Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/373

Rh SYSTEM.] M A M M ALIA 351 often happens, their surface wears away in mastication, it is never renewed. The open cavity at the base of the imperfectly developed rooted tooth (fig. 1, II.) causes it to resemble the persistent condition of the rootless tooth. The latter is therefore a more primitive tion, the condi- forma- tion of the root being a comple tion of the process of tooth develop ment. Function ally it is, however, difficult to say that the one is a higher form than the other, as they both serve important and different purposes in the animal economy. As is almost always the case in nature, inter mediate condi tions between these two forms of teeth are met with. Some, as the molars of the Horse, and of many Rodents, are for a time rootless, and have FIG. 1. Diagrammatic Sections of various forms of Teeth. I. Incisor of Elephant, with pulp cavity per sistently open at base. II. Human incisor during development, with root imperfectly formed, and pulp cavity widely open at base. III. Completely formed human incisor, with pulp cavity contracted to a small aperture at the end of the root. IV. Human molar, with broad crown and two roots. V. Molar of Ox, with enamel covering the crown deeply folded and with the depressions filled up with cementum. The surface is worn by use ; otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white, the dentine represented by horizontal lines, and the cc- mentum by dots. growing pulps producing a very long crown with parallel sides, the summit of which may b3 in u.se and beginning to wear away while the base is still growing, but ultimately the pulp contracts, forms a neck and distinct roots, and ceases to grow. The canine tusks of the Musk Deer and of the Walrus have persistent pulps, and are open at their base until the animal is of advanced age, when they close, and the pulp ceases to be renewed. The simplest form of the crown of a tooth is that of a cone ; but this may be variously modified. It may be flattened, with its edges sharp and cutting, and pointed at the apex, as in the laterally compressed premolars of most Garnivora, or it may be chisel- or awl-shaped, with a straight truncated edge, as in the human incisors; or it may be broad, with a flat or rounded upper surface. Very often there is a more or less prominent ridge encircling the whole or part of the base of the crown just above the neck, called the cingulum, which serves as a protection to the edge of the gum in masticating, and is best developed in flesh- eating and insectivorous animals, in which the gums are liable to be injured by splinters of bone or other hard frag ments of their food. The form of the crown is frequently rendered complex by the development upon its surface of elevations or tubercles called cusps, or by ridges usually transverse, but sometimes variously curved or folded. When the crown is broad and the ridges greatly developed, as in the molars of the Elephant, Horse, and Ox (fig. 1, V.), the interspaces between them are filled with cementum, which supports them and makes a solid compact mass of tbe whole tooth. When such a tooth wears away at the surface by friction against the opposed tooth of the other jaw, the different density of the layers of the substances of which it is composed enamel, dentine, and cementum - arranged in characteristic patterns, causes them to wear unequally, the hard enamel ridges projecting beyond the others, thus giving rise to a grinding surface of great mechanical advantage. Succession of Teeth. The dentition of all mammals con- Suoces- sists of a definite set of teeth almost always of constant and siou of determinate number, form, and situation, and, with few teeth&amp;gt; exceptions, persisting in a functional condition throughout the natural term of the animal s life. In many species these are the only teeth which the animal ever possesses. the set which is first formed being permanent, or, if accidentally lost, or decaying in extreme old age, not being replaced by others. These animals are called Monophyodont. But, in the larger number of mammals, certain of the teeth are preceded by others, which may be only of a very transient, rudimentary, and functionless character (being in the Seals, for example, shed either before or within :; few days after birth), or may be considerably developed, and functionally occupy the place of the permanent teeth for a somewhat lengthened period, during the growth and development of the latter and of the jaws. In all cases these teeth disappear (by the absorption of their roots and shedding of the crowns) before the frame of the animal has acquired complete maturity as evidenced by the coalescence of the epiphyses of the osseous system. As these teeth are, as a general rule, present during the period in which the animal is nourished by the milk of the mother, the name of &quot;milk teeth&quot; (French dents delait, German Mtlchzdkne)ha& been commonly accorded to them, although it must be understood that the epoch of their presence is by no means necessarily synchronous with that of lactations Animals which possess such teeth are called Diphyodont. No mammal is known to have more than two sets of teeth ; and the definite and orderly replace ment of certain members of the series is a process of quite a different nature from the indefinite succession which takes place in all the teeth continuously throughout the lifetime of the lower vertebrates. When the milk teeth are well developed, and continue in place during the greater part of the animal s growth, as is especially the case with the Unf/ulata, and, though to a less degree, with the Primates and Garnivora, their use is obvious, as taken all together they form structurally a complete epitome on a small scale of the more numerous and larger permanent set (see fig. 3), and, consequently, are ablo to perform the same functions, while time is allowed for the gradual maturation of the latter, and espe cially while the jaws of the growing animal are acquiring the size and strength sufficient to support the permanent teeth. Those animals, therefore, that have a well-developed and tolerably persistent set of milk teeth may be considered to be in a higher state of development, quoad dentition, than those that have the milk teeth absent or rudimentary. It is a very general rule that individual teeth of the milk and permanent set have a close rL4ationship to one another, being originally formed, as mentioned above, in exceedingly near proximity, and with, at all events as far as the enamel germ is concerned, a direct connexion. More over, as the latter ultimately come to occupy the position in the alveolar border temporarily held by the former, they are spoken of respectively as the predecessors or successors of each other. But it must be understood that milk teeth may be present which have no successors in the permanent series, and, what is far more general, permanent teeth may have no predecessors in the milk series.