Paper - The nature of the soft palate (1940)
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The Nature of the Soft Palate
By Frederic Wood Jones
Introduction and Nomenclature
In attempting to determine the true nature of any anatomical structure two factors, which at first sight may seem trivial, must be taken into account. In the first place our conception of any anatomical entity is likely to be influenced by the form of the entity that was first described and to which the distinctive name of the structure was first given. This, in its turn, will obviously depend upon what was the type of animal in which the structure was originally described. There are, as is well known, many instances of classical errors in human anatomy caused by the fact that descriptions of certain organs were founded originally upon the dissection of specialized mammals representative of Orders widely separated from the Primates. Some of these errors are, unfortunately, not cleared completely away from human descriptive anatomy even at the present day. On the other hand, the fact that so much anatomical description was first stereotyped from human dissection has led to a loss of the true conception of the nature of certain structures that have their complete expression only in other mammals.
The second factor that must be taken into account is the circumstance that the nomenclature will almost certainly be standardized in accordance with the type of structure to which it was first applied and that, therefore, it may be completely misleading when applied to the structure as seen in more perfected condition in other animals. The preservation of a nomenclature that is not applicable to the true nature of a structure is bound to lead to a lack of realization of the essential characters of any part or organ of the body. Since an incorrect nomenclature may so easily lead to a complete misconception of the nature of a structure, it is obvious that the perpetuation, or the alteration, of a terminology is a matter of considerable importance when the question of scientific accuracy is considered. I have elsewhere in this Journal (1939) called attention to the steady deterioration in the precision and accuracy of the nomenclature of the superior alveolar nerves that has taken place during the last century and, in that instance, have cited the latest (Anat. Soc. 1933) revision of anatomical terminology as being a retrograde step in the progress of precise nomenclature.
It is, therefore, worth while to inquire if in any way nomenclature has influenced our conception of the nature of the palate and if such changes as have been made constitute an advance in our realization of its true character or have helped to obscure it. The first step in this inquiry involves the term pharynx. When Galen introduced this term into descriptive anatomy he doubtless did so, as in so many other instances, from his knowledge of the anatomy of mammals other than man. By him the term “pharynx” is used for the throat —the passage by which the food passes from the mouth to the stomach. As signifying the food passage of the throat the term “pharynx” is employed by all the older anatomists. Only a few instances of this usage need be cited. Thomas Gibson (1688) writes of “‘the throat or the beginning of the oesophagus called pharynx”. James Keill (1703) defines ‘‘the oesophagus or gullet” as “‘a long, large, and round canal, which descends from the mouth” and further states that “‘the upper end of the gullet is call’d Pharynx”. Dionis (1716) speaks of ‘“‘the Pharynx, which is only the orifice of the oesophagus dilated to a great extent. ’Tis like a funnel, and some call it the gula or gullet. ’Tis seated at the bottom of the mouth that it may receive what is swallowed.” Heister (1752) says ‘‘the oesophagus or gula. . .isa membranous canal reaching from the fauces to the stomach. . .its upper part is called by anatomists the pharynx”. There can be no doubt whatever as to what was originally meant, and what the older anatomists understood, by the term “‘pharynx”’. There can also be no doubt that, were our knowledge of these parts still to be confined to the primitive (and more perfectly developed) condition seen in most mammals, the term “pharynx” would still be employed in this simple and unambiguous fashion. For the older anatomists there was a mouth or oral or buccal cavity leading into a funnel-shaped pharynx which passed directly into the oesophagus. Above the mouth and pharynx there was a palate which separated them from the nasal chambers above. This palate does not at all times cut off communication between the nasal chambers and the pharynx, since it has an aperture in it that leads from one cavity to the other. This older conception is the only one that can with any propriety be applied to the condition found in any primitive mammal: it is the only conception that can rightly be applied to the condition present in man. Unfortunately this correct and simple terminology has been confused by human anatomists, who, having failed to realize the true nature of the, somewhat modified, human palate, have extended the term “ pharynx” to the posterior part of the nasal chambers, a region to which the term can in no proper sense be applied. How far modern usage has led anatomists astray from the true conception of the connotation of the terms may be gathered from the following statement taken from the monograph of Negus (1929): “‘In most mammals the oral cavity has its axis placed directly in line with that of the pharynx, if the latter can be said to exist at all.” It is perhaps too late to make a plea that the terms “nasopharynx” and nasal parts or nasal portions of the pharynx should be abandoned: but it must be pointed out that their employment involves endless confusion and gives currency to a conception of the parts that is wholly erroneous. No part of the nasal chambers can possibly constitute any part of the true pharynx, for always, in all animals, they are separated from it by the palate which forms the roof of the mouth and the roof of the true pharynx. Although such a state of affairs is made abundantly clear from the merest inspection of the condition present in any primitive mammal, the typical disposition of the corresponding parts in man has undergone some change and the true nature and entire extent of the palate have not generally been recognized by human anatomists. There is no question involved in the anatomical limits of the hard palate. As the roof of the oral cavity and the floor of the nasal cavities, it is immediately recognized and homologized in all vertebrates. It is when a definition of the limits of the soft palate is sought for that there is uncertainty in human anatomy. The soft palate, now orthodoxly known as palatum molle, has been variously designated velum palati, velum pendulum palati, septum palati, valvula palati, or claustrum of the palate; and by most human anatomists has been considered as that part of the roof of the mouth and ‘oral pharynx” that stretches from the hinder edge of the hard palate to a posterior free margin, culminating in the middle line in the uvula. From the hinder end of this palatum molle three separate folds pass to the sides of the tongue, to the pharyngeal wall and to the thyroid cartilage respectively, and these folds appear to be regarded as being much upon the same morphological level, as ridges of mucous membrane raised up by muscles passing from the palate to other parts. Few human anatomists have differentiated in marked degree the morphological significance of the so-called