All these are hut temporary teeth ; for, as the jaws gradually extend, while the teeth, limn the hardness of their substance, admit, when fully formed, of but little increase in size, it was necessary that larger teeth should he provided, to fill up the enlarged spaces of the sock ets. For this purpose, an admirable organization has been contrived by nature within the body of the jaw bones. A second row of pulpy bodies is found at an early period in the channel between the two plates of the jaw-bones, and more deeply situated than the rudi ments of the first teeth, each enveloped in its proper membrane, and gradually advancing towards the gum, as that substance is penetrated by the first or temporary teeth. Not only is each of these second teeth furnished with a distinct investing membrane, but each of them is inclosed within a distinct socket, separate d from the sockets of the temporary teeth by bony fibres. When the rudiments of the second or permanent teeth first appear, they are nearly in contact with the temporary teeth ; but, as these latter advance beyond the gum, their sockets appear to rise, leaving the rudiments of the permanent teeth at a considerable distance. When a child has reached its fourth year, these permanent teeth are far advanced in ossification, (Plate X IX. Fig. 8 ) and, at this period, according to Dr Blake, there are more teeth formed and forming, than at any other age, as 26 may now be counted in each jaw.
The shedding of the temporary teeth generally begins when a child is about six or seven years of age, by which time the jaws are considerably extended. The temporary sockets are now gradually absorbed ; the bony partition between these and the permanent sockets is thus removed, and these latter begin to assume their proper form. The appearance of the second set of teeth takes place nearly in the same order as that of the tem porary teeth, except that the grinders are cut before the hicn.qpidrs. By the twelfth or fourteenth year, the cutting of the permanent teeth is in general completed, except with respect to the last grinders, or draft's. sapi enfix. We need scarcely remark, that, in declining age, such teeth as have not been broken, worn down, or extracted, gradually fall out; but it is extraordina:y, that. in a few cases, some few new teeth appear, even in extreme old age.
The form of the mouth is much influenced by the presence or absence of the teeth. In the infant soon after birth, the alveolar margin of the palate, and the upper edge of the lower jaw, arc very flat, and, from this circumstance, the distance between the nose and the chin is small, and the. facial angle larger than at any future period. A similar effect takes place when the teeth are lost in old age ; but, at this time, from the greater projection of the base of the lower jaw, the nose and chin appear to approach much nearer to each other.
The state of the gums, both before dentition, and after the loss of the teeth, is nearly the same. At both periods a callous rim is formed on the edges of the gum. calcu lated in infancy for pressing on the nipple, and, in old age, for supplying, in some measure, the want of teeth.
We have thus traced the progress of the teeth, from their first appearance to their final loss. We purposely avoid detailing the -ymptonts that accompany dentition, and the theories by which they have been explained, as these belong more properly to future articles. To complete our account or the organs of mastication, it remains only to notice the structure and connections of the lower jaw-bone ; but, as these have been sufficiently explained in our table of the bones, and as the motions of the lower jaw may be understood from the attach ments and functions ol its muscles, as given in the table or muscles, it is to enter more minutely into this part of the subject.
Of the morbid appearances of the organs of mastica tion, those of the teeth and gums are chiefly entitled to notice. The a eth are often irregularly placed, so as to interfere w ith each other, and require some of them to be removed ; awl, in some cases, this removal is rendered necessary, by the teeth having superfluous fangs, or fangs w ith projecting excrescences. It now and then happens, that one or more of the fangs of a tooth is wanting, uw ing to its having been absorbed in consequence of inflammation. The caries or rottenness of the teeth is too common to require particular notice ; but we may remark, that the teeth are sometimes dis coloured, and even rendered almost black, where no previous inflammation or toothach appears to have taken place. The substance called tartar, that usually collects about the neck of the teeth, when proper attention is not paid to keeping them clean, appears to be a deposition from the saliva. The gums are often spongy, and bleed on the slightest touch ; sometimes they arc ulcerated, and in a few cases affected with fungous excrescences.
For the best anatomical account of the mouth and its appendages, we may refer our readers to Bichat's .1na tomie Descriptive, tom. ii. p. 563 ; and for the best ac count of the teeth, see Albinus's .4nnotationes .lcade mica', lib. ii.; Cuvicr's Le ions d'.4natomie Comparee, tom. iii. ; Soemmering De Corpo•is Humans Fabrica, vol. i. ; the works of Hunter and Fox on the .Vaturul History of the Human Teeth ; Blake's Essay on the .S'iruct ;Ire and Formation of the Teeth in Man and vari ous ; Hoffman De Denbho• eorumque Morhis et Cura ; and Bell's ..lizatomy of the Human Lody, vol. iv.