On harelip and cleft palate

Before discussing from an embryological standpoint the various deformities which we have already described, it is essential for us to consider the normal process of development of the parts entering into their formation.

About the end of the third week of intra-uterine life, the anterior cerebral vesicle becomes acutely bent over the end of the notochord, and a marked depression is seen on the ventral aspect of this, constituting what is known as the Stomodæum, or primary buccal cavity. This, however, is formed rather by the outgrowth of surrounding processes entering into the formation of the facial elements than by any definite or distinct involution of epiblast. The cavity is bounded posteriorly and superiorly by the cephalic flexure of the cerebral vesicles, and inferiorly is separated from the cephalic portion of the intestine by a septum. This becomes perforated at a slightly later date (eighth or ninth week), and communication is thus established between the stomodæum and intestine. Anteriorly, the opening is at first stellate in shape (Fig. 29), but soon assumes the form of a transverse cleft by the union in the middle line of the first pair of post-oral branchial arches, in which are developed the primary cartilaginous bars on either side, known as Meckel’s cartilage, the anterior part of which goes to form the inferior maxilla, and the posterior part the malleus. The soft parts around develop into the lower portion of the cheek, the lower lip and chin (Fig. 30).

At the same time that this mandibular arch is being developed, other changes are occurring around the upper part of the stomodæum, viz. the shutting off of sacs lined with epiblast to assist in the formation of the organs of special sense, and the outgrowth between them of fleshy processes which by their later amalgamation form the facial skeleton and coverings. Three of these involutions of epiblast occur, two communicating more or less with the stomodæum, viz. the nasal and ocular; whilst the third, or auditory, is separate. Expansions from the cerebral vesicles meet them, and by further changes, unnecessary to particularize here, the organs of special sense are elaborated.

The most anterior of these depressions are the primary Olfactory pits which appear on either side on the lower surface of the wall of the anterior cerebral vesicle at a very early date. They are at first merely depressions surrounded by a raised margin; but subsequently they become pyriform by the extension of the lower end as a groove into the stomodæum. Each pit and groove is bounded laterally by thickened rounded outgrowths developed from a broad median fleshy protrusion from the same part of the cerebral vesicle, the fronto-nasal process; these are termed respectively the external and internal nasal processes, the latter being also named by His the globular processes (Fig. 30).

These Globular processes are separated in the median line by a groove which is subsequently obliterated by their amalgamation to form the central portion of the upper lip (“Philtrum” of German authors), and from their deep aspect the inner segments of the intermaxilla (endognathia). Above this groove is a central flattened median portion of the fronto-nasal process, from which subsequently the prominence of the nose is developed (Figs. 31A, 32, and 33), a result of the continued growth from its deeper aspect of the ethmo-vomerine plate.

The external nasal process forms the ala nasi and the soft parts in its immediate neighbourhood, extending downwards a little distance, but probably not so far as the red margin of the lip. From its deeper aspect the outer portion of the intermaxilla (mesognathion) is developed, and in it appears the germ of the lateral incisor. It also serves to separate the nasal pit from the second epiblastic intrusion which assists in the formation of the eye. The lower boundary of the naso-orbital fissure which passes from the primary ocular involution to the stomodæum is formed by the maxillary process, which is usually described as an upward extension of the mandibular process, but which probably arises separately as a pre-oral branchial outgrowth. From this is developed the whole of the superior maxilla, except that portion which is intermaxillary, and also the greater part of the cheek.

About the sixth week the stomodæum shows signs of division into upper and lower segments by the outgrowth from the deep aspect of each maxillary process of horizontal palatal plates, which by their junction in the median line form the rudiments of the hard and soft palate, separating thus the nasal and buccal cavities. The anterior portion of these unite with the lateral aspects of the deeper parts of the fronto-nasal process, leaving a space of greater or less dimensions in the median line, known as the anterior palatine canal, which serves subsequently for the passage of nerves and vessels, and for the lodgment of the “organ of Jacobson” in animals in which it occurs.

The upper or nasal cavity is again subdivided into lateral halves by the growth downwards from the under surface of the fronto-nasal process of a central vertical septum, to become in time the cartilaginous septum nasi and the bony ethmo-vomerine plate, uniting at its lower border with the primary fleshy palatine processes (Fig. 34).