anterior pillars and posterior pillars, which mark respectively the palatoglossus and _palatothyroideus muscles, and the third fold, which marks the true palatopharyngeus. A few exceptions may be quoted to demonstrate that some few human anatomists have recognized that the two sets of pillars are themselves on very different morphological planes and that the posterior pillars are in reality a part of the velum palati which in man has undergone a very definite change from the primitive mammalian condition. Winslow (1788) describes the velum palati as continuous with the posterior arch which has “‘a continuation with the sides of the pharynx”. Monro (1881) describes the posterior pillars as ‘“‘the lateral edges of the palate”. Trew (1840) says ‘‘The posterior pillars constitute in fact the free border of the velum.” The best description is probably that of Dionis (1716) for he says that “‘upon the sides of the uvula we meet with two arches,? which are the mouth of the Rima Nasalis: these arches consist of semi-circular fibres...when the semi-circular arches stretch themselves lengthwise, they become strait, in order to confine the air within the mouth when we blow or heave up the cheeks”. It would be difficult to furnish a better description of the parts: and the introduction of the term Rima Nasalis is a noteworthy detail. Ail these authors have realized that the posterior pillars are on an altogether different plane from the anterior pillars and that they are in fact a part of the backward continuations of the velum palati to join the posterior wall of the pharynx. Winslow even went a stage further in describing the muscles of the posterior arches (m. pharyngostaphylini=m. palatopharyngeus as distinct from the m. palatothyroideus) for he states that they “‘seem to form an entire arch by the union of their fibres”. The description of the entire arch, by Winslow, makes the remarkable illustration of Samuel Collins (1685, Table II, Fig. 1) more easy to understand (see Fig. 1). Turning again to modern descriptive anatomy and terminology, it would seem that in human anatomy the fact that the velum palati with the posterior pillars and the meeting of these pillars on the posterior pharyngeal wall (as will be shown later) constitute a single continuous entity has been somewhat lost sight of. Also the fact that this continuous entity forms the cephalic limit of the true pharynx has not been appreciated by human anatomists for almost two centuries. The term “Rima Nasalis”” employed by Dionis for the communication between the true pharynx and the nasal cavity, made via the. velum, is an excellent one and surely more accurate and more informative than the orthodox pharyngeal isthmus which is, morphologically, meaningless. If the term oropharyngeal isthmus, to denote the presumed meeting of the oral and pharyngeal cavities at the anterior pillars, is to replace the older isthmus faucium because of the “lack of agreement concerning its precise significance” (Nomenclature report, 1933), itis surely time that some such term as nasopharyngeal isthmus, or, better still, hiatus nasopharyngeus, should be employed, instead of the wholly incorrect pharyngeal isthmus, for the aperture in the soft palate. Since the examination of the parts in any generalized mammal makes it clear that the velum always separates the nasal chambers above from the pharynx below, it is remarkable that orthodox human descriptive anatomy should still include a definition of the soft palate so strikingly incorrect as that which is current. No morphological justification whatever exists for the statement that “‘the soft palate is a movable fold, suspended from the posterior border of the hard palate, and forming an incomplete septum between the mouth and pharynx” (Gray, 1938). To define it thus is to lead to a total misconception of its morphological nature and of its function. In no mammal does the soft palate intervene between the mouth and the pharynx.
1 He is speaking of bilateral arches, not of the two sets of “pillars of the fauces”’.
Fig. 1. (Fig. 1, Table II, Samuel Collins, Vol. 2, 1685.) “m.m. the cavity, seated beyond the uvula, is encircled every way with conglomerated glands, which discharge themselves into it as into a common duct or cistern: this cavity receiveth air from the two cavities of the nostrils, and transmitteth it through the cavity of the mouth to the larynx, aspera arteria, and bronchia of the lungs. n.n. the entrance of the cavity is encircled by divers minute glands.”
The Intranarial Epiglottis
It is impossible to discuss the true nature of the palate without some reference to the subject of the intranarial epiglottis around which a considerable amount of interest centred just over half a century ago. In vol. 28 of this Journal (1888-9) G. B. Howes, R. L. Bowles, Bland Sutton, Symington and others made a series of contributions to the discussion. The question had an added interest at that time in consequence of the very varying statements that had been made with regard to the intranarial position of the epiglottis in the human embryo and foetus. It was expected that a true evolutionary trend would be displayed in the human foetal condition, and only the controversial atmosphere that became woven about the question succeeded in masking this real evolutionary nature in the conditions recorded. Bland Sutton was quoted by Howes as the authority for the statement that ‘‘he had detected the truly intranarial condition in the human foetus at five months and at birth”: but at the meeting of the Society held on 80 May 1889, Bland Sutton retreated in some measure from this position and stated that ‘“‘he would not be responsible for the statement that the epiglottis was always intranarial in the human foetus, although it often was in the fifth month of intrauterine life”. Meanwhile Symington had shown a developmental series of human embryos in which the epiglottis was in all states intrapharyngeal. The work of Bowles was adversely criticized by Dr Benjamin Howard and, as the evolutionary significance of the human embryonic condition appeared to be somewhat uncertain and the evidence cited from examination of the foetal pig unreliable, the subject became less attractive. Howes had, however, studied the problem at first hand in many mammals, but his attention was directed far more to considerations of the relative levels of the epiglottis and the velum than to the characters of the velum itself. In the whole of the discussions at this time, and in subsequent writings on this question, there has been a tendency to regard the intranarial epiglottis merely as an epiglottis thrust up behind the posterior border of the velum palati into the nasal chambers. Such an outlook is obviously dictated by the conception of the velum as a flap, added to the hind end of the hard palate and having itself a free posterior margin, which was assumed to be its morphological termination. Howes gives no detailed description of the palates of any of the many animals in which he determined the presence of an intranarial epiglottis: moreover, after writing his first paper, he became aware of the work of Riickert, published six years earlier, and learned that his own discovery of the intranarial epiglottis in the rabbit—the discovery that started his investigations—was not original. Interest in the whole subject waned and even in the great monograph of Negus published in 1929 far more attention is paid, in this connexion, to the features of the larynx than to the condition of the velum that is associated with the intranarial epiglottis.
