THE DEGENERATE TEETH AND JAWS
Next to the ears, the jaws and teeth (as was to be expected from the
variability of these organs in allied animals) are most affected by
degeneracy. This is particularly true of the vertebrates, especially
mammals, as might have been anticipated from their phylogeny. At the head
of the vertebrates is man; at the foot is the lancelet (amphioxus), which
is perhaps most akin to those semi-vertebrates the ascidians, who, in
their larval phase, are higher than when adult, and whose life-history
excellently illustrates that potent phase of evolution, degeneracy.
The lancelet has a spinal cord enclosed in a half-gristly canal (the
notochord). It is practically destitute of a brain. The cerebral vesicle
which represents this is a plain cavity without true subdivision into
ventricles. There is no cranium. The eye (central in position) is a mere
pigment spot with which it is able to distinguish light from darkness. The
nose (behind this) is a small pit, lined with cilia, for purposes of
smell. Into this the cerebral vesicle of the larval lancelet opens. The
mouth is well guarded against the intrusion of noxious substances which
have to pass through a vestibule richly provided with sensitive cells,
resembling the taste buds of the human mouth. There is no heart. In this,
as in the case of the eye, the lancelet is lower than the ascidians, the
insects, crustaceans, and many molluscs. It approximates those worms
which, despite a very elaborate vascular system, are destitute of a heart,
the function of which is performed by contractile blood-vessels. From an
embryologic and morphologic standpoint the proximate ancestor of the
vertebrates may have been a free swimming animal, intermediate between an
ascidian tadpole and the lancelet, and the primordial ancestor, a
worm-like animal organised on a level with the star-fish. The vertebrates
embryologically develop from this stage to the lampreys; thence to the
cartilaginous fish (shark); to the amphibia (frog, toad, axolotl); to the
reptiles; and thence to the oviparous mammals (duck-bill and echidna or
spiny ant-eater); to the lemurs, and through forms like the
_Pithecanthropus erectus_ to man. Mammal teeth pass, in evolution, from
the simple types found in that oviparous edentate, the spiny ant-eater of
Australia, to those of the indeciduous ancestors of the sloths and
armadilloes, and their descendants, inclusive of the dolphins and whales,
whose teeth, both in the fetal Greenland and adult sperm whale, preserve
this old type. (The whales have degenerated from the hoofed mammals to
suit their environment.) While, as in the edentates, these teeth may be
few, they may also, as in the insectivorous marsupials, approximate those
of the reptilia in number (sixty or seventy on a side) and characteristic
location.
The evolution of this primitive tooth to the bicuspid and molar type has
been explained by two theories: that of concrescence and that of
differentiation.[216]
A number of conical teeth, in line as they lie in the jaws of the sperm
whale, represent the primitive dentition.[217] In time a number of these
teeth, according to the concresent theory, cluster together so as to form
the four cusps of a human molar, each one of the whale tooth points
forming one of the cusps of the mammalian tooth. Vertically succeeding
teeth might also be grouped. What evidence is there in favour of this
theory? and what is there against it? All primitive reptiles from which
the mammals have descended, and many of the existing mammals, have a large
number of isolated teeth of a conical form. Further, by shortening of the
jaws, the embryonic germ from which each of the numerous tooth-caps is
budded off in course of development could have been brought together in
such a manner that any cusps originally stretched out in a line would form
groups of a variable number of cusps, according to the more or less
complex pattern of the crown. Against the acceptance of this theory stands
the fact that cusps quite similar in all respects to each of the cusps
which form the angles of the human molar are even now being added to the
teeth in certain animals, such as the elephant, whose molar teeth cusps
are being thus complicated. In the mesozoic period certain animals with
tricuspid teeth occur. According to the theory of concrescence these teeth
ought not to show any increase of cusps in later geologic periods, but
down through the ages to the present time successors of those animals
continue to present a very much larger number of cusps. How is this
increase of cusps to be accounted for? Has there been a reserve store of
conical teeth to increase the number? Most obviously to every student of
the fossil history of cusps there is no reserve store, but new cusps are
constantly rising upon the original crown itself by cusp addition.
In the Triassic occur the first mammalia with conical, round, reptilian
teeth. There are also some aberrant types which possess complex or
multitubercular teeth.
These teeth begin to show the first trace of cusp addition.
