Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/254

236 236 DIGESTIVE ORGANS After the tooth has become sacculated, and coincident with the transformation of the odontoblast cells of the dental papilla into dentine, calcification begins in the elongated prismatic cells of the internal or enamel epithelium ; their protoplasm becomes calcified, Fio. 21. Vertical section through the cum in the region of the molar teeth. p, the papilla of a milk molar; 1, the inner, 2, the middle, and 3, the outer layers of the enamel organ ; n, the neck of the enamel organ ;, the superficial epithelium; ct, ct, ct, the sul-epithelial connective tissue which subsequently forms the sac of the tooth; r, the cavity of reserve occupied by epithelium, iii connection with which the permanent successional tooth is formed. X 300. and they become the rods or prisms of the enamel. As the hardening takes place from the periphery to the centre of each cell, the axial portion may, as Tomes pointed out, remain soft for some time in the axis of the enamel rod. With the increase in length, and with the calcification of the cells of the enamel epithelium, the stellate gelatinous cells disappear, and the outer ends of the enamel rods come in contact with the cells of the external enamel epithelium. By some observers the external epithelium is sup posed to disappear without undergoing any special differentiation, but by others it is believed to undergo conversion into Nasmyth s membrane. In this manner the crown of a tooth is formed, and it is lodged in a membranous sac formed by the differentiation into a fibro- vascular membrane of the surrounding connective tissue. Whilst within its sac, the crown of the tooth possesses the characteristic, form of the group of teeth to which it belongs. After the calcifica tion of the enamel rods is completed, it can undergo no further change either in shape or in increase of size. Whilst the crown of the tooth is being formed, ossification of the jaws has been going on, and the tooth, with its membranous sac, has become lodged in an alveolus or socket in the jaw, which alveolus is closed in by the gum. In order that the crown of the tooth may come into use as a masticatory organ, it has to be elevated to the level of the gum, which is absorbed by the pressure, and the crown then erupts into the cavity of the mouth. The process of eruption is due to the development of the fang, which, as it grows in length, elevates the crown of the tooth and forces it outward. The dentine of the fang is developed from the odontoblast cells of the pulp in a manner similar to that already described for the development of the dentine of the crown. The cement or crusta petrosa i.-j developed from the connective tissue lining the alveolus, which forms the alveolo-dental periosteum. It is therefore an ossifica tion in membrane. As the temporary or milk teeth precede the permanent teeth, their papilloe are naturally the first to form. The series of milk- papillae are not, however,, simultaneously produced. From the observations of Goodsir, it has been shown that the milk-papilla of the anterior molar in the upper jaw appears about the seventh week ; then the canine papilla, the two incisor papillae, and the posterior molar papilla are sucessively formed, the last making its appear ance about the end of the tenth week. The dental papillae in the upper jaw immediately precede the papilloe of the corresponding teeth in the lower jaw. The eruption of the milk teeth into the mouth does not begin to take place until the latter half of the first year of extra-uterine life, and is not completed until betweccn the second and third year. Though variations occur in the date of eruption of each tooth in different children, it may be stated that the incisors usually appear from the seventh to the ninth month, the anterior molars from the twelfth to the sixteenth month, the canines during the seven teenth or eighteenth month, the posterior milk molars from two to two and a half years. The milk teeth begin to be shed about the sixth year by the dropping out of the incisors. The last to be shed are the canines, which do not fall out till the tenth or eleventh year. The shedding of the milk teeth is preceded by the absorption of the fangs. This is effected, as was satisfactorily shown by J. Fio. 22. One-half the lower jaw of afortu* about the llth or 12th week, showing the dental papillae in the order of their appearance. 1, the first milk molar: i the canine; 3 and 4, the two incisors; 5, the second milk molar. From Goodsir. FIG. 23. Posterior part of the lower jaw of a child at birth. 5, the crown and sac of the posterior milk molar; 6, the crown and sac of the first permanent molar; 6, the cavity in connection wilh which the papilla of the second per manent molar ultimately forms, y, shows a temporary and permanent incisor from the same foetus. From Goodsir. Tomes, by the agency of a group of cells situated at the bottom of the sockets. As these cells occasion absorption of the tooth tissue, similar to that occurring in the bone tissue from the action of the large multi-nucleated osteo-klast cells, they may appropriately be called odonto-klnsts. The development of the permanent teeth will now be considered. In the description of the arrangement of the teeth it has been pointed out that the number of teeth in the permanent set exceeds that of the temporary set. The permanent incisors and canines come into the place of the temporary incisors and canines, and the permanent bicuspids succeed the temporary molars, but the permanent molars have no milk predecessors, and are superadded at the back of the dental series. The development of the successional permanent teeth, which are the ten anterior teeth in each jaw, will first be examined. Prior to the period when the lips of the primitive dental groove- meet, to produce the saccular stage of dentition of the severa/ temporary teeth, an indentation, or furrow, takes place in the connec tive tissue adjoining the string of epithelial cells which form the neck of the enamel organ. This furrow constitutes what Goodsir termed the cavity of reserve, and it is filled up by epithelial cells continuous with the epithelium of the neck of the enamel organ. As a cavity of reserve is formed immediately behind (i.e., on tbe lingual side of) each milk tooth, they are ten in number in each jaw, and, except that for the anterior molar, are formed successively from before backwards. The cavities of reserve are concerned in the production of the per manent successional teeth, and each temporary tooth is replaced by the permanent tooth formed in connection with the cavity of re serve situated immediately behind it (fig 21). The cavities of re serve become elongated, and widened, and pass above the tem porary teeth in the upper jaw, and below those in the lower jaw. At the bottom of each a dental papilla forms, the apex of which indcntates and becomes covered by the epithelium contained in the cavity, which forms a cap for the papilla, and constitutes the enamel organ for the permanent tooth. The cavity becomes completely closed by the growth of the surrounding connective tissue, and the embryo permanent tooth becomes sacculated. The process of calcification then goes on, in both the enamel organ and dental papilla, in a man ner similar to that already described in the temporary teeth. The permanent teeth then become lodged in sockets in the jaw distinct from those of the temporary teeth. The sac of each permanent tooth re mains connected with the fibrous tissue of the gum by a slender fibrous band, or gubcrnaculum, which passes through a hole in the jaw immediately behind the corresponding milk tooth. Before the successional permanent tooth erupts, not only should the temporary tooth be shed, but the bony partition between their respective sockets must bo absorbed. The superadded permanent teeth, or permanent molars, three in number on each side, lie behind the successional teeth. Their mode of origin is similar to that of the temporary teeth. The primitive groove, occupied by an involution of the epithelial cover ing of the gum, is prolonged backwards. Three dental papilla successively appear at the bottom of this groove, and the epithelium covering each papilla forms its enamel organ. Legros and Magitot, however, state that the second permanent molar arises in connection with a diverticulum (cavity of reserve) proceeding from the epithelial string of the enamel organ of the first permanent molar, and that the wisdom tooth is formed in connection with a similar diverticulum from the second permanent molar. The embryo tooth becomes sacculated, and goes through the process of calcification similar to what has been described in the other teeth