Paper - A preliminary communication on the formation of the nasal cavities (1911)
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Frazer JE. A preliminary communication on the formation of the nasal cavities.(1911) J Anat. 45(4): 347–356. PMID 17232894
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A Preliminary Communication on the Formation of the Nasal Cavities
By J. Ernest Frazer, F.R.C.S., Lecturer on Anatomy, St Mary’s Hospital Medical School.
In this paper I will endeavour to give a short general account of the maxillary structures in the early mouth cavity, showing their relations to the future nasal tissues and neighbouring parts: after this I propose to proceed with a more detailed account of the development of the septum, reserving that of the outer wall, and cavity as a whole, for future consideration.
The drawings and diagrams are constructed from the models on which the research was largely founded, and are intended to show the various points dealt with in the communication.
The first five figures show the maxillary process of the mandibular arch in its relation to the nasal cavity before the closure of the palate ridges.
In fig. 1 the facial aspect of an embryo early in the fifth week is shown, exhibiting the anterior nasal opening with its bounding processes. In fig. 2 the nasal region of the same embryo is seen from below, the mandibular arch and floor of the mouth cavity being cut away, and the very small posterior nasal opening—partly closed by a bucco-nasal membrane— is seen to open into a shallow groove bounded externally by the maxillary process.
In the sixth week (fig. 3) the maxillary mass has grown considerably in depth, as has also the thick primitive septum, and there is a corresponding increase in size of the posterior opening: a palatine ridge is overhanging the situation of the original groove, and alveolar and labial parts are easily distinguishable in the mass.
In the seventh-week embryo (fig. 4) the inter-maxillary space is deeper still, owing to further growth of the maxillary processes: the labial portions have completed the labial ridge in front, the alveolar parts have nearly met below the median process, and the palatine ridges are well marked but do not reach the median process in front.
Fig, 3. : Fie, 4.
Fig. 5 shows the condition in the eighth week: the alveolar ridges are nearly complete in front, and the palate folds have extended so that their front extremities reach the sides of the median process or primitive septum. The various regions of the palatine ridge are distinguishable, so that one can see how much of the structure goes to form the hard and soft palate and the posterior pillar of the fauces respectively.
Later than this the “hard-palate” regions of the palate folds meet in the middle line and are applied to the back of the septum for the greater part of its extent, so that the posterior nares are small channels corresponding in their height with that part of the septum which is not covered by the fused palate-folds.
The alveolar and labial ridges that meet their fellows in the middle line do so by a process of sub-epithelial invasion, and not by overlapping the underlying “globular processes.” Behind the alveolar ridge this globular region remains uncovered, continuous laterally with the free palatine folds, and these, when they meet across, do so behind the globular region.
It is apparent, then, that the maxillary mass deepens by its growth the originally shallow groove into which the nasal pit opened, leads to a consequent increase in height of the primitive posterior nasal opening, and with its fellow forms the lateral boundaries of a deep space into which the cavities open in front, and which is occupied largely by the developing tongue. The palatine ridge is about halfway down the mass, and the inner aspect of the mass deep to (above) this will form in its front part the outer wall of the back part of the nasal cavity when the palate closes, and behind this will be continued into the side wall of the nasopharynx.
Fig. 5. — Eighth week.
But there is another process of the maxillary mass which has not been mentioned so far: this is a solid ingrowth from its highest or deepest part, extending inwards over the roof of the general cavity, and turning down- wards and forwards on the lateral and back aspect of the septum to reach the globular region.
This septal ingrowth is found in the early stage figured in this paper, and, by its lower and front enlargement, seems to be responsible for the existence of the globular processes.