In Fig. 1, Plate A, the teeth of the dromatherium of the coal beds of
North Carolina occur on the sides of the main cone, cusps or rudimentary
cuspules. On either side of the main cone are two cuspules. In the same
deposit occurred another animal represented by a single tooth (Fig. 3), in
which these cusps are slightly larger. These cusps have obviously been
added to the side of the teeth and are now growing. In teeth of the
Jurassic period, found in large numbers both in America and in England,
but still of very minute size, are observed the same three cusps. These
cusps have now taken two different positions; in one case they have the
arrangement presented in Plate B. The middle cusp is relatively lower, and
the lateral cusps are relatively higher; in fact these cones are almost
equal in size. These teeth are termed triconodont, as having three nearly
equal cones. But associated with this is the spalacotherium, the teeth of
which are represented in Plate A, Fig. 4. This tooth illustrates the
transformation of a tooth (triconodont) with three cusps in line into a
tooth with three cusps forming a triangle. Here the primitive cusp is
the apex of a triangle of which the two lateral cusps are the base. This
tooth, in this single genus, is the key of comparison of the teeth of all
mammalia. By this can be determined that part of a human molar which
corresponds with a conical reptilian tooth. This stage is the triangle
stage; the next stage is the development of a heel or spur upon this
triangle (see in the amphitherium, Fig. 5). The opossum still distinctly
preserves the ancient triangle. Look at it in profile, inside or in top
view, and see that the anterior part of the tooth is unmodified. This
triangle is traceable through a number of intermediate types. In Miacia
(Fig. 6), a primitive carnivore, is a high triangle and a heel; looked at
from above (Fig. 6a), the heel is seen to have spread out broader so that
it is as broad as the triangle. The three molars of this animal illustrate
a most important principle, namely, that the anterior, triangular portion
of the crown has been simply levelled down to the posterior portion.
[Illustration: PLATE A.]
[Illustration: PLATE B.]
These three teeth form a series of intermediate steps between a most
ancient molar and the modern molar of the human type. The second tooth is
halfway between the first and third. The second molar, seen from above,
has exactly the same cusps as the first, so it is not difficult to
recognise that each cusp has been directly derived from its fellow. The
third tooth of the series (Fig. 7) has lost one of its cusps; it has lost
a cusp of the triangle. It is now a tooth where only half the triangle is
left on the anterior side and with a very long heel. That tooth has
exactly the same pattern as the lower human molar tooth (Fig. 8), the only
difference is that the heel is somewhat more prolonged. These teeth belong
to one of the oldest fossil monkeys, anaptomorphus. Human lower molars,
not very exceptionally, instead of four cusps, have five. The fifth cusp
always appears in the middle of the heel, or between the posterior lingual
and the posterior buccal. This occurs in monkeys and other animals, but no
record exists of the ancient anterior lingual reappearing. The human lower
molar, with its low, quadritubercular crown, has hence evolved by addition
of cusps and by gradual modelling from a high-crowned, simple, pointed
tooth.
Human teeth are of excellent service in the initial determination of
degeneracy in the child. For this purpose the teeth should be studied from
the first evidence of their development until they are all in place, which
occurs normally, in most cases, by the twenty-second year.
Teeth-enamel is formed from the epiblast, and dentine, cementum, pulp
(except as to nerve tissue) from the mesoblast. The enamel organs of the
first set appear during the seventh week of foetal life; the dentine bulb
during the ninth week. At this period the tooth obtains its periphery.
This models the enamel cap which fits over the dentine like a glove. When
imperfections in hand or fingers exist these deformities are distinctly
observed upon the glove, and in precisely the same manner are observed the
different shapes and sizes of the incisors, cuspids, and molars.
Calcification of the teeth begins at the seventeenth week of foetal life.
The illustration (Fig. 50) shows the progress of calcification and
development of the temporary set of teeth. Examination will show that any
defect in nutrition, from conception to birth (due to inherited states or
maternal impressions), has been registered upon the teeth. The state of
the constitution and the locality register the date of such defects. Thus
if the tooth, as a whole, be larger or smaller than normal, or abnormally
irregular, taint is undoubtedly inherited from one or both parents. If, on
the other hand, there be defect at any part on the crowns of the teeth,
and the contour be perfect, the date of malnutrition can be easily
determined from this chart. More or less than the normal number of teeth,
abnormally placed, demonstrates the existence of inherited defect, since
the germs must have been deposited at the period mentioned. No absolute
rule can be laid down as to date of the eruption of the teeth. The teeth
of the temporary set erupt nearly as follows:
_After Birth._ _Time of Eruption._
Lower Central Incisors 7 months 1 to 10 weeks.
Upper " " 9 months 4 to 6 weeks.
Upper and Lower Lateral 12 months 4 to 6 weeks.
First Molars 14 months 1 to 2 months.
Cuspids 18 months 2 to 3 months.
Second Molars 26 months 3 to 5 months.
[Illustration: FIG. 50.]
