In All The Higher Animals The Teeth Are Developed Directly From The Mucous Membrane Dentition, and are therefore, like hair, nails, feathers, etc, appendages of the skin, and form no part of the true osseous system. As early as the fifth week of foetal life, according to the observations of Prof. Goodsir, a deep, narrow groove between the lip and the rudimentary palate in the upper jaw indicates the future situation of the teeth. "Within the next three weeks papillae developed at the bottcm of the groove become the germs of the future milk or temporary teeth. In the progress of development the papillae are enveloped in open follicles, and these again are converted into shut sacs; contemporaneously with these changes, the edges of the dental groove arc themselves growing, so that by the 14th week they meet, enclosing the tooth sacs. "Within the sacs the papillary pulp is gradually converted into dentine, of which the body of the tooth is composed, while the enamel is probably formed by calcification of the epithelium abundantly produced from the inner surface of the sacs.

As teeth are required before the jaws have attained their growth, and yet from their structure are incapable of enlarging pari passu with the bones in which they are placed, provision is made for a temporary set, which when they have served their purpose are replaced by the permanent teeth. As early as the 14th week minute crescentic depressions of mucous membrane may be discovered above and at the inner part of the opercula of the milk teeth; these depressions soon become converted into minute compressed sacs, which gradually sink behind and below the sacs of the milk teeth, and in these sacs are developed the first ten permanent teeth of each jaw; the other six are developed in sacs placed posterior to those of the last milk teeth, which are formed in a manner precisely similar to those of the milk teeth themselves. The ossification of the permanent teeth commences a little before birth with that of the first molar, and proceeds during the first three years of infancy successively in the incisors, the canines, and the bicuspids. The approach of the time for the eruption of the temporary teeth is announced by an increased secretion of saliva.

In the earlier months of infancy the mouth is comparatively dry, but as the teeth shoot into the gums the mouth becomes moist and the child begins to drivel. The progress of dentition is not apparently continuous, but after the eruption of each successive pair a pause of one or two months generally follows. The central incisors commonly pierce the gum in the course of the 7th month after birth, those of the lower jaw preceding the upper ones by a short interval; between the 7th and 10th months the lateral incisors make their appearance; from the 12th to the 14th month the anterior molars, and between the 14th and 20th the canines are cut; and the first dentition is completed between the 18th and 36th months by the protrusion of the posterior molars. Both the time and the order of appearance of the first set of teeth admit of a good deal of variation, their progress being hastened or delayed sometimes six or seven months by a lateral incisor, or even a molar or canine tooth, cutting the gum before the appearance of the central incisors. The period of primary dentition is attended with increased risk to the life of the infant. During its continuance the proportionate mortality becomes much increased, and in the bills of mortality numerous deaths are ascribed to teething alone.

It must be remembered, however, that at this time all the functions of the young being are in a state of great activity, and that teething is but one in a series of changes by which the infant is prepared to substitute for the milk provided by its mother, food suitable to the conditions of its future existence. In a healthy infant dentition in itself is attended with little inconvenience and no danger; when the teeth come to distend and stretch the mucous membrane lining the gums, there is probably a little tenderness and pain, some fretfulness, and perhaps slight febrile excitement; but in the absence of other causes of disease, this soon passes over. If, however, the nervous system is unduly excitable, dentition may seriously complicate other maladies. When the process of dentition is advancing normally, it should never be interfered with; when the gum is red, swollen, and painful, scarification may be resorted to with advantage, and may be repeated if necessary, the trilling loss of blood affording relief to the inflamed gum. When the tooth is evidently about to pierce the gum, if the child appears to sutler, it may be freed by cutting down to it with the gum lancet.

In cases where convulsions supervene suddenly without an evident cause, if dentition is proceeding actively and the gums are tense and swollen, the gum lancet may be resorted to. Occasionally dentition is attended with much fever and derangement of the digestive organs, while a sloughy unhealthy ulceration makes its appearance on the gum over the teeth just about to protrude, or at the edge of the gum of those which have recently been cut. In these cases the gum lancet does positive harm, while they readily yield to a properly regulated diet, and to the use of the chlorate of potash in solution, in doses of one or two grains repeated every four hours. During the earlier period of childhood a bony plate or partition separates the permanent from the fangs of the temporary teeth; as the period approaches in which the former are to replace the latter, this partition disappears, and the crown of the enlarged permanent tooth makes its way into the cavity of the temporary fang. As the permanent tooth advances, the fang of the milk tooth is absorbed, not however from any pressure exercised by the one upon the other, the two never coming in contact; and as the crown of the milk tooth falls off, the permanent tooth is ready to replace it.

The first anterior or true molar usually appears at about 6 1/2 years; about the same time or a few months later the central permanent incisors appear; the lateral ones are developed at 8, the anterior and posterior bicuspids at 9 and 10, the canines from 11 to 12, the second true molars from 12 to 13, and the wisdom teeth from 17 to 19. From the investigations of Mr. Edwin Saunders ("The Teeth a Test of Age, considered with reference to the Factory Children"), it would appear that the second dentition furnishes the best physical evidence of the age of children within our reach; in the majority of instances he found its indications coincided closely with the real age of the children, and when they failed the extreme deviation was but a year.