The Comparative Anatomy of the Soft Palate
It has been mentioned that one factor that may tend to obscure a realization of the essential characters of any anatomical structure is the failure to recognize the structure as it presents itself under rather different guises in varying animal types. The statement that “‘the soft palate, or velum palati, which is only found in mammals, is a new structure” is taken from Gegenbauer (1878), but it may be accepted as representative of the views of comparative anatomists generally. The accepted teaching is that there is originally a primitive palate and to this primitive palate there is added a velum palati, as a neomorph, in the mammals alone among the vertebrates. This orthodox view is well stated by Keith (1918) as follows: ‘‘In amphibians, reptiles and birds the naso-buccal channel becomes dilated to form a true respiratory nasal passage and the parts bounding the passage unite on the roof of the mouth to form the primitive palate.” After enumerating the elements involved in the formation of the primitive palate, he proceeds: ‘‘In mammals a fourth element is added to the primitive or reptilian palate, and in this way the mouth is separated from the nasal respiratory passage and can serve the purpose of mastication and suction.”’ The new (fourth) element added in the mammals is the velum palati and it may be mentioned incidentally that the cavity of the mouth, ever since its most primitive inception, has served the purpose of mastication and that this is certainly no new function begot by the development of a velum.
As a matter of fact, the orthodox statement that the velum palati is a new structure found only in the mammals stands in need of revision. It seems strange that such a statement should ever have been made and still more remarkable that it should still be maintained. For the velum palati was certainly recognized as being present in submammalian vertebrates as early as the seventeenth century and its presence in these classes was accepted as axiomatic by John Hunter a century later. It would seem that more recent anatomists are not content with claiming that the velum palati is a structure peculiar to mammals, for in a recently published work on human anatomy we have the remarkable assertion that “‘a palate is peculiar to mammals including man” (Grant, 1937), and the context leaves no doubt in the reader’s mind that the whole of the palate is included in the dictum.
The Velum Palati in Reptiles
In the Reptilia, as a phylum, the nasal chambers are separated from the oral cavity by a bony palate of the type known as the primitive palate. This primitive palate may be defined, on orthodox lines, as consisting of: (1) “a premaxillary and vomerine part developed between the nasal passages; (2) a right and left maxillary part, laid down on the lateral or outer aspect of each passage”” (Keith, 1913). In the majority of reptiles the primitive palate does not extend far back into the oral cavity and the posterior orifice of the nasal chambers is therefore situated in the fore part of the mouth. In the crocodiles, however, this bony palate is of great extent and is carried backwards on the roof of the mouth by the mid-line meeting of the palatines and pterygoids. This extensive bony union on the floor of the nasal chambers causes the hiatus nasopharyngeus to be displaced backwards until it is carried to the posterior end of the cranium and is situated only a short distance anterior to the occipital condyle. But in addition to this backward shifting of the hiatus nasopharyngeus there is another development of the palate peculiar to the Crocodilia. John Hunter (1861) described the “‘velum palati” of the crocodile. Owen (1866) wrote that “‘a velum palati is developed only in the Crocodilia”, and Mivart (1873) speaks of the ‘transverse fold, or soft palate which hangs down in front of the posterior nares”. Negus (1929), however, introduces a note of uncertainty, for he says “‘a velum of the palate is said to be present in the crocodiles, although it is scarcely to be seen in the specimens I have examined”. In his Fig. 12 he names the “‘velum” of Hunter and Owen as “‘mouth flap” and it is to be presumed that he did not homologize this structure with the velum of the mammals.
The so-called velum palati of the crocodiles differs from the avian and mammalian structure known by the same name in that it is not perforated by the hiatus nasopharyngeus, but lies altogether anterior to it (see Fig. 2). John Hunter (1861) observed of the crocodilian velum that it arises ‘half an inch further forwards than (the posterior end of) the bony palate, so that it arises from the roof of the mouth”. It seems quite obvious, therefore, that the crocodilian velum cannot be included in the definition of the true velum as that structure is recognized here. In no sense can it be considered as a continuity passing from the posterior extremity of the hard palate to form the roof of the pharynx and the dorsal wall of the oesophagus. The palatal flap of the crocodile contains no muscle between its reduplicated layers of palatal mucous membrane and no muscles pass to or from it to connect it with any adjacent structures. Its functional role is an entirely passive one, for in the action of closing the mouth cavity from the pharynx, the ‘“‘valve of the tongue” (John Hunter), which is constituted by the anterior margin of the hyoid, is thrust up in front of it by muscular action (see Fig. 3).
Fig. 2. Palate and dorsal wall of pharynx of Crocodilus niloticus. The palatal flap lies anterior to the hiatus nasopharyngeus and has no relation to it.