The tongue grows as a fleshy protuberance from the floor of the stomodæum; antero-lateral segments on either side from the conjoined second and third branchial arches unite with a central posterior growth from the tuberculum impar in an inverted 𝖸-shaped manner. At the point of junction of the segments is a depression from which the thyroid gland develops, indicated in later life by the foramen cæcum.

Each of these primary epiblastic pits is at a later date almost entirely cut off from its connection with the buccal cavity. The external and internal nasal processes of the fronto-nasal outgrowth unite below the anterior olfactory pits, and thus surround the anterior nasal apertures, and separate the nares from the mouth. The external nasal and superior maxillary processes are also freely amalgamated except along one small deep track, which remains patent to form the nasal duct and lachrymal passages; and probably the internal nasal and maxillary processes unite below the external nasal process to establish the continuity of the red margin of the upper lip. The union of all these various parts has been completed by the sixth to the tenth week of normal fœtal life; the external nasal and superior maxillary processes unite first, and by the sixth week are becoming closely approximated to the central portion of the fronto-nasal process, a time when the palatal processes are only indicated as slight ridges. By the ninth week the alveolus and upper lip are complete, and union of the palate is commencing from before backwards, being usually completed even to an indication of the uvula by the tenth week.

Whilst the later stages of these developmental processes are in progress, points of ossification have been appearing in many places to form the cranial and facial skeleton. A full knowledge of this subject is still unattained, but the researches of Goodsir, His, and others have thrown much light on hitherto dark passages. There are two main sources of origin of the bones of the skull, viz. from cartilage and from membrane, and it is important to appreciate the portions of the skull which originate from each of these sources respectively. The bones laid down primarily in cartilage are mainly those forming the base of the skull and their anterior prolongations. Thus about the fourth week of intra-uterine life the basis cranii consists of a cartilaginous mass surrounding the upper end of the notochord, and prolonged anteriorly around the pituitary fossa as two cartilaginous bars, the trabeculæ cranii, into the fronto-nasal process. From the anterior extremity of this the nasal bones and cartilages are developed, and from the under surface the ethmo-vomerine plate. The transformation of the primordial mesoblastic undifferentiated tissue into recognisable cartilage is occurring from about the fourth or fifth week until the eighth, when ossification at different centres is apparent. To the development of the intermaxilla we shall refer in detail later; suffice it to say here that the ossifying centres appear about the eighth week, and by the twelfth to the fourteenth the whole process is ossified, and the space between the maxillæ closed except posteriorly, where the anterior palatine canal remains permanently patent; the component parts of the bone, however, are not united until a later date. There are two other cartilaginous foci from which ossification ensues, viz. the pterygo-palatine cartilage in the superior maxillary process, a delicate bar from which arise in part the pterygoid and palatine plates; and Meckel’s cartilage in the mandibular process for the production of the mandible and malleus. All the other facial bones are developed from membrane, more or less in connection with these bars. The vomer is ossified from a single nucleus appearing in the upper part of the ethmo-vomerine plate, about the ninth week; from this two laminæ are developed, which, passing downwards and forwards on either side of the middle line, embrace the septal cartilage. The amount of the osseous material increases from behind forwards, until at maturity a median osseous lamina remains which is grooved only anteriorly.

The palate bone develops from a single centre appearing about the eighth week at the junction of the horizontal and perpendicular portions. The superior maxilla is supposed to arise from four separate foci of ossification, viz. for the alveolar arch, for the palate, for the orbito-malar portion, and for the naso-facial segment. All these are united together by the third month.

It is unnecessary here to discuss the development of the teeth beyond stating that the thickening of the epiblast covering the gums, which occurs as the earliest sign of the production of the milk teeth, is to be seen about the forty-fifth day, when as yet there are no signs of ossification of the maxilla, and by two and a half months a distinct involution filled with cells is evident. Calcification commences about the eighteenth week of intra-uterine life, and extending from crown to fang is usually not completed until from twelve to twenty months after birth.

By the fifteenth week of embryonic life preparation is being made for the development of the first four permanent molars, and soon afterwards in the sixteenth week occur the inflections of the mucous membrane giving rise to the enamel organs for the twenty anterior permanent teeth; and from this period until the birth of the infant the germs of the twenty-four permanent teeth are passing through the various stages preparatory to calcification, so that at birth the child has not only twenty milk teeth with calcification nearly complete, but also the germs of twenty-four permanent teeth. Calcification commences in twelve of these latter during the first year of life, viz. in the first molars and the incisors, and spreads from the crown in which it starts to the fang. In the case of the incisors this process is not completed until the tenth year.[43]

The question as to the ossification of the intermaxilla has been purposely omitted hitherto, that the subject and its morphological relationship to congenital deformities might be more fully discussed.

