Fig. 76 furnishes excellent ill.u.s.tration 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.

[Ill.u.s.tration: FIG. 76.]

[Ill.u.s.tration: FIG. 77.]

Fig. 77 ill.u.s.trates 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.

[Ill.u.s.tration: FIG. 78.]

There is a gradation from central incisors toward the bicuspids, in evolution. This grading of form is not observed in pa.s.sing 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.

[Ill.u.s.tration: 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 ill.u.s.trates two bicuspids united at the crowns. Fig. 85 shows two molars perfectly united. Fig. 86 ill.u.s.trates 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.

[Ill.u.s.tration: FIG. 80.]

[Ill.u.s.tration: FIG. 81.]

[Ill.u.s.tration: FIG. 82.]

[Ill.u.s.tration: FIG. 83.]

[Ill.u.s.tration: FIG. 84.]

[Ill.u.s.tration: FIG. 85.]

[Ill.u.s.tration: 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 212 inches, the highest 262, with an average of 250. The width of jaws of 402 British soldiers to-day is: narrowest, 188; widest, 263; average 228. The highest width was very rare; only eight measured 250. The jaws of the mound builders, compared with the existing cliff dwellers, show similar results. The average width is about 250 inches. This is also true of nearly pure negro races.

Measurements of normal jaws of 855 Italians of Central Italy were: narrowest, 188; widest, 263; average 217. Measurements of normal jaws of 4,935 Americans gave the following results: narrowest, 175; widest, only one case, 256; average 213. 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 250 inches in diameter, the jaw of people now living in the same locality is from 025 to 033 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 circ.u.mference 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 a.s.sumes the perpendicular line or arrested face, as ill.u.s.trated 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.

[Ill.u.s.tration: FIG. 87.]

[Ill.u.s.tration: FIG. 88.]

[Ill.u.s.tration: FIG. 89.]

[Ill.u.s.tration: 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 const.i.tutional 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 s.p.a.ce 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 s.p.a.ce, the bicuspid must erupt either within or without the arch.

[Ill.u.s.tration: 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 dent.i.tion, 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 a.s.sociated 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).

[Ill.u.s.tration: FIG. 95.]