Fig. 3. Section of nasal chamber (black), palate, palatal flap, pharynx and larynx of Crocodilus niloticus.
Although this palatal flap of the Crocodilia was, presumably, homologized by Hunter, Owen, Cuvier, Mivart and others with the velum palati of mammals, it seems obvious that no such homology can be defended. In abandoning this incorrect homology—as undoubtedly it must be abandoned—one curious piece of evidence which might be raised in its support must be noted. The human anatomist, in modern text-book descriptive anatomy, has added to the confusion by claiming that the human velum forms a ‘“‘septum between the mouth and the pharynx”. This is exactly the role that the crocodilian palatal flap plays in the specialized economy of the crocodiles, but which, in fact, the true mammalian velum does not subserve.
There can be no doubt that the palatal flap is a specialized development in the crocodiles and that it subserves a very obvious and essential function that is demanded by the peculiar and specialized mode of life of these animals. It cannot be considered as being in any way a reptilian expression of any phase of development of the true velum palati.
The Velum Palati in Birds
Of the avian palate it may be said in general that although a vast amount of detailed work has been done upon the arrangement of the bones constituting the palate, the descriptive accounts of the roof of the mouth and pharynx in its entirety are by no means so detailed. Following the lead of Huxley in 1867, systematic ornithologists became habituated to the study of the bones entering into the formation of the avian palate as a basis of the well-known scheme of classification: but no comparable study of the soft parts of the palate accompanied this anatomical activity. In the year preceding that in which Huxley outlined his scheme of avian classification by reference to the structure of the bones of the palate, Owen (1866) had ‘declared that “‘the posterior apertures of the nostrils being generally in the form of narrow fissures are undefended by a soft palate or uvula”’.
It must at once be admitted that if anything in any way resembling the human soft palate is looked for in the horny roof of the avian mouth, it is looked for in vain: but if by a velum palati we mean a continuity passing from the posterior end of the hard palate to form the roof of the pharynx and dorsal wall of the oesophagus, then it cannot be denied that such a structure exists in birds.
The opening of the nasal chambers into the roof of the mouth takes very different forms in the avian series and the details become further complicated by the varying relations taken by the Eustachian chamber: but in general it may be said that from the bony palate and around the nasal and Eustachian orifice, the roof of the mouth and pharynx—or palate—is continuous with the dorsal wall of the oesophagus, and this is the essential feature of the velum palati in its full development. In the Tubinares, in which the nasal chambers, the turbinates, and all the parts subserving the olfactory Sense are particularly well developed, an obvious velum exists. The orifice of the nasal chambers is ovoid in shape and it may, or may not, be continued forwards as a linear cleft in the mid-line of the hard palate. Around the orifice and extending backwards is a frill-like extension of the palate which passes back to the dorsal wall of the gullet. This velum shares in the somewhat horny character of the whole of the roof of the mouth, nevertheless it is without doubt the homologue of the soft palate such as is generally recognized in mammals. The velum is perforated in a typical manner by the orifice that leads into the nasal and Eustachian chambers this is the true hiatus nasopharyngeus or rima nasalis of Dionis. It is of interest to note that the elevated tubercle, upon which the laryngeal orifice opens, is reciprocally adapted to the concavity of the rima nasalis. It is impossible to disregard the basal similarity of this condition to that seen in primitive mammals. The likeness to the condition present in Ornithorhynchus is particularly striking. The palate of Prion turtur is illustrated here (Fig. 4): elsewhere (1937), I have described and figured the palates of other members of the Tubinares, Puffinus tenuirostris and Diomedea cauta, which in all essential details are quite similar. Despite the assertion, made by Owen in 1866, that no homologue of the soft palate exists in birds, there were several accounts in the literature in which no doubt was left by the writer that he accepted the avian structures as being akin to the soft palate of mammals. In particular the work of Samuel Collins should be mentioned. In his Systeme of Anatomy, etc. (1685) he gives excellent figures and descriptions of the palates of several birds. Especially noteworthy are his figures and descriptions of the palate of the swan (Table 4, Fig. 1) in which he says that “the termination of the palate is made in two oblong glandulous processes”, which extend behind the frilled edge that marks the posterior margin of the rima nasalis.
Fig. 4 Folate and dorsal wall of pharynx opened along its left margin margin and the tongue and larynx reflected to the right.
It may be said that in birds the bony palate, which is the floor of the nasal chambers and the roof of the mouth, is continued backwards by a velum palati so as to join the dorsal wall of the pharynx and oesophagus and that this continuous velum is pierced by a hiatus nasopharyngeus or rima nasalis, beneath which, on the floor of the mouth, behind the tongue, the opening of the larynx is seated. It must, therefore, be claimed that a true velum palati exists, in its whole continuity, in birds.
The Velum Palati in Mammals
In all primitive mammals, no matter if they be members of the Ornithodelphia, Didelphia or Monodelphia, the velum palati is in identical form. This primitive form is most readily appreciated by saying that the roof of the mouth is continuous with the roof of the pharynx and the dorsal wall of the oesophagus, which in primitive mammals are extended in nearly the same axis. This continuity between the roof of the mouth and the dorsal wall of the oesophagus is interrupted only by the aperture that permits the nasal chambers to communicate with the cavity of the pharynx. Beneath this aperture the orifice of the larynx projects behind the base of the tongue and through the aperture the orifice of the larynx may be thrust into and retained within the nasal chamber. In all primitive mammals the hiatus nasopharyngeus is surrounded by a sphincteric ring of muscle (m. palatopharyngeus) and it depends, almost certainly, upon the conditions that immediately preceded death whether the epiglottis and laryngeal orifice remain in the nasal chamber or are withdrawn from it.
