DEVELOPMENT.[42]
_Normal development of mouth, face, nose, and
teeth—Ossification—Development of intermaxilla; old ideas
(Goethe’s, &c.); Albrecht’s theory—Harelip; position of cleft
in alveolus, and in lip—Dentition; accessory teeth—Development
of other deformities._
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).
[Illustration: FIG. 29.—Head of fœtus, of about 5 weeks, from ventral
aspect (after His), showing the primitive stomodæum bounded above by the
undivided fronto-nasal process, laterally by the maxillary, and below by
the still separate mandibular processes. The quinqueradiate appearance is
well represented. (_Sutton._)]
[Illustration: FIG. 30.—Head of fœtus from ventral aspect of a little
later date (6-7 weeks). The mandibular processes have now united; the
orbito-nasal fissure has come in contact with the ocular vesicle, and the
fronto-nasal process has developed into external and internal nasal (or
globular) processes around the nasal fossæ (after His). (_Sutton._)]
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.
[Illustration: FIG. 31A.—Head of fœtus at a somewhat later date (8 weeks)
as seen from the front, showing the nasal and maxillary processes in
close apposition, and the clefts between them diminishing in size.
FIG. 31B.—The same, seen from below, the mandibular process having been
removed.
_i.m._ Central portion of fronto-nasal process, _m.n.pr._ Internal nasal
process. _pr.gl._ Globular process, or the lower rounded extremity of
the former. _l.n.pr._ External nasal process. _m.x._ Maxillary process.
_m.n._ Mandibular process.]
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.
[Illustration: FIGS. 32 AND 33.—Later stages of development of fœtal
head.]
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.
[Illustration: FIG. 34.—Diagrammatic representation of the development of
the palatal processes and of the ethmo-vomerine plate, seen in vertical
section. (After _Gegenbaur_.)
_a._ Ethmo-vomerine plate. _b._ Palate processes. _c._ Tongue. _d._
Buccal cavity. _e._ Nasal cavity.]
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.
[Illustration: FIG. 35.—Bony palate of a fœtus of six months, showing the
development of the intermaxilla in four portions.
_a._ Endognathion. _b._ Mesognathion. _c._ Exognathion. _d._ Anterior
palatine canal. _e._ Endo-mesognathic suture.[50] _f._ Exo-mesognathic
suture. (After _Gilis_.)]
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).
[Illustration: FIG. 36.—Diagram representing the old or “Goethe” theory
of alveolar harelip, indicating both central and lateral incisors as
developed from one intermaxilla on either side, and the alveolar cleft
between the maxilla and intermaxilla.
M. Maxilla. IM. Intermaxilla. _i₁._ Central incisor. _i₂._ Lateral
incisor. _c._ Canine. _m₁._ First molar. _m₂._ Second molar.]
[Illustration: FIGS. 37 AND 38.—Diagrams to represent the “Albrecht”
theory of harelip, in conditions where the alveolus contained four or six
incisors respectively.
EG. Endognathion. MG. Mesognathion. XG. Exognathion. _i₁._ Central
incisor. _iₐ._ Accessory incisor. _i₂._ Lateral incisor. _c._ Canine.
_m₁, m₂._ 1st and 2nd molars.]
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:
[Illustration: FIG. 39.—Drawing of an adult upper jaw with right-sided
alveolar harelip and cleft palate, and the vomer attached to the left
palatal margin. A rudimentary mesognathion bearing the stunted lateral
incisor is shown on the outer side of the cleft.
_i₁, i₂._ Central and lateral incisors, _x._ Inter-endognathic suture.
_y._ Exo-mesognathic suture. (After _Albrecht_.)]
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.
[Illustration: FIG. 40.—Drawing of a case of double alveolar and palatine
cleft with projecting os incisivum, to show the lateral incisor on outer
side of the cleft.
_i₁_, _i₂_, _c_, _m₁_, _m_₂, represent the alveoli of the teeth as in
Figs. 26 and 27. _v._ Vomer. B. Palatal process of superior maxilla
united to the meso- and exo-gnathion. A. Os incisivum, consisting of the
two endognathia, and bearing the alveoli of the central incisors. (After
_Albrecht_.)]
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.