In the time of Galen[44] the presence of the intermaxilla as a separate bone had been demonstrated in apes and lower animals, and its existence in man inferred, although probably not actually seen. This opinion held its ground till the sixteenth century, when Vesalius attacked it, maintaining that no such bone existed in man, and its absence was even claimed as a distinguishing feature from the lower animals. The first to actually discover and notify the separate existence of the bone in the human skull was Dr. Robert Nesbitt,[45] who, in a lecture before the Royal College of Surgeons in 1736, described and figured the suture crossing the anterior part of the palate at all times of life, and maintained that during intra-uterine life each superior maxilla “is generally divided into two distinct parts, the sutural line running from between the dentes canini and incisivi up to the bottom of the nose.” But the merit of appreciating the importance of this fact belongs to Goethe[46] and Vicq d’Azyr,[47] the former of whom, in the year 1779, demonstrated the existence of the intermaxilla in the human fœtus, and, as the outcome of this discovery, promulgated in 1819 the theory that in alveolar harelip the cleft in the alveolus occurs at the maxillo-intermaxillary suture, i. e. between the lateral incisor tooth and the canine. This opinion has been believed and handed down through successive generations of surgeons until the present day, but more recent and exact research has so increased our knowledge that it cannot now be considered correct. Many painstaking embryologists have, during the last ten years, been investigating this subject; but the honour of raising the question as to the morphological position of the cleft in harelip lies with Professor Albrecht of Brussels, who in a masterly series of papers has fully established the fact, observed also by others, that the intermaxilla is not developed en masse, but is formed by the coalescence of four segments, two on either side; and he maintains that the cleft in alveolar harelip lies not between the maxilla and intermaxilla, but between the inner and outer intermaxillary segments. These have been named respectively the endo- and meso-gnathion, whilst the maxilla proper is called the exognathion. According to Albrecht, therefore, the cleft is not situated along the exo-mesognathic, but along the endo-mesognathic suture.

Much controversy has been lighted up by this pronouncement, but here only a few of the points of interest and importance will be discussed.

The development of the intermaxilla from two centres on each side may be accepted as a proven fact. It was first maintained by the late Mr. Callender,[48] who stated that these bones have a lateral wedge-shaped sutural surface, fitting into a groove in either superior maxilla, and that the alveolar processes of the latter extend forwards, forming the anterior walls of the sockets of the central incisors, and so fix the bones in position. A confirmation of the idea that the anterior alveolar walls of the incisor teeth are formed in this way was sought in the well-established fact that these particular parts are very imperfectly developed in those cases of alveolar harelip in which the os incisivum is isolated from the superior maxillæ; but such is probably due to the abnormal condition and position in which the bone is developed, rather than to the loss of the maxillary “clip.” And certainly the most recent researches tend to prove that the maxillæ have no share in the formation of the alveoli of the incisor teeth.

Sir William Turner and other anatomists have fully confirmed this method of development from four ossific centres, and important additions to our knowledge of the subject have been made recently. Thus M. Gilis[49] describes and figures the condition of the palate as seen in a six-months fœtus, where it was clearly demonstrated that the intermaxillary portion formed a sort of lozenge-shaped prism fitting in between the two maxillæ, and consisting of four portions of bone, the sutures between these being perfectly clear and distinct (Fig. 35). The posterior extremity of the short axis of the intermaxillary segment corresponds to the anterior palatine canal, and the anterior surface forms the median anterior alveolar border, no process of the superior maxilla closing in the alveoli in front. The upper border of the bone forms the floor of the nasal apertures.

Biondi, of Breslau,[51] has completed the observations necessary for the establishment of this fact by demonstrating the four actual points of ossification in many fœtal skulls of different dates, which had been specially prepared for the purpose. Moreover, as mentioned before (p. 27), traces of the five intermaxillary sutures, when looked for, may be found in many adult and in the majority of young skulls.

The fourfold division of the intermaxilla being granted, it is obvious that a cleft through the alveolus such as that occurring in alveolar harelip happens at one of the two following situations, either along the endo-mesognathic, or through the exo-mesognathic suture, i. e. between the component elements of the intermaxilla, as Albrecht declares, or between the maxilla and intermaxilla, as Goethe suggested—a claim which has been vigorously defended by Kölliker against its newer rival.