The whole story of the old discussion of the intranarial epiglottis almost certainly depends upon the conditions prevailing at the time of death, for the thrusting up and withdrawal of the larynx through the velar aperture are active and vital muscular processes dictated by the needs of the moment. Just as a snake can herniate its laryngeal orifice from its mouth when engaged in swallowing something so large that it blocks up the whole of its oral cavity and pharynx, so can the primitive. mammal herniate its laryngeal orifice into its nasal chambers and thereby establish a direct airway from its nostrils to its lungs. This subject is referred to again in considering the functional significance of the intranarial epiglottis.
The condition of the velum palati and the nasopharyngeal aperture is illustrated here in Ornithorhynchus (Fig. 5) as an example of the Monotremes, in Dasycercus (Figs. 6, '7) and Dasyurus (Fig. 8) among the Marsupials: in Crocidura (Fig. 10) representative of the Insectivora, Pedetes (Fig. 11) as a Rodent, and in Lemur (Fig. 12). But exactly comparable conditions are present in all mammals with the exception of the higher Haplorine Primates (Pithecoidea). Among the Ornithodelphia, both Ornithorhynchus and Echidna are typical of the condition described and figured: and the outstanding features of the horse. certain deer, and the cetaceans, have been frequently referred to in the literature. It may be accepted that the typical mammalian velum is the anterior portion of the dorsal pharyngeal wall, separated from the dorsal wall of the rest of the pharynx and oesophagus, in the mid-line only, by the hiatus nasopharyngeus, but continuous with them upon either side of the hiatus.
Fig. 6. Palate, hiatus, larynx and tongue of Dasycercus.
Fig. 7. Section, slightly to the right of the mid-line, of the head of Dasycercus to show the normal position of the palate and larynx.
Fig. 8. Palate and dorsal wall of pharynx of Dasyurus. The pharynx has been opened along its right margin and the tongue and larynx reflected to the left.
The Velum Palati in Man
How is the classical velum palati as described and defined in human anatomy to be correlated with the condition present in all mammals more generalized in this regard than the Pithecoidea? It is at once obvious that the velum palati of human anatomy is only a part of a continuous structure separated, in arbitrary fashion in nomenclature and description, from its continuations by a misconception of its true nature. There is no such thing as a separate “soft palate”, with a free posterior margin in any mammal: but there is always a continuous dorsal oropharyngeal wall perforated by a nasopharyngeal aperture in every member of the mammalian phylum. The nasopharyngeal aperture is enlarged in man and the laryngeal orifice has ceased to occupy its primitive relation to it: nevertheless, the primitive dorsal oropharyngeal wall is present despite the undue size of the aperture that perforates it (Fig. 14). Quite obviously the posterior margin of the human “soft palate” marks the anterior boundary of the orifice; and equally obviously the palatopharyngeal folds or ‘* posterior pillars” (as Dionis, Winslow, Monro, Trew and certain other human anatomists have recognized) constitute its lateral free borders (Fig. 15). But it does not seem to have occurred, even to Gustav Passavant, that the ridge on the posterior pharyngeal wall, now named after him, constitutes its posterior margin. Perhaps to Samuel Collins (1685) belongs the credit for having first illustrated the complete circumference of the nasopharyngeal hiatus in man. The figures of the condition in the human foetus given in this paper should be compared with Zuckerkandl’s excellent figure of the same parts in the human adult (1904).
The Hiatus Nasopharyngeus
(a) Anterior margin
One well-known distinction of the human velum is its richness in glands that pour their secretions into the buccal cavity. In particular these glands are aggregated at the hinder end of the human soft palate, or at the anterior margin of the hiatus nasopharyngeus. When Owen observed that in birds the apertures of the nasal chambers. were ‘“‘undefended by a soft palate or uvula”, he overlooked the similarity in glandular structure between the velum of birds and mammals. Samuel Collins described in 1685 the “two oblong glandulous processes”’ in the palate of the swan and this is perhaps the first reference in the literature of the well-known glandulae sphenopterygoidae in birds. In all the mammals that I have examined the anterior margin of the hiatus is prominent and tumescent and usually this tumescence is continued forwards in the midline of the velum towards the hard palate. This is particularly conspicuous in Crocidura (Fig. 10) but in most forms it is evident in some degree. The swelling is also particularly well marked in the mid-line of the anterior margin of the hiatus itself. In order to ascertain the nature of this tumescence, the hiatus and its immediate surroundings were excised in a series of mammals (Homo (foetalis), Lemur, Crocidura, Pedetes, Dasyurus and Dasycercus). The excised portions were flattened between two slides, cleared with xylol and mounted in Canada balsam. Under the microscope the tumescence of the anterior margin of the hiatus was clearly revealed as being due to glandular masses in the thickness of the velum. These glandular masses conformed to a general type. In the more anterior part of the velum the main masses tended to be bilateral, but in the posterior part, at the anterior margin of the hiatus nasopharyngeus, they became confluent in the mid-line and extended some distance around the anterior margin, causing the elevation seen on the surface. It appears to be this mid-line aggregation of glandular tissue, which in some animals (see Fig. 11) causes a median thickening in the anterior margin of the hiatus, that ultimately culminates in the development of a uvula. The development of a definite uvula is a question of degree and it is by no means a simple matter to define exactly what degree of development warrants the assertion that an animal does or does not possess a uvula. In Pedetes the aggregation of glandular tissue in the mid-line produces a definite prominence or tubercle on the anterior margin of the hiatus nasopharyngeus. This median tubercle, which is present in many mammals, might not be generally accepted as being a declared uvula, but at least it appears to be the anatomical basis from which a definitive uvula is developed in certain forms. It might be suggested that the more completely the hiatus is occupied by the orifice of the larynx, the less would be the irregularity in the margin of the hiatus caused by the mid-line aggregation of glandular tissue. Conversely, we might expect that when the larynx is permanently withdrawn from the hiatus, this mid-line development of glandular tissue might culminate in the production of a pendulous prolongation thereby developing into what is generally recognized as a uvula. In general such an expectation is fulfilled in the mammalian series, for it is in the Pithecoid Primates alone that the uvula is generally accepted as being present.
