494 OF NUTRITION. 



backwards and upwards, into the maxillary tuberosity; and thus it not only draws the 

 surface of the gum in the same direction, but lengthens out the great cavity of reserve 

 (Fig. 131 d). During the few months which succeed birth, however, the jaw is greatly- 

 lengthened; and when the infant is eight or nine months old, the Anterior Permanent 

 Molar resumes its former position in the posterior part of the dental arch: and the great 

 cavity of reserve returns to its original size and situation (c). This cavity, however, 

 soon begins to bulge out at its posterior side, and projects itself, as a sac, into the max- 

 illary tuberosity (/); a papilla or pulp appears in its fundus; and a process of con- 

 traction separates it from the remainder of the cavity of reserve. Thus the formation 

 of the Second Permanent Molar from the first, takes place on precisely the same plan 

 with the formation of the Permanent Bicuspids from the Temporary Molars. The new 

 sac at first occupies the maxillary tuberosity (g) ; but the lengthening of the jaw gradually 

 allows it to fall downwards and forwards, into the same line, and on a level, with the rest 

 (A). Before it leaves the tuberosity altogether, the posterior extremity of the remainder 

 of the cavity of reserve sends backwards and upwards its last offset the sac and pulp 

 of the Wisdom-tooth (t); this speedily occupies the tuberosity after the second molar has 

 left it (j) ; and ultimately, when the jaw lengthens for the last time, at the age of nine- 

 teen or twenty, it takes its place at theposterior extremity of the range of the adult teeth (k). 

 Thus, the Wisdom-teeth are the second products of the posterior y great cavities of 

 reserve ; and the final effects of development in the secondary dental groove. In the 

 Elephant, in which there is a continual new production of molar teeth at the back of 

 the jaw, it is probable that from each sac a cavity of reserve is formed, which produces 

 the succeeding tooth; and thus the only essential difference between its dentition and 

 that of Man, consists in the degree of continuance of this gemmiparous process ; which 

 ceases in Man after being twice performed, but is repeated in the Elephant until nearly 

 the close of its life. 



e. We have thus sketched the history of the Development of the Teeth, up to the time 

 when they prepare to make their way through the gum. The first stage of this deve- 

 lopment may be termed the papillary ,- and the second, the follicular. The latter termi- 

 nates when the papillae are completely hidden by the closure of the mouths of the folli- 

 cles, and of the groove itself. The succeeding stage, which has long been known as the 

 saccular,is the one during which the whole formation of the Tooth-substance, and of the 

 Enamel, takes place. It is during this period, also, that the ossification of the jaw is 

 taking place; and that the bony sockets are formed for the teeth, by the consolidation of 

 the anterior and posterior ridges bounding the alveolar groove (in which the dental groove 

 was originally imbedded), and of the interfollicular septa, which are produced by the 

 meeting of transverse projections from these ridges. The history of development in the 

 Lower Jaw is very nearly the same; the chief difference being in the origin and situation 

 of the primitive dental groove. 



/. We have now only to consider the fourth or eruptive stage, that in which the Teeth 

 make their way through the gum. This process chiefly results from the lengthening of 

 the fang, by the addition of a new bony matter; and the crown of the tooth is thus 

 made to press against the closed mouth of the sac (Fig. 130, m). This at last gives way, 

 so that the sac assumes its previous condition of an open follicle. When the edge of 

 the tooth has once made its way through the gum, it advances more rapidly than can 

 well be accounted for, by the usual rate of lengthening of its fang; and this appears to 

 be due to the separation of the bottom of the sac from the fundus of the alveolus ; so 

 that the whole tooth-apparatus is carried nearer to the surface, leaving a space at the 

 bottom of the alveolar cavity, in which the further lengthening of the root can take 

 place (n). The open portion of the sac remains as the narrow portion of the gum, 

 which forms a vascular border and groove round the neck of the perfected tooth (o). The 

 deeper portion of the sac adheres to the fang of the tooth, and is converted by ossifi- 

 cation into the Cementum or Crusta Petrosa ( 634). What is commonly denominated 

 the Periosteum of the Tooth, really belongs as much to the Alveolus. It is connected 

 with the tooth by the submucous cellular tissue, which originally intervened between the 

 tooth-sac and the walls of the osseous cavity. It appears from Mr. Nasmyth's researches, 

 that the inner layer of the portion of the capsule, which covered the crown of the tooth, 

 remains adherent to it ; forming a thin coating of Crusta Petrosa (most of which is, 

 however, soon worn off) over the Enamel. During the period that the Milk-teeth have 

 been advancing, along with their sockets, to their perfect state and ultimate position, the 

 Permanent sacs have been receding in an opposite direction, and have with their bony 

 crypts been enlarging; and at last they occupy a position almost exactly below the 

 former (n and o). They still retain a communication with the gum, however; the 

 channel by which they descended not having completely closed up, and the neck of the 

 sac being elongated into a cord which passes through this. The channels may after- 



