The following phenomena take place about the sixth or seventh month after birth, very rarely sooner, still more rarely before birth, though there are examples of this, as is proved by the history of Louis XIV. At first the two small incisor teeth of the lower jaw appear, sometimes together, sometimes separately; soon after the corresponding incisors of the superior jaw. A month or two later, the four other incisors are cut. At the end of the first year, the four canine teeth usually appear. At the end of the second, or often later, two molar are cut in each jaw and two others soon follow. Each cutting almost always begins in the lower jaw. At the age of four years, four and a half, sometimes five or six, always at a very uncertain period, there appear below two other molars and then two above, which complete the number of twenty-four teeth forming the first dentition; all these except the last four fall out and are replaced by new ones. The following is the mechanism of this first dentition; the ossification extending constantly towards the root, the tooth can no longer be contained in the socket; it pierces the alveolar portion of the membrane and the mucous membrane of the mouth and an intermediate medullary texture that separates them, with much ease, as this triple layer gradually becomes thinner as the cutting approaches. Is this phenomenon owing only to the mechanical pressure of the tooth? I think that there is another cause; for observe that here the membranes are very little raised before rupturing; whilst that in polypi and other tumours that sometimes arise under the membrane of the gums, it is infinitely more stretched, still it does not break, but is only lifted up. The mechanism of the opening of the gums is not more known than the principle of the severe accidents which are sometimes connected with it. The sac which formed the original membrane of the follicle being thus open, its portion which lines the socket unites to the membrane of the mouth, becomes continuous with it and at the same time adheres intimately to the neck of the tooth; and as during the development of the root, the internal face of this membranous portion, at first loose as we have seen, has gradually contracted adhesions with it, it follows that this root is found fastened between the alveolar portion which lines its exterior, and the medullary portion which covers the interior; it is this which gives it solidity. As the adhesions of the membrane increase, we can less easily distinguish it. It is rare that in the first dentition the formation of the root is finished as completely as in the second; its internal cavity remains also very broad, and the marrow is more developed. _Second Dentition considered before Cutting._