The relative position of the clefts is indicated in the diagrams appended (Figs. 36, 37, and 38).

Albrecht’s papers on the subject are numerous, and contain a large amount of interesting material which space forbids us to introduce here; and one must refer readers, desirous of knowing more, to the appended list of his chief works.[52] But the arguments in favour of his theory may be briefly stated to be drawn from the following facts:

1. That in cases of alveolar harelip, a small portion of bone has been found in many instances on the outer side of the cleft, quite distinct from the maxilla. This is claimed to be the mesognathion, separated by the cleft from the endognathion, and by a distinct suture from the exognathion. A picture (Fig. 39) is appended of an adult skull taken from the museum of the Royal Anatomical Institute of Kiel, which clearly shows a small distinct flake of bone in the required position, extending back as far as the ordinary site of the anterior palatine canal. Such has been also found in children’s skulls, and very distinctly in a series of horses’ skulls with alveolar harelip. (For figures of such, v. ‘Langenbeck’s Archiv,’ xxxi, 2.) But this condition, indicating the distinct entity of the mesognathion, is not very commonly to be demonstrated in human pathology, inasmuch as the suture is early obliterated. But its existence is indicated by the dentition, to which we must now turn.

2. That in alveolar harelip there is in a large number of cases a distinct precanine or incisor on the outer side of the cleft. This is well shown in Figs. 39 and 40. In the former, an adult skull, the mesognathion is distinctly seen bearing the alveolus of a precanine tooth, the lateral incisor; whilst the central bony portion (the endognathion) bears but the socket for one tooth, the central incisor. The latter is a picture of a child’s skull with double alveolar harelip and cleft palate; the os incisivum is seen separate and projecting forwards at the end of the nasal septum; it bears the sockets of the two central incisors. Outside the cleft the alveolus bears four teeth on either side, viz. two molars, one canine, and one precanine, which we cannot but recognise as the normal lateral incisor. So that the dental formula of the upper jaw might be represented thus:

In normal jaw—CI₂I₁I₁I₂C;

in double alveolar harelip—

CI₂=I₁I₁=I₂C,

where C represents the canine, I₁ and I₂ the central and lateral incisors, and the double lines indicate the position of the clefts. Careful examination of a considerable number of skulls has brought much confirmatory evidence to light, indicating the truth of the above proposition. Thus, to pick out a few facts from the mass of material available:—Sabourand[53] records two cases of unilateral harelip with cleft palate, one of which died at the age of thirty-three days. In each the dentition was typically that described by Albrecht, viz. four teeth on the side of the cleft (two molar, one canine, and one precanine), and six on the opposite (two median incisors, one lateral incisor, one canine, and two molars). Broca[54] has reported a case of a much deformed fœtus stillborn at seven months. In this there was cleft palate and double harelip with the os incisivum freely moveable, but not displaced. The bone was found to consist of two little masses, mobile on each other, and each containing two incisor germs; and on each side external to the cleft there was a precanine similar in shape to the incisors. The middle one of these three incisors was distinctly the least developed.

Again, Sir William Turner[55] has carefully investigated the dentition, as seen in casts obtained from various hospitals, of fifteen specimens of alveolar harelip, eight of which were single left-sided, four single right-sided, and three double clefts. To these he adds the records of forty-nine preparations examined and reported on by Kölliker;[56] we can here, therefore, discuss the dentition of sixty-four cases. They may be divided into two groups:

(α) In which no precanine intervened between the cleft and the canine—thirteen cases.

(β) In which a precanine existed between the cleft and the canine—fifty-one cases.

In not a few instances the os incisivum contained four teeth, and yet a precanine existed external to the cleft, i. e. in hexaprodontous jaws the cleft passed between the middle and outer precanines.

A similar condition is described by Albrecht[57] as occurring in an adult human skull in the museum of the University of Kiel. In this a cleft palate exists, with the fissure extending through the alveolus of the right side, i. e. a right-sided alveolar harelip with cleft palate. The mesognathion is plainly seen on the outer side of the cleft, bearing an incisor tooth. On the inner side of the cleft (i. e. on the left side) are the alveoli for five incisors before reaching the canine socket of the left side, so that here is a skull with six incisor teeth, and with a cleft between the alveoli of the right middle and outer precanines. And not a few similar preparations are indicated by Biondi[58] as occurring in the Berlin and Breslau collections. The condition of parts is represented diagrammatically in Fig. 38. Albrecht’s explanation is that the middle of the three precanines, i. e. the outer tooth in the os incisivum, is an accessory development; whilst that on the outer side of the cleft is the normal lateral incisor springing from the mesognathion.

