent.
- Thus the olfactory fossa is made from the olfactory pit, and has side and front walls of nasal processes, a back wall of maxillary process, and a ﬂoor separating the anterior and posterior primitive nares, and consisting of maxillary process, and, in its upper and front part, a “ﬂoor process” of fused mesial and lateral nasal plates.
- The increase in vertical height and antero-posterior length is brought about by the growth of the maxillary process mainly; the nasal processes do not grow so much in proportion.
- The increase in vertical height takes place in an upward direction, and not in a downward direction: this is probably due to displacement of the lower maxillary margin downwards being prevented by the resistance of the mandible and pericardium, etc.
- As a result of this mode of vertical increase, the level of the roof of the cavity and of its posterior opening is raised as regards the roof of the pharnyx, so that in the third month the cavities, originally low down, are opening on a level with the upper part of the pharynx.
- As a result of this rise in the level of the posterior openings the back part of the septum comes into existence, being uncovered, as it were, by the retreat upwards of the upper parts of the openings on each side. There is no “ downgrowth ” of the septum.
- The vertical. increase upward goes on throughout the second month into the third month. Probably after this the increase in height is only pcwi passu with the general growth.
- The nasal cavity is formed by the addition of a secondary maxillary part to the privnary cavity of the olfactory pit. The secondary part lies between secondary areas on the outer wall and septum, which are added to the back and upper part of these by the growth of the maxillary process.
- The primary part of the outer wall lies in front and includes that part of the wall that lies in front of a line drawn from behind the downturned anterior end of the interior turbinal (ﬂoor process) to the" lower end of the hiatus semilunaris, then up along this, to a point on the roof corresponding to the position of the back part of the crista galli: the area only just extends down over the limen nasi.
- The inferior turbinal, which is at ﬁrst only primary, is added to from behind, so that its greater part is ultimately secondary: the middle turbinal is secondary, but has a later extension into the primary area: the upper turbinal is probably altogether secondary in nature.
- The upper and middle meatuses are formed by the growth of the turbinals. The lower meatus possibly owes its depth to the growth of the maxillary region below the level of the primary area, and its anterior or primary part may be formed separately.
Cite this page: Hill, M.A. (2019, November 15) Embryology Paper - A further communication on the formation of the nasal cavities. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_A_further_communication_on_the_formation_of_the_nasal_cavities
- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G