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. [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.]