Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/372

Rh MAMMALIA [DENTAL Elephant, it is a structure which plays a very important part, covering and filling in the interstices between the folds of the enamel. In appearance, hiatological structure, and chemical composition it is closely allied to osseous tissue, containing lacunae and canaliculi, though only when it is of considerable thickness are Haversian canals present in it. Develop- Development of the Teeth. The two principal constituents meiit. O f the teeth, the dentine and the enamel, are developed from the two layers of the buccal mucous membrane, the dentine from the submucous, the enamel from the epithelial layer. The latter dips down into the substance of the gum, and forms the enamel organ or germ, the first rudiment of the future tooth, which is constantly present even in those animals in which the enamel is not found as a constituent of the perfectly-formed tooth. Below the mass of epithelial cells thus embedded in the substance of the gum, and re maining connected by a narrow neck of similar structure with the epithelium of the surface, a portion of the vascular submucous areolar tissue becomes gradually separated and defined from that which surrounds it, and assumes a distinct form, which is that of the crown of the future tooth, a single cone in the case of simple teeth, or with two or more eminences in the complex forms. This is called the dental papilla or dentine germ, and by the gradual conversion of its tissue into dsntine the bulk of the future tooth is formed, the uncalcified central portion remaining as the pulp. The conversion of the papilla into hard tissue com mences at the outer surface of the apex, and gradually proceeds downwards and inwards, so that the form of the papilla exactly determines the form of the future dentine, and no alteration either in shape or size of this portion of the tooth, when once calcified, can take place by addition to its outer surface. In the meanwhile calcification of portion of the cells of the enamel organ, which adapts itself like a cap round the top of the dentinal papilla, and has assumed a somewhat complex structure, results in the formation of the enamel coating of the crown of the tooth. While these changes are taking place the tissues immediately surrounding the tooth germ become condensed and differen tiated into a capsule, which appears to grow up from the base of the dental papilla, and encloses both this and the enamel germ, constituting the follicle or tooth sac. By the ossification of the inner layer of this follicle, the cementum is formed. This substance therefore, unlike the dentine, increases from within outwards, and its growth may therefore be the cause of considerable modi fication of form and enlargement, especially of the roots, of certain teeth, as those of Seals and some Cetacea. The delicate homogeneous layer which coats the enamel surface of newly-formed teeth, in which cementum is not found in the adult state, and known as Nasmyth s membrane, is considered by Tomes as probably a film of this substance, too thin to exhibit its characteristic structure, though by others it is believed to be derived from the external layer of the enamel organ. The homology of the teeth with the dermal appendages, hairs, scales, and claws has already been alluded to, arid it will now be seen that in both cases two of the primary embryonic layers are concerned in their development, the mesoblast and epiblast, although in very different proportions. In the hair or nail the part derived from the epiblast forms the principal bulk of the organ, the mesoblast only constituting the papilla or matrix. In the tooth the epiblastic portion is limited to the enamel, always of relatively small bulk and often absent, while the dentine (the principal constituent of the tooth) and the cementum are formed from the mesoblast. When more than one set of teeth occur in mammals, those of the second set are developed in a precisely similar manner to the first, but the enamel germ, instead of being derived directly from an independent part of the oral epithelium, is formed from a budding out of the neck of the germ of the tooth succeeded. In the case of the true molars which have no predecessors, the germ of the first has an independent origin, but that of the others is derived from the neck of the germ of the tooth preceding it in the series. The foundations of ths permanent teeth are thus laid as it were almost simultaneously with those of their predecessors, although they remain in many cases for years before they are developed into functional activity. Although the commencement of the formation of teeth takes place at an early period of embryonic life, they are in nearly all mammals still concealed beneath the gum at the time of birth. The period of eruption, or &quot; cutting &quot; of the teeth as it is called, that is, their piercing through and rising above the surface of the mucous membrane, varies much in different species. In some, as Seals, the whole series of teeth appear almost simultaneously ; but more often there are considerable intervals between their appearance, the front teeth usually coming into place first, and those at the back of the mouth afe a later period. 1 General Characters. The simplest form of tooth may be Forms ol exemplified on a large scale by the tusk of the Elephant teeth. (fig. 1, I.). It is a hard mass almost entirely composed of dentine, of a conical shape at first, but during growth becoming more and more cylindrical or uniform in width. The enamel covering, present on the apex in its earliest condition, soon disappears, but a thin layer of cementum covers the circumference of the tooth throughout life. On section it will be seen that the basal portion is hollow, and contains a large conical pulp, as broad at the base as the tooth itself, and deeply imbedded in the bottom of a recess or socket in the upper maxillary bone. This pulp continues to grow during the lifetime of the animal, and to be converted at its surface into dentine. The tooth therefore continually elongates, but the use to which the animal subjects it in its natural state causes the apex to wear away, at a rate generally proportionate to the growth at the base, otherwise it would become of inconvenient length and weight. Such teeth of indefinite growth are said to be &quot;rootless,&quot; or to have &quot;persistent pulps.&quot; One of the corresponding front teeth of man (fig. 1, II. and III.) may be taken as an example of a very different condition. After its crown is fully formed by calcification of the germ, the pulp, though continuing to elongate, begins to contract in diameter ; a neck or slight constriction is formed ; and the remainder of the pulp is converted into the root (or fang), a tapering conical process which is imbedded in the alveolar cavity of the bone, and has at its extremity a minute perfora tion, through which the vessels and nerves required to maintain the vitality of the tooth enter the pulp cavity, very different from the widely open cavity at the base of the growing tooth. When the crown of the tooth is broad and complex in character, instead of having a single root, it may be supported by two or more roots, each of which is implanted in a distinct alveolar recess or socket, and to the apex of which a branch of the common pulp cavity is continued (fig. 1, IV.). Such teeth are called &quot;rooted teeth.&quot; When they have once attained their position in the jaw, with the neck a little way above the level of the upper margin of the alveolus, and embraced by the gum or tough fibro-vascular membrane which covers the alveolar border, and having the root fully formed, they can never increase in length or alter their position. If they appear to do so in old age it is only in consequence of absorption and retrocession of the surrounding alveolar margins. If, as 1 See the conclusion of the article DIGESTIVE ORGANS, vol. vii. p. 233 sq., for a more detailed and illustrated account of the structure and development, especially of the human teeth.