[Illustration: FIG. 51. SHOWS LINES OF DEVELOPMENT OF THE PERMANENT
TEETH.]
The enamel organs and dentine bulb for the permanent teeth form just
before birth (Fig. 51) in like manner with the temporary set. They form
just above the temporary set on the upper and below on the lower jaw. The
permanent molars begin to calcify at the twenty-fifth week of foetal life.
The permanent incisors do not calcify until a year after birth. Any
deviation in size or contour of the permanent teeth from the normal must
hence be due to defect in nutrition in the dentine bulb, between the
fifteenth and twenty-fifth week of foetal life. Any deviation in
calcification (except the cusps of the first permanent molars) must occur
after birth. At the third year twenty-four teeth are fairly well
calcified. At the fifth year the second permanent molars, and at the
eighth year the third molars or wisdom teeth, begin to calcify.
The following table gives the age of eruption of permanent teeth:
First Permanent Molars Circa 6 years.
Upper and Lower Central Incisors " 7 years.
Upper and Lower Lateral " " 8 years.
First Bicuspids " 9 years.
Second Bicuspids " 10 years.
Cuspids " 11 years.
Second Permanent Molars " 12 years.
Third Permanent Molars " 17 to 24 years.
Man, at this present stage of evolution, has twenty teeth in his temporary
and thirty-two in his permanent set. Any deviation in number is the result
of embryonic change occurring between the sixth and fifteenth week, for
the temporary teeth, and the fifteenth week and birth for the permanent.
The germs of teeth which erupt late in life, and are called third sets, of
necessity appear ere birth and are completely formed at the beginning of
the second year, although they remain protected in the jaw until
eruption.
More than twenty teeth in the temporary set, or thirty-two in the
permanent set, is hence an atavistic abnormality. From the maxillary and
dental standpoint man reached his highest development when well-developed
jaws held twenty temporary and thirty-two permanent teeth. Decrease in the
numbers of teeth meant, from the dental standpoint, degeneracy, albeit it
might mark advance in man's evolution as a complete being. In the New
Mexican Lower Eocene occur monkeys like the lemurarius and limnotherium,
each the type of a distinct family. The lemurarius, most nearly allied to
the lemurs, is the most generalised monkey yet found. It had forty-four
teeth in continuous series, above and below. The limnotherium, while
related to the lemurs, had some affinities with the American marmosets.
These solved the problem of the origin of the extra teeth (known as
supernumeraries) that sometimes occur in man, and demonstrated that man,
during his evolution from the lowest monkey, lost twelve teeth. These
supernumerary teeth assume two forms; either they resemble the adjoining
teeth or are cone-shaped. While they are rarely exactly counterparts,
every tooth can be duplicated, as the following illustrations show.
Fig. 52 illustrates fairly well-formed duplicate central incisors, the
normal incisors being outside the dental arch. They are crowded laterally
by the large roots of the supernumerary incisors.
[Illustration: FIG. 52.]
[Illustration: FIG. 53.]
[Illustration: FIG. 54.]
Fig. 53 shows an extra right lateral in a temporary set in the upper jaw.
Fig. 54 an extra right lateral in the permanent set. Fig. 55 illustrates
normally developed supernumerary cuspids which are all grouped together
upon the right side, the bicuspid being also duplicated on each side;
indeed, all but the molars are duplicate. Fig. 56 shows supernumerary
third molars, easily demarcated from the normal molars. The teeth which
fail to approximate their normal neighbours assume the cone shape of the
primitive tooth.
[Illustration: FIG. 55.]
[Illustration: FIG. 56.]
The fact that the cone-shaped tooth, as a rule perfect in construction, is
found everywhere in the jaw, but especially in the anterior and posterior
part of the mouth, is of much value in outlining tooth and jaw evolution,
especially in the degeneracy phase. The upper jaw, being an integral part
of the skull and fixed, is, of necessity, influenced by brain and skull
growth; hence degeneracy is more detectable in it than in the lower.
[Illustration: FIG. 57.]
[Illustration: FIG. 58.]
[Illustration: FIG. 59.]
[Illustration: FIG. 60.]
[Illustration: FIG. 61.]