Fig. 9. Palate and dorsal wall of pharynx of Dasyurus, prepared as described in text, to show glandular tissue and sphincteric fibres of m. palatopharyngeus.
Fig. 10. Palate and dorsal wall of pharynx of Crocidura. The tongue and larynx are reflected to the left.
(b) Lateral margins
In the cleared specimens prepared as described above, it was apparent that the prominent lateral margins of the hiatus were composed of mucous membrane folded around well-defined strands of muscular fibres which, arising from the bony palate, continued around the margins of the hiatus, to meet behind it and join the posterior (dorsal) wall of the pharynx. These fibres are undoubtedly those of the m. palatopharyngeus and their posterior merging into the dorsal pharyngeal wall is discussed in the next section. In such marsupials as Dasyurus (Fig. 9) and Dasycercus the meeting of the fibres behind the hiatus constitutes a definite median elevation on the dorsal wall of the pharynx. In most generalized mammals the muscles of the two lateral margins meet as a sphincteric ring within the encircling pharyngeal musculature.
(c) Posterior margin
In 1869 Passavant described a “‘cross-roll” on the posterior wall of the pharynx at the level of the soft palate. This ridge he considered as a normal feature, produced by efforts to close the hiatus nasopharyngeus in the ordinary phonation of all vocal sounds other than those having a nasal tone. He particularly stressed its high development and the ease with which it could be demonstrated in cases of cleft palate where the functional need for its presence was peculiarly necessary. As for its causation, he realized that it was produced by muscular fibres encircling the back of the pharynx, but he did not share Luschka’s opinion that the palatopharyngeus was of great importance in this regard and considered that it was ‘“‘an isolated function of the superior constrictor muscle”’.
Fig. 11. Palate and dorsal wall of pharynx of a neonatal specimen of Pedetes,
Fig. 12. Palate and dorsal wall of pharynx of Lemur.
Fig. 13. Section of the head of a baboon to show relation of the larynx to the hiatus nasopharyngeus, the posterior margin of which is “‘ Passavant’s bar”’.
Fig. 15. Palate and dorsal wall of pharynx of famines a human embryo of 100 mm. R.V. length.
Fig. 14. Palate and dorsal wall of pharynx of The dorsal wall of the larynx remains in a human embryo of 74 mm. R.V. length. situ.
It would appear that, with the exception of Jonnesco’s work, but little attention was paid to Passavant’s bar from an anatomical point of view until Wardhill revived the subject in 1928 and Whillis published his description of the musculature in 1930. In this last paper it is stated that ‘“‘the presence of the ridge led us (Wardhill and Whillis), in discussing the problem, to the conclusion that there must be some muscular mechanism producing it, and that the muscle fibres must run transversely. No mention was made in Passavant’s original paper of such fibres.” This last statement is a somewhat remarkable one. Whillis concluded that the muscle responsible for the production of the ridge was a definite slip of the superior constrictor muscle and herein, of course, he restated Passavant’s original conclusion. This muscular slip he named as the palatopharyngeal sphincter, a designation reminiscent of Luschka’s (1862) constrictor isthmi pharyngo-nasalis. The nature of these fibres can only be made clear by a review of the disposition of the palato-pharyngeus muscle of human anatomy. The confusion that has arisen with regard to the actual muscle that pulls the posterior wall of the pharynx forwards “‘at the level of the base of the uvula” is due to the very varying ideas that have been held by anatomists concerning the muscles that, attached to the palate, pass to other parts of the mouth and pharynx. In brief, there are three separate muscles that radiate to the tongue, the larynx and the posterior pharyngeal wall respectively. The fibres passing from the palate to the tongue—the m. palatoglossus—may be dismissed from the discussion since no great discrepancy in their description has been conspicuous in the literature. In generalized mammalian types there is, posterior to the m. palatoglossus, a separate muscle passing from the palate to the larynx. This is the m. thyrostaphylinus or m. thyropalatinus of Winslow, Cloquet, Jonnesco and other anatomists. In the interests of uniform nomenclature this muscle is here termed m. palatothyroideus. The third muscle, more or less confluent with the others at its attachment to the palate, passes around the hiatus nasopharyngeus and embraces the pharynx “at the level of the base of the uvula”’. This muscle is the m. pharyngostaphylini of Winslow, the m. pharyngostaphylinus plus the m. peristaphylopharyngeus of Cloquet. It was more definitely named by Luschka as m. pharyngopalatinus seu m. constrictor isthmi pharyngonasalis. For this muscle the best name is that of Albinus—m. palatopharyngeus. Unfortunately it has been the custom of most modern anatomists to combine the laryngeal and pharyngeal muscles under the name given by Albinus to the pharyngeal portion only. It matters not that this muscle is usually divided in modern descriptive anatomy into dorsal and ventral or medial and lateral parts, the correct conception of two separate morphological and functional elements is lost sight of by the employment of our present terminology. In the generalized mammalian type these three muscles, the one passing from the palate to the tongue, the second from the palate to the larynx and the third surrounding the hiatus nasopharyngeus, are not widely sundered even at their insertions, since the tongue, the larynx and the posterior pharyngeal wall are situated at nearly the same level. In man the parts have become readjusted to different levels. The m. palatoglossus remains in a more or less primitive condition and there is no problem concerning its identity in human or generalized mammalian anatomy. The m. palatothyroideus has undergone a marked change in man consequent on the sinking of the level of the larynx below the velum. The m. palatopharyngeus has, to a certain extent, become involved in this change, for its lower fibres are pulled down the posterior pharyngeal wall in the wake of the descending larynx. Nevertheless its upper fibres retain their primitive position and continue to carry out their original sphincteric function. This fact was recognized by many earlier anatomists, and John Innes (1804) especially notes that the fibres of the palatopharyngeus “run backwards to the top and lateral parts of the pharynx”. There is no need to discuss those fibres that passing above the level of the m. palatopharyngeus have been. described as a separate muscle—the m. salpingopharyngeus. By some anatomists they have been regarded as part of the m. palatopharyngeus, and by some as part of the superior constrictor of the pharynx. It matters little to which morphological plane they are assigned; but probably the clearest definition of their morphology is that of Macalister (1889). As a consequence of confusing