With such facts one necessarily collates the accredited teaching respecting the number and character of the incisor teeth in man.

Normally one finds two incisors on each side, occupying the space between the canines, but it is a fact perfectly well recognised by dentists that occasionally an extra precanine or incisor is present (Fig. 27); and very rarely are there more than three incisors on either side. My colleague, Professor Underwood, tells me that once he has seen the cast of a jaw with at least five precanine teeth on one side only, but that was an absolute exception, and only to be explained as a vagary of nature. The more common existence of three incisors can scarcely be placed in the same category, especially when one considers that although not constant by any means throughout the series, yet amongst the mammalia one does find three incisors as an oft-repeated formula; and certainly the typical mammalian dental formula would indicate the occurrence of three incisors on each side. Hence it is possible that the occasional occurrence of three incisors in man is an illustration of the so-called “recurrence to type,” and that, under ordinary circumstances, one incisor has been suppressed; and the majority of anatomists fully concur in the belief that it is the middle one of the three which has disappeared. The occasional failure of the wisdom teeth to erupt, an occurrence which dentists tell us is increasing in frequency, is additional evidence as to the possibility of the disappearance of an incisor.

The accessory tooth in the os incisivum met with in some cases of alveolar harelip is maintained by Albrecht to be a reappearance of this lost middle incisor; and his explanation of such an occurrence seems very feasible, viz. that the existence of the alveolar cleft prevents the naso-palatine artery from anastomosing with the posterior palatine, and thus the vascular supply to the os incisivum is greater than it should be under normal circumstances; hence, there being a superabundance of nutrient material, nature uses such in the restoration of a structural unit ordinarily suppressed. The same fact (viz. the absence of the usual anastomosis) may explain why the mesognathion is (even when demonstrably present) always small and the lateral incisor not infrequently stunted or absent, and so answers the objection to this theory which has been raised on the ground that in cases of alveolar harelip an incisor external to the cleft is not invariably present.

Hence the dental formula in cases of alveolar harelip will vary as follows:

In tetraprodontous jaws—

CI₂=I₁I₁=I₂C,

or

C=I₁I₁=C;

in hexaprodontous jaws—

CI₂=IₐI₁I₁Iₐ=I₂C,

or

C=IₐI₁I₁Iₐ=C,

where Iₐ represents the accessory incisor, the other letters as on p. 51. The former of each of these pairs of formulæ represent the mesognathion and lateral incisor as present, the latter as absent.[59]

Notwithstanding the mass of positive evidence which is steadily accumulating in favour of Albrecht’s theory, there are still some careful observers who will not admit its truth. Some attack it on the ground that the intermaxilla does not consist of four portions (Kölliker, &c.); with such we have already dealt. Others object to it on the ground that any teeth existing external to the cleft are merely accessory teeth, or due to a bifidity of the canine. The arguments with which they support their opinion are derived from the following considerations:

(i) That in normal development accessory teeth do certainly occur, as in the case mentioned above (p. 53), whilst in a few instances of abnormality the same condition has been noted (e. g. a few cases recorded by Kölliker;[60] see also Fig. 19, showing a facial cleft in which several accessory teeth are present along the inner margin). And the explanation given of such facts is that the involution of mucous membrane from which the teeth are developed is continuous along the alveolar ridge, and not localised to the definite spots from which the teeth subsequently erupt. This, however, is merely an opinion still sub judice, and not absolutely proven.

(ii) That in other defects accessory structures are sometimes produced in the neighbourhood, as if Nature, being baulked in her efforts of development at one spot, expends her energies in some less useful addition elsewhere. The accessory auricles and auricular appendages seen in macrostoma are cited as illustrations of this; and any precanine external to the cleft is maintained to be of a similar character. Whilst fully admitting the plausibility of such teaching, I cannot see that it explains such osseous development as occurs in Albrecht’s Kiel skulls, or such a regular appearance of an incisor external to the cleft as that indicated by the figures quoted above. The less frequent existence of an accessory tooth in the os incisivum seems much more readily explicable on such a ground.

