CHAPTER XIII. The Degenerate Teeth and Jaws

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.

A number of conical teeth, in line as they lie in the jaws of the sperm whale, represent the primitive dentition. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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. 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.

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.

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 cebidæ 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 canidæ 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.

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.

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.

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 alæ 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).

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 | | | | | | | | | | | | | | | 514|1·5|24·4|0·3|9·3|13·2|7·7|25·4|...|...|...|..|... |... -----------+----+---+----+---+---+----+---+----+---+---+---+--+----+----