In the sixth week the ingrowth is a thick condensation with a well- marked border, forming the roof of the deepened groove and approaching the middle line as it is traced forwards and downwards: it meets its fellow in the globular region, where it is much enlarged, and becomes continuous 350, Mr J. Ernest S. Frazer
externally with the maxillary condensations. In succeeding stages the development of this ingrowth keeps pace with that of the maxillary mass and of the central septal basis, and forms the rounded edge to the back of the broad “septum.” It tends to meet its fellow at higher levels, behind the central basis of the septum. The pharyngeal end of Rathke’s pouch is at first between the ingrowths, some distance behind their meeting, but in the seventh week they catch it, as it were, between them, and, in the subsequent growth of the septum, seem to cause its dislocation forward on to the back of the septum that I have already described in this Journal.
The short general account just given of the distribution of the processes of the maxillary mass can be summed up for present purposes by saying that the maxillary process of the mandibular arch practically forms the side wall and roof of the mouth cavity as far forward as the olfactory pit, and constitutes the immediate wall of this below and behind: it follows that, as the nasal cavities extend backwards, they must do so at the expense, so to speak, of this maxillary condensation, which will form the bed on which their lining membrane will lie.
(a) Superficial Aspect
If the nasal cavity is opened, at the beginning of the second month, by removing the outer wall, the appearance of the inner wall or primitive septum is as shown in fig. 6.
The most striking object on this wall is the rounded upper prominence A. This is really at this stage part of the roof, looking more downwards than outwards; the outline of section shows this.
Comparison of this figure with fig. 1 makes it clear that the lower area B is the upper surface of the globular process of that side: in front B is continued into a wider surface, the outer aspect of the median fronto-nasal process, an area whose further development can be better considered with that of the whole cavity on some future occasion.
The cavity has extended back very slightly beyond the level of the posterior nasal opening, and this posterior area C is necessarily covering the maxillary mesenchyme of the septal ingrowth which runs into the globular process below.
The early septum can thus be said to consist of three areas—leaving the front region out of account for the time. The areas are—an upper (A), originally roof, or floor of the primitive olfactory pit, and a lower (B), part of the boundary of the pit. These two might be described as primary areas, and, as the cavity extends back, they will have a third secondary area added to them, already seen in this figure as a small region (C) continuous, as follows from its development, with the lower area B.
A and B are separated by a deep sulcus, which at one spot forms a wide fossa (X), the earliest indication of the “organ of Jacobson.”
In the sixth week (fig. 7) the upper area A is still prominent and rounded, but is now more definitely part of the inner wall; the section tracing shows this change very well. This is associated with the down- growth of the whole septal process, and increase in depth of the cavity, and, as a result, the posterior nasal opening is much larger and the area C is correspondingly increased, but of course remains continuous with the lower area B.
Fig. 6. — Inner wall of left nasal cavity at beginning of fifth week. The upper area A looks downwards and slightly outwards, the lower area B looks upwards and outwards: these directions are shown in the outline of a transverse section above the main figure. The anterior opening of the cavity, on the left, is large, but the posterior opening, below the small area C, is very small, and partly closed by the bucco-nasal membrane, indicated by the “dotted line. X, deep depression, the first rudiment of Jacobson’s organ.
Jacobson’s organ is now a tubular recess placed in a position correspond- ing with that of its earlier stage in fig. 6.
As might be expected from its method of development, the area A is olfactory. At this stage olfactory fibres are distributed all over it, most markedly over its back part down to the groove and Jacobson’s organ, to which some large and well-marked fibres go.
The growth of the nasal cavity continues in the eighth week (fig. 8) and ninth week (fig. 9), when the palatine folds close below the greater part of the posterior opening.
In the eighth week the olfactory area A is still rounded, though this is not so apparent as in the earlier stages. There is an increase in the size of the posterior area C, as also in the depth of the posterior opening.
Fig. 9. — A, B, C, X as in preceding figures. n.p. is placed in front of the naso- palatine canal. P marks the posterior limit of the posterior nasal opening, which extends from the naso-palatine canal to P. It has been largely closed in by the meeting of the palate folds, which fuse with the edge of the septum: they are shown in the figure as lying below it but not fused with it. The posterior nares are those small parts of the openings left, between the extreme limit of the fusion with the septum and the level P.