the m. palatothyroideus and the m. palatopharyngeus anatomists have come to assume that the fibres of this latter muscle are disposed almost vertically in man. This being so there has been a tendency to assign to the stratum of the superior constrictor of the pharynx those upper fibres of the palatopharyngeus that encircle the pharynx in a nearly horizontal axis. It was these fibres that, in 1869, Passavant assigned to the superior constrictor and which were rightly held by him to be responsible for the production of the sphincteric action of the hiatus nasopharyngeus. In 1879 Sappey drew attention to these fibres and described them as a separate muscle—his m. occipitostaphylinus—but considered them to be the highest portion of the superior constrictor of the pharynx, although he noted their common origin with the palatopharyngeus. It is these same fibres that were redescribed by Whillis in 1980 as part of the superior constrictor and to which he gave the name palatopharyngeal sphincter. More recently (unpublished work, 19389) R. H. Townshend has re-examined the whole question and has determined that. Passavant’s bar, which he examined exhaustively in a patient with cleft palate, is undoubtedly due to this sheet of muscle and that there is no doubt whatever that the muscle is the upper part of the palatopharyngeus and is no part of the superior constrictor. Such a finding justifies Luschka’s name, constrictor isthmi pharyngo-nasalis, as a designation for the upper fibres of the palatopharyngeus. It also confirms Jonnesco’s account of the action of the palatopharyngeus, when he says: “‘Cette occlusion due, d’aprés Passavant, uniquement a Ja contraction des constricteurs supérieurs, est produite surtout par laction des muscles pharyngo-staphylinus, dont les constricteurs ne sont que les auxiliaires” (1912, p. 170). It is evident that the human palatopharyngeus has not departed widely either in morphological type or in function from the condition prevailing in the more generalized mammalian types.
The Functional Implications of the Primitive Mammalian Velum
Dr R. L. Bowles, in his paper “‘ Observations upon the mammalian pharynx, with especial reference to the epiglottis” (1889), was much concerned with the inconstancy of his findings and those of his assistant on the actual position of the larynx in the young pig. ‘“‘My assistant, Mr Stainer, has introduced his hand into the pharynx of many pigs of six months and upwards and has always found the velum to lie anteriorly to (below) the epiglottis and firmly clasping its root. This being so, I was not a little surprised to find that in one animal (said to be five months old) the epiglottis lay much more within the buccal cavity than I should have suspected.” This last finding was made by the dissection of the dead animal and not, like Stainer’s observations, by examination of the living. All the variations in position of the larynx with relation to the velum detected by Bowles were looked upon by him as due to age changes and to relative growth; and he regarded the position attained at any particular age as a definitive one. Howes, however, realized that age changes alone could not account for the varying conditions met with and he appreciated the fact that the variations found post-mortem might be due to displacement of the larynx by traction and manipulation during dissection. The fact that there was no sort of abnormality of the larynx or velum in the two rabbits in which he found the epiglottis intranarial in position on dissection justified such a supposition. But it is quite certain that the manipulation of the parts during dissection does not wholly account for the varying positions found, for the examination of several specimens of any species will reveal wide variations even when the utmost precautions are taken against disturbance of the parts. It is far more likely that in the act of death the larynx is commonly withdrawn from the nasal chambers in most animals having an intranarial epiglottis: it is only occasionally that an animal dies with its larynx in its nasal chambers and this event only happens with any frequency in those animals in which the larynx is particularly tightly held within the sphincteric ring of the hiatus nasopharyngeus. In Dasycercus I have found the larynx in the nasal chambers in the majority of specimens examined: but in Dasyurus and other marsupials it is commonly buccal in position in the dead animal. It is obvious and it is generally recognized that the intranarial position of the larynx is a primitive mammalian mechanism for securing an airway direct from the anterior nares, to the nasal chambers, larynx, trachea and lungs. From this direct air passage the buccal cavity and pharynx are altogether shut off. Food, passed from the mouth and via the pharynx to the oesophagus, is guided upon either side of the larynx through bilateral channels, the remnants of which are represented by the piriform fossae in man. Although the intranarial position of the glottis admits to perfection the functions of olfactation and respiration, it is apparently incompatible with vocalization. In such mammals as are capable of producing vocal sounds, the glottis must be withdrawn from the nasal chambers so as to take up a buccal position during vocalization. No doubt the relaxation of the sphincteric muscle of the hiatus nasopharyngeus, combined with the action of muscles passing to the larynx, is concerned in this movement of withdrawal: but other factors are certainly involved. The actual site of the hiatus nasopharyngeus varies rather widely in different species of primitive mammals, being in some considerably farther back than in others. But apparently its position depends entirely upon the build of the skull, for it would seem to be a general rule that it lies between the temporomandibular and the occipitoatlantal articulations. It is obvious, therefore, that any extension of the head on the occipito-atlantal articulation will tend to withdraw the glottis from the nasal chambers and the completion of the withdrawal will be effected by opening the mouth (see Fig. 18). It will be recalled that most mammals that are capable of producing vocal sounds have, as a preliminary to any vocal effort, a general movement of extending the head on the neck. This raising of the head appears to be regarded in a vague sort of way as an aid to the dissemination of the voice: but I imagine that it is a necessary preliminary in animals having an intranarial epiglottis in order to insure a buccal position for the larynx. The opening of the mouth completes the movement and the animal is free to produce its vocal sound. A dog may growl with a flexed head: but I doubt if it can bark or especially howl to much effect without some degree of extension of the head on the neck. It is noteworthy that the primitive marsupials, in which the larynx is most completely intranarial, are for the most part incapable of producing vocal sounds. It is also to be remembered that man, in whom the larynx is most completely and permanently extranarial, is the most continuously vocal of all mammals.