To summarize the principal points as to the development of ordinary harelip:

1. That the intermaxilla is derived from the union of four ossific portions, two on either side of the median line, and that these are to be known respectively as the endo- and meso-gnathion, whilst the superior maxilla is termed the exognathion.

2. That these ossific portions are developed from the internal and external nasal projections of the fronto-nasal process respectively, and that ordinarily the central and lateral incisors are developed one from each segment. Occasionally an accessory incisor is developed between the other two from the endognathion.

3. That the external nasal process does not enter into the formation of the upper lip, but terminates superficially in the depression immediately below the ala nasi (see p. 57).

4. That simple harelip, where the cleft is limited to the soft parts, is due to the non-union of the superficial portions entering into the formation of the lip.

5. That in alveolar harelip the cleft passes between the endo- and meso-gnathion.

6. That the os incisivum consists merely of the two united endognathia, and normally carries only the two central incisors. Any additional tooth is not the normal lateral incisor, but an accessory one, probably due to the reappearance of an old suppressed member.

7. That any precanine existing on the outer side of the cleft is the normal lateral incisor, springing from the mesognathion; but that the latter portion of bone is rarely seen as a separate entity in human skulls possessing such deformity, from early obliteration of the suture between it and the maxilla, and that it is often badly developed and the lateral incisor stunted or undeveloped from defective vascular supply.

Having entered thus fully upon the question as to the situation of the cleft in alveolar harelip, we must now turn to the consideration of the morphological position of the oblique facial clefts, and their relations to the above.

It has been already pointed out that they commence at about the same spot in the lip margin as ordinary harelip, and thence run upwards and outwards clear of the ala nasi towards the centre of lower eyelid, following along the line of the naso-orbital fissure. Such a condition, coupled with the developmental facts already mentioned, viz. the existence of four segments in the intermaxilla and their relations to the internal and external nasal processes, suggests the following conclusions:

1. That the situation of the cleft in the lip margin is in all probability between the internal nasal and maxillary processes. The truth of this proposition depends on whether or not the external nasal process has any share in the formation of the lip. His, in his diagrams (Fig. 30), seems to indicate that it does not, whilst Biondi[61] claims that it does. The fact that these facial clefts commence at about the same spot in the lip margin as do the clefts in harelip seems distinctly to point to the conclusion that the superficial portion of the external nasal process is limited to the development of the ala nasi, and of the soft parts immediately around it. And this opinion goes far to explain the dimpling of the skin around and the consequent definition of the ala nasi.

2. That the situation of the cleft in the alveolus is between the meso- and exo-gnathion, so that the first tooth on the outer side, if developed, will be the canine.

3. That the upper extremity of the cleft should typically be located at either the inner canthus or about the middle of the lower eyelid; but this is not always the case. The association with coloboma iridis is readily explained by an imperfect closure of the choroidal cleft which normally occurs at the lower and inner segment of the globe.

Median harelip in its two forms is readily explained. The true median cleft is due to the non-union of the two globular processes developed from the median portion of the fronto-nasal outgrowth. If prolonged between the bones, it occupies the position of the median inter-intermaxillary suture, and so passes between the two endognathia.

The more serious and complete form is due to the non-development of the globular processes, and hence absence of the central portion of the lip, the endognathia, and of the ethmo-vomerine plate. The contour and size of the alæ nasi in pictures of this deformity suggest strongly that the external nasal processes are developed, but no record of the dentition of these rare cases is to be found.

Macrostoma is due to the non-union of the maxillary and mandibular processes, or possibly in some instances to the imperfect development of the former.

Mandibular clefts are due to the non-union of the separately developed lateral segments of the mandibular process—a deformity which must result from an earlier interference with the normal conditions of development than any of the others. Prof. Wölfler has pointed out that at the period when the branchial arches are being formed, the aortic bulb lies between their ventral extremities, reaching up even as high as the mandibular processes. If from any cause the retrogression of the heart and aortic bulb into the thorax is interfered with, then non-union of the visceral arches may result, and even a cleft mandible may thus be caused. This ingenious theory is stated only to apply to the more severe cases.[62]

Cleft palate is due to non-union of the palatal outgrowths of the maxillary processes. When the cleft extends beyond the anterior palatine canal it may pass along any of the intermaxillary sutures, but usually between the endo- and meso-gnathion on one or both sides. Inasmuch as the palate closes normally from before backwards, it is most common to find the deficiencies at the posterior rather than at the anterior end.