In the ninth week the olfactory area still shows a somewhat rounded front part, but otherwise is more or less flattened out to the general level of the septum. The posterior area C is again proportionately larger, and the large size of the posterior opening corresponds with this. But this large opening is blocked up in a great part of its extent by the palate fold closing in below it to meet its fellow, and only a small part of it remains behind this fold as the posterior naris.
(b) The Deeper Structures of the Septum
If the different areas just described are considered in connection with the first part of this paper, it becomes apparent that corresponding differ- ences should exist in the nature of the deeper tissues that form the bases of these areas, and this is found to be the case. There is a central basis made up of a front part underlying the area A, and a back part supporting the area C and continued into B, and on each side of this in C is a thick lateral layer of maxillary septal ingrowth extending into B and spreading out here into the mass that forms the floor of this part of the cavity.
These septal ingrowths form, therefore, what one might term “lateral layers” lying on thé central basis and continuous with each other round its free lower edge.
Looking at the deeper structures of the septum from this point of view, they might be described as divided into central septal or basic structures, and those formed in the lateral layers, and it will be convenient to give a description of them in this way.
The Central Basis
A reference to the outline section of the cavity in the fifth week makes it apparent that the mesenchymal mass that supports the area A is continuous with, and in fact is part of, the enclosing capsule of that part of the brain that lies immediately over it—z.c. the olfactory part of the cerebral outgrowth.
As this area becomes more vertical in position, the condensation that is formed between them is therefore directly continuous with the brain capsule, between and below the olfactory parts, and, in front of this, the overlapping remainder of the cerebral vesicle. In this way a thick rounded bar of condensed tissue comes to represent the upper and front part of the central septal basis.
On the other hand, the back part of the early septum has as its central support a bar continuous above and behind with that part of the membran- ous basis cranii that lies under the telencephalon, immediately in front of the pituitary formation. This bar is not so thick antero-posteriorly as the first, and is therefore more definitely rounded. It goes downwards and forwards, growing smaller as it is traced forward, till it is lost between the upper parts of the globular processes. The two bars would thus meet at an angle if prolonged, and this angular intervening district is filled by a thinner plate of condensed mesenchyme, continuous in front and behind with the main bars, but being lost above, where it is separated from the brain structures by an interval that seems only to contain vascular tissue.
Chondrification apparently occurs separately in the two regions, about the seventh week. So far as I have observed the process, it seems to begin as a distinct change in the front bar, and as an extension from the chondrifying basis cranii in the hinder one. I cannot say with any degree of certainty how the intermediate area becomes cartilaginous As the whole structure increases its height, the original thickness of the bars is gradually flattened out, remaining longest at their upper ends: here the front bar is continuous with the primary cartilaginous capsule of the cavity, and the hinder bar, slowly differentiating from the basis cranii proper, stands out more and more posteriorly. In this way the intermediate area increases in size, being added to from the top, where it forms a secondary connection with the primary capsule.
It is possible then to distinguish, in the early cartilaginous central septal basis, three areas, an upper and front, a lower and back, and an intermediate region between these: this is in addition to the front region, which is not being considered in the present paper.
The Structures Formed in the Lateral Layers
As has been already shown, the septal ingrowth from the maxillary mass forms a “lateral layer” applied to the surface of the hinder and lower part of the central septal basis, and continuous with its fellow round the lower free border of this: below and in front it underlies the area B and passes out into the floor, forming the mass of the globular region.
The area C on the septum would lie on the lateral layer, and the extent to which it covers the septal basis increases, as shown in the figures and in the description given.
This lateral layer (as follows from its mode of origin) is continuous over the roof of the cavity with the outer maxillary mass, and in this mass the back part of the nasal capsule is formed. The upper and back part of the capsule extends over the roof and turns down on the side of the septal central cartilage, in the plane of the lateral layer, with which it is of course directly continuous: it forms a hook-like projection of cartilage applied to the side of the hinder bar of central cartilage, and has the mucous lining of the back part of the cavity below, outside, and in front of it.