The evolution of the jaw is toward shortening in both directions. This
shortening will continue so long as the jaw must be adjusted to a varying
environment. The jaw of man having originally contained more teeth than at
present, lack of adjustment to environment produces, from the shortening,
degeneracy of the jaw and atavism of the teeth. While this may coincide
with general advances of the individual, it indicates that he is not yet
adjusted to his new environment. The shortening of the upper jaw causes
supernumerary, cone-shaped teeth to erupt, in mass, at the extreme ends of
the jaw, as shown in the following figures. Fig. 57 illustrates a
cone-shaped tooth between the two central incisors, forcing them out of
position. Fig. 58 shows three supernumerary teeth--a cone-shaped tooth
between the central laterals, and the cuspids out of position. The left
permanent lateral is at the median line; another cone-shaped tooth remains
in the vault, while the supernumerary left lateral is in place. As many as
eight are at times to be observed in the anterior vault. Posteriorly these
teeth are most often noticed in connection with the third molars, usually
on a line with other teeth posterior to the last molar. Fig. 59 shows two
supernumerary teeth in the anterior and two in the posterior part of the
left arch; the molars have been extracted. Supernumerary teeth are not
confined to these localities, but may be observed at any point in the
dental arch (Figs. 60 and 61). The primitive cone-shaped tooth is rarely
observed in the lower jaw. In twenty-six years' practice I have not seen a
case. The mobility of the lower jaw prevents that mal-adjustment to
environment present in the upper. The continual shortening, in both
directions, of the jaw causes the third molars frequently to wedge in
between the angle of the jaw and the second molars, so that eruption, if
possible, is difficult.
[Illustration: FIG. 62.]
[Illustration: FIG. 63.]
[Illustration: FIG. 64.]
The third molar is often absent in the English-speaking and Scandinavian
races. In 46 per cent. of 670 patients it was missing. Frequently its
development is abortive. This tooth, in the struggle for existence, seems
destined to disappear. It is more often absent from the upper than the
lower jaw. When absent, or badly developed, the jaw is smaller and
frequently teeth irregularities, nasal stenosis, hypertrophy of nasal bone
and mucous membrane, adenoids and eye disorders coexist. Fig. 62 shows
absence of the left third molar with irregularities of that side of the
arch. In Fig. 63 both third molars are seen to be missing. Anteriorly,
the lateral incisors are most often wanting; 14 per cent. of the laterals
were wanting in 670 patients. In the progress of evolution man has lost
one lateral upon each side of the mouth and the second lateral seems also
destined to disappear. In Fig. 64 the left lateral incisor has
disappeared; and in Fig. 65 both lateral incisors are absent. Not
infrequently does it occur that centrals, cuspids, bicuspids, and even
molars are absent, even their germs not being detectable. Fig. 66 shows
three supernumeraries in the anterior part of the mouth and but two
molars. The absence of the teeth indicates lack of development of germs,
due either to heredity or defective maternal nutrition at the time of
conception or during early pregnancy.
[Illustration: FIG. 65.]
[Illustration: FIG. 66.]
[Illustration: FIG. 67.]
Crescent-shaped, bitubercular, and tribucular as well as deformed teeth,
tend to be cone-shaped. The malformation of these teeth results from
precongenital trophic change in dentine development, dwarfing and notching
the cutting and grinding edges of the second set of teeth, of which a
familiar example is the so-called Hutchinson's teeth, usually referred to
a syphilitic causation. Hutchinson's position has, however, been more
strongly stated than his words justify, since he admits that in at least
one-tenth of the cases this cause could be excluded.
Syphilis only plays the part of a diathetic state profoundly affecting the
maternal constitution at the time of dentine development; while these
teeth may be due to secondary results of syphilis, they do not
demonstrate syphilitic heredity.
In Fig. 67 are seen the teeth of an individual affected with
constitutional disease (referring to Fig. 51 it becomes evident that the
defective lines represent the respective ages, 2-1/2, 4, and 5 years). The
degree of pitting will depend, as a rule, on the severity of the
constitutional disorder. In the case just cited, however, although
nutrition was but slightly disordered, each tooth shows a tendency to
conate. Not infrequently cavities extend completely through the tooth. The
cusps of the (permanent) first molars, calcifying at the first year, are
usually attacked also, and arrested in development, producing the cone
shape. These data, together with the dates of eruption of the temporary
and permanent teeth, furnish an absolute basis for calculation as to
malnutrition producing excessive or arrested development, not only of the
teeth and jaws but all parts of the body.
[Illustration: FIG. 68.]
[Illustration: FIG. 69.]
[Illustration: FIGS. 70, 71.]
Fig. 68 shows a very degenerate jaw with cone-shaped, malformed
bicuspids. The right lateral missing, the cuspids are erupting in the
vault and the dental arch is assuming a V-shape. The jaw shows, as a
whole, marked arrest in development. Fig. 69 shows Hutchinson's teeth.
Were the first molars visible, they would present marked contraction of
the outer surface with a malformed centre. Referring again to Fig. 51, it
is observable that trophic changes affected the system at the age of
birth. The outer surface exhibits a tendency to take the cone shape. Figs.
