The matrix of the Mammalian tooth sinks into a furrow and soon becomes inclosed in a cell in the substance of the jaw-bone, from which the crown of the growing tooth extri cates itself by exciting the absorbent process, whilst the cell is deepened by the same pro cess and by the growth of the jaw into an alveolus for the root of tne tooth. Where the formative parts of the tooth are repro duced indefinitely to repair by their progres sive calcification the waste to which the work ing surface of the crown of the tooth has been subject, the alveolus is of unusual depth, and to its bottom or base. In teeth of limited growth, the dentinal pulp is reproduced in progressively decreasing quantity after the completion of the exterior wall of the crown, and forms by its calcification one or more roots or fangs, which taper more or less ra pidly to their free extremity. The alveolus is closely moulded upon the implanted part of the tooth ; and it is worthy of special remark that the complicated form of socket which results from the development of two or more fangs is peculiar to animals of the class Mam malia.
In the formation of a single fang the ac tivity of the reproductive process becomes enfeebled at the circumference, and is pro gressively contracted within narrower limits in relation to a single centre, until it ceases at the completion of the apex of the fang ; which, though for a long time perforated for the admission of the vessels and nerves to the interior of the tooth, is, in many cases, finally closed by the ossification of the remaining part of the capsule.
- When a tooth is destined to be implanted by two or more fangs, the reproduction of the pulp is restricted to two or more parts of the base of the coronal portion of the pulp, around the centre of which parts the sphere of its reproductive activity is progressively contracted. The intervening parts of the base of the coronal pulp adhere to the cap sule, which is simultaneously calcified with them, covering those parts of the base of the crown of the tooth with a layer of cement. The ossification of the surrounding jaw being governed by the changes in the soft, but highly organised, dental matrix, fills up the spaces unoccupied by the contracted and divided pulp, and affords, by its periosteum, a surface for the adhesion of the cement or ossified capsule covering the completed part of the tooth.
The matrix of certain teeth does not give rise during any period of their formation to the germ of a second tooth, destined to suc ceed the first ; this, therefore, when com pleted and worn down, is not replaced : all the true Cetacea are limited to this simple provision of teeth. In the Armadillos, Me gatherioids, and Sloths, the want of germi native power, as it may be called, in the matrix is compensated by the persistence of the matrix, and by the uninterrupted growth of the teeth.
In most other Mammalia, the matrix of the first developed tooth gives origin to the germ of a second tooth, which sometimes dis places, sometimes takes its place by the side of, its predecessor and parent. All those teeth which are displaced by their progeny are called temporary, deciduous, or milk teeth ; the mode and direction in which they are displaced and succeeded,— viz., from above downwards in the upper, from below upwards in the lower, jaw ; in both jaws vertically—are the same as in the Crocodile ; but the process is never repeated more than once in any mammiferous animal. A con siderable proportion of the dental series is thus changed; the second, or permanent teeth, having a size and form as suitable to the jaws of the adult, as the displaced tem porary teeth were adapted to those of the young, animal. The permanent teeth, which assume places not previously occupied by deciduous ones, are always the most poste rior in their position, and generally the most complex in their form. The successors of the deciduous incisors and canines differ from them chiefly in size ; the successors of the deciduous molars may differ likewise in shape, in which case they have always less complex crowns than their predecessors.* The " bicuspids," in Human Anatomy, and the corresponding teeth, called " pre molars," in the lower Mammals, illustrate this law.
The first true molar owes the germ of its matrix to a vegetation or bud, separated by the fissiparous process from the matrix of the last deciduous tooth ; but the backward elongation of the jaw affords space for its development by the side of its progenitor, during which process it may in like manner give origin to a second, and this to a third, molar, succeeding each other from before backwards or horizontally.
In this successive germ-production, we find repeated the multiparous property of the dental matrix of the crocodile; but the concomitant growth of the jaw allows the second, third, and sometimes fourth genera tion of true molars to co-exist, and come into place side by side. In the unguiculate, and most of the Ungulate, species of the placental division of the Mammalian class, the fissiparous reproduction of horizontally succeeding teeth stops at the third genera tion; in other words, they have not more than three true molars on each side of the upper and lower jaws. In the Marsupial series, the same process extends to a fourth generation of true or horizontally succeeding molars* ; and in most of the species, the four true molars are in use and place at the same time ; but in certain Kangaroos, the anterior ones are shed before the posterior ones are developed. This successive de cadence is still more characteristic of the grinding teeth of the Elephant, which are finally reduced to a single molar tooth on each side of both jaws.