Fig. 10 shows this cartilaginous process applied to the side of the central cartilage. The area in front of it and below it (C) is covered by thick tissue continuous with it and forming the lateral layer, and in this tissue the vomer and the vomerine cartilage develop about the ninth week.
The vomer, showing as two bony strips more or less sagittally placed behind the hinder bar, seems to me to be formed in this early stage in the lateral layer, where this turns behind the bar. It is limited for some time to that part of the septum which is covered by the palate folds, where it forms; these facts, and the time of its appearance, suggest that in its early form it may have some connection—mechanical or other—with the pre- sence of the palate folds, which are separated by its two halves from the back of the septal basis.
1 Possibly by extension from the front bar. Preliminary Communication on Formation of the Nasal Cavities 355
The vomerine cartilage is a chondrification in the lateral part of the lateral layer near its lower end, so that it lies external to the vomer, where its back part comes into relation with this bone. It is continued down into the former globular region, lying internal to and in front of the naso-palatine canal. This canal is bounded in front and internally by the globular mass and behind and externally by the palate ridge of its side.
Fig, 10 shows semi-diagrammatically the structures lying against the back area (C) of the septal cartilaginous bases on the right side. The line m indicates in places the approximate position of the edge of the mucous lining ; ¢.b. is the upper end of the posterior carti- laginous bar ; caps. is the downturned part of the capsule ; v. the right half of the early vomer ; and v.c. the vomerine cartilage. The other letters are as in the previous figures.
A tracing showing the relations of the structures in the septum is seen in fig. 11, representing the right side of the inner wall, etc. in the tenth week. In this the vomer is still practically confined to the palate region of the septum, but has increased in depth below, where the two halves have joined.
The vomerine cartilage shows an extension in front of the region of the naso-palatine canal, and its appearance suggests that it develops originally as a support to the inner and front walls of the canal.
The process of the nasal capsule turning down on the septum is seen to be much longer—no doubt in correspondence with the growth of this structure—but to preserve the same general relations.
It is possible, then, to describe all these paraseptal structures as lying in one plane—that of the nasal capsule formed in the maxillary tissues. They are all situated in the continuous areas C and B of the septum, and comprise the upper process of the capsule, the layer of tissue underlying the areas, the vomer, and the vomerine cartilage—and, in fact, these structures might be considered to be modifications or specialised parts of the capsule where it lies or would lie in the inner wall of the cavity.
I have not yet been able to make complete observations on the later fate of the structures I have described as existing in the early septum. Possibly the sphenoidal turbinals may be derived from the upper spur of the capsule, and the vomer, in the stages in which it has been shown in this paper, may not be of the same value as the bone found in the adult skull. I have paid no particular attention to this last point, which lies somewhat outside the sphere of the present investigation, but I have little doubt that the bone increases its depth, largely, by late addition to its lower border.
Fig. 11. — A tracing from a linear reconstruction. 48mm. Seen from the right. The soft palate is represented as cut, and the naso-pharynx is shaded. OJ. shows the position of the olfactory lobe, and the other letters are as before. The slight oblique shading in the areas C and B indicate the extent of the “lateral-layer” tissue, and its continuity with the septal process of the capsule (caps,) is shown above, The vomerine cartilage (v.c.) has a long wing directed outwards underlying at some little distance the naso-palatine opening and the floor of the cavity just in front of this. The vomer (v.) is deeper than in the last figure, but is still con- fined to the region of the septum that is in contact with the palate.
The vomerine cartilage has a wing—seen in the last figure—thrown out in front of the naso-palatine canal, a little distance away from it. In the fourth month I have noticed these two pieces of cartilage nipped between the ossification of the maxilla and its premaxillary extension, the bony parts being continuous below the cartilages; but I am unable to say when these pieces of cartilage disappear and leave the upper part free to assume its final position.
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