70, 71, 72, 73, and the molars in Fig. 66, exhibit malformations that
assume the cone shape and the centre frequently associated with this type
of teeth. The coincidence in form between Hutchinson's and malformed teeth
and those of the chameleon suggests that tropho-neurotic change produces
atavistic teeth. Fig. 74 illustrates the tendency of human bicuspids (when
there is no antagonism) to rotate one-fourth round, thus again indicating
an atavistic tendency toward the teeth of the chameleon. Fig. 75 exhibits
extreme atavism; all teeth anterior to the molars are cone-shaped. The
third molars are missing and would, probably, never erupt. In Fig. 76
appears more marked atavism. The upper and lower are both cone-shaped, and
the superior first bicuspid exhibits tendency thereto. The right superior
second bicuspid, second and third molars, the right inferior first and
second bicuspids, with second and third molars are missing. The same
condition, probably, exists on the left side. The space in the upper jaw
is due to the insufficient width of the teeth. Alternation of teeth in the
upper and lower jaw is a reptilian feature.
[Illustration: FIG. 72.]
[Illustration: FIG. 73.]
[Illustration: FIG. 74.]
[Illustration: FIG. 75.]
Fig. 76 furnishes excellent illustration of the principles already stated.
In degenerate jaws every tooth in the jaw, at one point or another, may
display rudimentary cusps. On the incisors they are always to be found on
the lingual surface.
[Illustration: FIG. 76.]
[Illustration: FIG. 77.]
Fig. 77 illustrates the centrals with two rudimentary cusps, the laterals
with one, and the cuspids with one also. Fig. 78 represents cusps upon the
lingual surface of the molars. The cuspids are not unlike the lower
cuspids with a rudimentary lingual cusp.
[Illustration: FIG. 78.]
There is a gradation from central incisors toward the bicuspids, in
evolution. This grading of form is not observed in passing from the cuspid
to the bicuspid in man. But the cuspid often presents a cingulum on the
lingual face that inclines it toward the bicuspid forms in lower mammals,
like the mole, and the first premolar, or bicuspid, is then more
caniniform, the inner tubercle being much more reduced. This inner
tubercle is very variable and erratic as to its position. It appears as
far front as the centrals and is often present on the lingual face of the
laterals of man. The lingual tubercle is very constant on the first
bicuspid of man and is as well developed as the buccal. But in some lower
forms, as in the lemurs, it is quite deficient. It attains the highest
development only in the anthropoids and man. Considering these stages of
development, the grading from the cuspid to the bicuspid forms was more
gradual in the earlier species than in the later, where the individual
teeth have taken on special development.
[Illustration: FIG. 79.]
The skull of a degenerate girl who died from tuberculosis, at thirteen
years, presented, among other stigmata, a cusp on the external surface of
a right inferior cuspid. In Fig. 79, where every tooth is present, a most
remarkable display of cusps occurs. The cusps upon the cutting and
grinding edges are not obliterated. Commencing with the left superior
central incisors, three cusps are present with a rudimentary palatine
cusp. The laterals also show three cusps, while the cuspid has two very
distinct. The first and second bicuspids have tubercular cusps, they
being in line. The buccal cusps upon the molars, two or three, and are
still in position. The palatine cusps are worn away. The same is the case
upon the opposite side, except that the cuspid has cusps that have fused
together, leaving a small projection upon the mesial side and a
rudimentary palatine cusp. The cusp upon the third molar is lost. In
another case (Fig. 61) the primitive cone teeth are seen trying to shape
themselves into incisors. The lateral incisors, cuspids and bicuspids are
still cone-shaped. The first permanent molar is fairly formed while the
second molars are still in a primitive condition.
Degenerate teeth unite in twos, threes, fours, and fives. These single,
cone-shaped teeth grow together and form bicuspids and molars. The germ of
any two normal teeth may intermingle and unite; not only are the crowns
found united with separate roots, but crowns and roots are united
throughout.
Figs. 80 and 81 show two superior, central and lateral incisors joined
together throughout the entire length of crown and root. In Fig. 82 two
lower incisors are united throughout. Fig. 83 shows a cuspid with two
roots. George T. Carpenter, of Chicago, has a right superior, second
bicuspid with three well formed roots. Fig. 84 illustrates two bicuspids
united at the crowns. Fig. 85 shows two molars perfectly united. Fig. 86
illustrates central and lateral incisors of the permanent set perfectly
united. Fig. 87 shows two molars united. Fig. 88 a molar and supernumerary
taking the cone-shape with deformed centre. Fig. 89 shows three malformed
teeth, each coated and completely united. It is not uncommon to find three
molars united together, as, for instance, the second, third, and
supernumerary molars. C. V. Rosser, of Atlanta, Georgia, has two small
molars and a supernumerary cuspid perfectly united, from crown to root,
and these three further united to the roots of a well-formed molar.