Note on the Work of Rückert
Rückert published his monograph Der Pharynx als Sprach- und Schluckapparat in 1882, and although it gives an exhaustive account of the palate of man and lower artimals, it appears to have been very generally overlooked by later writers. For the use of the copy that I have been able to consult I am indebted to the Zoological Library of the University of Bristol. Rückert was apparently unacquainted with the work of Passavant, for he describes the ridge to which Passavant called attention and names it indifferently as the “erista” or the “torus” of the dorsal pharyngeal wall. In describing the velum he mentions the “arcus”, which surrounds what is here termed the hiatus nasopharyngeus, and says “only the anterior part of this ring as far as it belongs to the velum palatinum contains muscular fibres, the posterior part consists only of two mucous membrane layers firmly connected with each other”. It is evident that he pictured his “crista” as being merely a mucous membrane ridge. Later on, however, in speaking of the crista of lower mammals he observes “‘ This connexion is not a casual one, as the fibres of the m. pharyngopalati continue to the posterior wall in order to unite in the middle line.”
Rückert was acquainted with the work of Luschka on the human velum: but it would seem that he did not realize that his “‘arcus pharyngo-palatinus”’, which in man clothes the m. palatothyroideus, had been separated (by the fall of the larynx) from the true arcus pharyngopalatinus (such as he describes in lower mammals) which contains the m. palatopharyngeus. Rückert failed to recognize the muscular nature of the crista pharyngea or pharyngeal torus that had been so well described by Passavant thirteen years previously : but all other characters of the true velum and the limits of the pharynx he described in masterly fashion.
Summary and Conclusions
- The velum palati is not confined to mammals, being present, in comparable form, in birds. The so-called velum of the Crocodilia, however, cannot be homologized with the mammalian velum.
- The primitive condition of the velum in mammals consists in a complete continuity from the roof of the mouth to the dorsal wall of the pharynx.
- This continuity is broken only by the presence of the hiatus nasopharyngeus by which the rima glottidis establishes continuity with the nasal chambers.
- It is violating the elementary principles of anatomical nomenclature to extend the term "pharynx" to any part of the nasal chambers: and therefore the term “pharyngeal isthmus” for the hiatus nasopharyngeus is misleading and should be deleted from nomenclature.
- The definition of the “soft palate” as “a movable fold, suspended from the posterior border of the hard palate, and forming an incomplete septum between the mouth and pharynx” (as in Gray, 1938, p. 1263, and most modern text-books of human anatomy) is absurd. The velum in no mammal separates the mouth from the pharynx: but it always separates the pharynx from the nasal chambers.
- The m. palatopharyngeus of modern text-books of human anatomy is composed of two separate muscles, confused in modern terminology — the m. palatothyroideus and the true m. palatopharyngeus.
- The true m. palatopharyngeus is the muscle that has been mistaken by many human anatomists for the upper fibres of the superior constrictor of the pharynx.
- The true m. palatopharyngeus is the sphincter of the hiatus nasopharyngeus and is responsible, in man, for what is known "Passavant’s bar".
- The anterior margin of the hiatus nasopharyngeus is glandular in all mammals. In mammals in which the rima glottidis is permanently, or even frequently, withdrawn from the hiatus this glandular tissue is aggregated in a median tubercle culminating in the development of the uvula.
- The position of the rima glottidis in relation to the velum is commonly altered by the conditions prevailing at death.
- In such animals as are capable of phonation and have the rima glottidis habitually in the nasal chambers, it must be withdrawn (by extension of the head, opening of the mouth or by other means as auxiliaries) during phonation.
- The condition of the Pithecoidea, and especially in Homo, is characterized by an enlargement of the hiatus nasopharyngeus and, especially in Homo, by the permanent withdrawal of the rima glottidis from the nasal chambers. In Homo this change in structural relations is definitely related to the constant vocalization characteristic of man.
For the three examples of the Crocodilia and for the specimens of Lemur and Pedetes that I have dissected, I am indebted to the Zoological Society of London. For specimens of Crocidura, I am indebted to Prof. Osman Hill.
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