[Illustration: FIG. 80.]
[Illustration: FIG. 81.]
[Illustration: FIG. 82.]
[Illustration: FIG. 83.]
[Illustration: FIG. 84.]
[Illustration: FIG. 85.]
[Illustration: FIG. 86.]
That human jaws, like human ears, are degenerating is demonstrable by
actual measurements. Mummery, who examined the skulls of 200 Briton and
Roman soldiers, found the narrowest 2.12 inches, the highest 2.62, with an
average of 2.50. The width of jaws of 402 British soldiers to-day is:
narrowest, 1.88; widest, 2.63; average 2.28. The highest width was very
rare; only eight measured 2.50. The jaws of the mound builders, compared
with the existing cliff dwellers, show similar results. The average width
is about 2.50 inches. This is also true of nearly pure negro races.
Measurements of normal jaws of 855 Italians of Central Italy were:
narrowest, 1.88; widest, 2.63; average 2.17. Measurements of normal jaws
of 4,935 Americans gave the following results: narrowest, 1.75; widest,
only one case, 2.56; average 2.13. If in the highest type of physical man
the width of the upper jaw from the outer surface of the permanent molars
near the gum margin was originally 2.50 inches in diameter, the jaw of
people now living in the same locality is from 0.25 to 0.33 inches
smaller. Although the jaw has thus been growing smaller, since there are
no breaks or deformities in the contour of the dental arch this must be
regarded simply as an adaptation to environment, and not degeneracy in the
proper sense of the term. The degeneracy of the jaws, on which I would lay
special stress, is that in which deformity has resulted from inability to
adjust structure to a changing environment. When arrest of development so
takes place that deformities of the dental arch result, the jaws vary from
two inches to one inch in width. As a rule, the teeth are the same size
to-day that they were thousands of years ago. This is due to the fact that
they are ante-natal and not influenced by post-natal systemic changes. The
jaws do not contract as a result of mouth breathing.[218] If the jaw be
arrested and be smaller in circumference than the teeth, a break takes
place in the dental arch and deformity results. Two types of deformity
occur, the V-shaped arch and the saddle arch. All other types of
deformity, not due to local causes, are modifications of these two. These
deformities always occur with the second teeth only. In these cases the
facial profile assumes the perpendicular line or arrested face, as
illustrated in the chapter on Degeneracy of the Face. They are never seen
before the sixth year, when the second set begin to erupt and are complete
with development of the second molars at 12. They may become exaggerated
later in life from want of room, the eruption of the third molar and want
of harmony in relation of the two jaws when closed.
[Illustration: FIG. 87.]
[Illustration: FIG. 88.]
[Illustration: FIG. 89.]
[Illustration: FIG. 90.]
There are three characteristics of the normal arch. Independent of
temperamental peculiarities, the line extending from one cuspid to the
other should be an arc of a circle, not an angle or straight line; the
lines from the cuspids to the third molar should be straight, curving
neither in nor out, the sides not approximating parallel lines. Absolute
bilateral uniformity is not implied in this, as the two sides of the
human jaw are rarely, if ever, wholly alike. A uniform arch necessitates
uniformity of development between the arch of the maxilla and the arch of
the teeth and a correct position of the individual teeth in their relation
to each other. When there is inharmonious development between the jaws and
the teeth, as may happen when one parent has a small maxilla with
correspondingly small teeth, and the other a large one, with
correspondingly large teeth, if the child inherit the jaw of one and the
teeth of the other, irregularities must follow. Such difference in
diameter between the arch of the maxilla and that of the crowns of the
teeth is a constitutional cause of irregularity. When there is a
difference between these diameters the line formed by the teeth must
either fall outside or within the arch of the maxilla and irregularities
of arrangements result. The primary division of irregularities is the
V-shaped and saddle-shaped arches. We have the V-shaped variety (Fig. 90,
one of the typical forms), where the apex of a triangle is formed by the
incisors, the base of the triangle being a line connecting the first two
molars. If, because of premature or tardy extraction, the first molars
move forward, or by coincidence of the arch of the maxilla and the arch of
the crown of the teeth in trying to accommodate itself to the lesser arch
of the maxilla, the arch becomes a broken line, forming an angle at the
incisors. This angle results from two causes: the thinness of the process
at this point and the diminution of resistance which must follow.
When the permanent bicuspids erupt under a favourable condition, so that
their greatest diameter is in a line with the greater diameter of both
cuspids and first molar, they will be held firmly in place, since the
greatest pressure is on this very line. On the other hand, when the
bicuspids are erupted after their proper time, while the cuspids progress
duly, and meeting no resistance fall into their proper places, but the
bicuspids adapt themselves as best they can to the space left for them,
and if the arch of the maxilla does not coincide with that of the crowns,
they must fall within or without the arch. Now, if the first molar have
moved forward, diminishing the space, the bicuspid must erupt either
within or without the arch.
[Illustration: FIG. 91.]
To understand why they are generally found within the arch, the shape of
the molar and cuspids must be kept in mind. A transverse section of their
crowns shows their proximal walls not to be parallel, but wedge-shaped,
their diameter being greater on the buccal than on the palatal side. When
the crowned bicuspid falls within the greatest diameter of these teeth,
finding more room within the arch, they naturally slip in the direction of
least resistance, _i.e._, toward the palate. A local cause for the same
condition is found in the fact that the crown of the bicuspids, before
their eruption, was held between the roots of the temporary molars, and,
as these form an arch of a smaller circle than that of the permanent
teeth, the bicuspids will be found generally inside the arch. From both
causes occurs an inward curvature, which is termed the saddle-shaped arch
(Fig. 91). It should be noted here that, since the V-shaped irregularity
is found anterior to the cuspid, the upper incisors are always projecting
beyond the lower; the saddle-shaped irregularity is invariably posterior
to the cuspid from an inward curve. The incisors never project. Both forms
contract the arch; the V-shaped anteriorly, the saddle-shaped posteriorly.
In both forms the forward movement of the first molar is the local cause.
Deformities of the dental arch are due, first, to arrest of development of
the jaws, and, second, in the nature of the deformity, to the order of
eruption of teeth, which rarely erupt twice alike. From an evolution
standpoint these deformities are atavistic. The V-shaped reverts to the
reptilian type; the saddle-shaped to the lower mammals. In the gorilla,
the nearest to man in dentition, there is a very distinct approach to the
saddle shape. In the chimpanzee it remains. The orang-outang exhibits less
of this tendency. The arch of some of the cebidae very nearly approaches
man. It all depends upon the extent of prognathism. When that is reduced
the arch appears and rectangular arrangements of the teeth are lost. Most
carnivors exhibit a distinct approach to the saddle shape. Some felines
have a shortening of the jaw, partly obliterating the tendency, but in
most canidae it is quite marked.
These are facts which cannot be overlooked, since, from the very nature of
development and eruption of the teeth, they cannot take any other form.
The arrangement of the crowns of the cuspid (canine) in the jaw before
eruption is such that, no matter what the local condition of the jaws or
teeth may be the V-shaped or saddle-shaped dental arch must be produced.
[Illustration: FIG. 92.]
In no symptoms is degeneracy so evident as in the stigmata resultant on
hypertrophy of the alveolar process. This occurs at all ages, but more
particularly at the period of development of the permanent set of teeth.
The entire alveolar process may become involved (Fig. 92), or only a
portion (Fig. 93).
Hypertrophy of the alveolar process is the result of irritation incident
upon eruption and the shedding of the temporary teeth, and eruption of the
permanent teeth.
[Illustration: FIG. 93.]
Laryngologists, rhinologists, and neurologists claim that certain vaults
are deformities; in reality the alveolar process is hypertrophied. The
jaws, as a whole, owing to an unstable and ill-balanced nervous system,
are liable to become excessively developed, as well as arrested in
development. Excessive development of the superior maxilla is evinced by a
fulness of the upper lip. In these cases the upper maxilla is too large
for the lower, and stands out beyond it. The lower may be quite normal.
When there is simply a want of proportion between the two jaws, it is due
to the diminutive or excessive size of one while the other is normal. The
criterion in these cases must be the facial angle. The upper jaw is
usually in harmony with the skeleton, while the lower jaw depends for its
size largely upon function, its size being the result of accident rather
than the result of general proportions.
[Illustration: FIG. 94.]
When the upper jaw is normal, or smaller than the lower, the extent of the
posterior portion is determined by the occlusion of the first permanent
molar, which keeps the alveolar processes in permanent relation to each
other at this point and allows freedom of development in front. If the
occlusion be not normal, the upper jaw and alveolar process will develop
laterally as well as anteriorly. The teeth of the anterior columns may
either stand vertically, or they may be turned in toward the lower
incisors. The latter defect is produced by the action of the lips. When
the cuspids are in their normal position the upper incisors form a larger
arch than the lower, and this permits of their being turned inward; but
when the cuspids have moved so far forward that they are not normally
interlocked with the lower teeth, the incisors are too crowded to permit
this. While the jaws are growing smaller the teeth tend to cause reversion
to the original form. Arrest of development of the superior maxilla is
always associated with marked depression at the alae of the nose, producing
the appearance of having been hollowed out from a point at the floor of
the orbit to the grinding surface of the lower teeth (Fig. 94).
[Illustration: FIG. 95.]
Arrest of the lower jaw (Fig. 95) is common among degenerates. This
consists of a shortening of the body of the jaw. Sometimes it is arrested
to such an extent that there is apparently no chin. About 50 per cent. of
criminals of Elmira, New York, have this deformity. The following table
shows the number of deformities of the jaws and teeth which I have found
among some of the degenerate classes.
-----------+------------------------------------------------------------
JAWS. |Number examined.
| +-------------------------------------------------------
| |V-shaped.
| | +---------------------------------------------------
| | |Partial V.
| | | +----------------------------------------------
| | | | Semi-V.
| | | | +------------------------------------------
| | | | |Saddle.
| | | | | +--------------------------------------
| | | | | |Partial Saddle.
| | | | | | +---------------------------------
| | | | | | |Semi-Saddle.
| | | | | | | +-----------------------------
| | | | | | | |Normal.
| | | | | | | | +------------------------
| | | | | | | | |Arrested Development.
| | | | | | | | | +--------------------
| | | | | | | | | |Excessive
| | | | | | | | | |Development.
-----------| | | | | | | | | | +----------------
TEETH. | | | | | | | | | | |Irregular.
| | | | | | | | | | | +------------
| | | | | | | | | | | |Tubercles
| | | | | | | | | | | |of teeth.
| | | | | | | | | | | |
| | | | | | | | | | | |Present.
| | | | | | | | | | | | +---------
| | | | | | | | | | | | |Tubercles
| | | | | | | | | | | | |of teeth.
| | | | | | | | | | | | |
| | | | | | | | | | | | |Absent.
| | | | | | | | | | | | | +----
| | | | | | | | | | | | | |Reg-
| | | | | | | | | | | | | |ular.
| | | | | | | | | | | | | |
| | | | | | | | | | | | | |
-----------+----+---+----+---+---+----+---+----+---+---+---+--+----+---
Criminals | | | | | | | | | | | | | |
at | | | | | | | | | | | | | |
Pontiac, | | | | | | | | | | | | | |
Ill. | 465| 75| 71| 3| 66| 63| 16| 171|...|...|123|13| 452|342
| | | | | | | | | | | | | |
Criminals | | | | | | | | | | | | | |
at | | | | | | | | | | | | | |
Elmira, | | | | | | | | | | | | | |
N. Y. |1041|381| 49| 1|157| 26|...| 422|...|...|220|26|1015|821
| | | | | | | | | | | | | |
Criminals | | | | | | | | | | | | | |
at | | | | | | | | | | | | | |
Joliet, | | | | | | | | | | | | | |
Ill. | 468| 13| 79| 19| 59| 92| 24| 163|...|...|...|..|... |...
| | | | | | | | | | | | | |
Prostitutes| | | | | | | | | | | | | |
at Chicago,| | | | | | | | | | | | | |
Bridewell | 30| 10| 17| 7| 27| 10| 10| ...| 1|...|...|..|... |...
| | | | | | | | | | | | | |
Insane at | | | | | | | | | | | | | |
Dunning, | | | | | | | | | | | | | |
Ill. | 700| 26| 47|...| 12| ...|...| 486|...|...|...|..|... |...
| | | | | | | | | | | | | |
Insane at | | | | | | | | | | | | | |
Kankakee, | | | | | | | | | | | | | |
Ill. | 613| 69| 107| 29| 89| 105| 61| 153|...|...|...|..|... |...
| | | | | | | | | | | | | |
Idiots, | | | | | | | | | | | | | |
imbeciles |1977|129| 236|...|207| ...|...|1095|...|...|...|..|... |...
| | | | | | | | | | | | | |
Deaf and | | | | | | | | | | | | | |
Dumb |1935|169| 192|...|203| ...|...| 901|...|...|...|..|... |...
| | | | | | | | | | | | | |
Blind | 207| 7| 9|...| 11| ...|...| 105|...|...|...|..|... |...
| | | | | | | | | | | | | |
Inebriates | | | | | | | | | | | | | |
[219] | 514|1.5|24.4|0.3|9.3|13.2|7.7|25.4|...|...|...|..|... |...
-----------+----+---+----+---+---+----+---+----+---+---+---+--+----+----