The dentition of mammals is highly variable, and the differentiation of the teeth in the various forms is to be explained by adaptation to the character of the food. All the teeth are im bedded in alveoli of the jaw-bones, to which the growth of teeth is confined. The teeth are so arranged that those of the upper and lower jaws alternate. The teeth of one jaw oppose the inter spaces between the teeth of the other jaw. Thus each tooth presses against a portion of two teeth of the opposite act. When all opposing pressure fails the tooth thus rendered functionless tends to be extruded. This alternating arrangement pre vents the loss of one tooth from greatly affecting the teeth of the opposite set. Most mammals are `diphyodont'—that is, possess two sets of dentition—namely the deciduous or 'milk' den tition and the permanent teeth. In most of the fishes as well as in many of the Ampbibia and reptiles the teeth can be renewed indefinitely. A few mammals, namely the Cetacea and Edentata, are `monophyodont,' that is, possess only one set of teeth. The milk teeth are supplanted by a process of absorption which goes on in certain cases to such an extent that the root is prac tically removed. This removal is effected by gi ant cells analogous to, if not identical with, the osteoclasts that tear down bone. The teeth usu ally cease to grow after being fully formed, hut in edentates and rodents growth continues throughout life, and is necessary in order to sup ply the wear on the teeth brought about by the gnawing habits of these animals.
Mammalian teeth are classified according to their shape and function into incisors, canines, premolars, and molars. The four front teeth in the case of man are blunt., flattened, and chisel shaped, and are fitted for cutting or dividing the food. They are the Incisors.' In recently cut incisors the cutting edge is raised in three scal lops, which soon wear away. The edge of the incisor is formed by a beveling off of the dentine on the inner or lingual surface. The two central upper incisors in man are slightly larger than the two laterals. This difference is more marked in anthropoid apes. The lower central incisors are narrower than those of the upper jaw. The lower lateral incisors are larger than the lower central pair. The two canines or 'eye-teeth' on each jaw are stronger both in root and crown than the incisors. The crown is blunt and pointed. The canines are succeeded by two pre molars or 'bicuspids' on each side of the jaw, which correspond to the three and four premo lars of the typical mammalian dentition. Their crowns possess two cusps. The root is' normally single, but is often grooved or cleft by a longitu dinal furrow into two or even three roots. The premolars of the lower jaw are smaller than those of the upper. The molars are three in number on each side of the jaw. The form of the first molar is the most constant. The roots are three in number. The third molar, or 'wisdom tooth,' is the most variable as to its time of appear ance; it may be cut almost as soon as the other molars, or many years later, or not at all.
In existing mammals there is a certain degree of gradation between the forms of the teeth. The typical number of mammalian teeth is 44. The human number is 32 in the permanent set and 20 in the milk set. In the human set the third incisor and the first two premolars of the typical mammalian set are absent. hence at the canines in man there is an abrupt change both in front and behind. For convenience and brevity the number and kinds of teeth of mammals are usu ally designated by means of a formula as follows: For marsupials: i c I- pm 3 = 44.
For eutherian mammals: 44.
For man: i c pm 3 la 3 = 32.
It is often very difficult to determine which teeth are to lie accounted incisors, canines, etc., and the following rule is sometimes employed: Incisors, upper jaw, on premaxilla.
Incisors, lower jaw, correspond to upper. Canines of upper jaw, next tooth immediately behind suture of maxilla and premaxilla. Canines of lower jaw, close in front of upper canine.
Premolars replace milk molars.
Molars, behind milk molars (when present).
A tooth is usually described as consisting of three parts—the crown, neck, and root—and is composed of three ]lard secretions—the dentine, enamel, and cement—and a central pulp cavity. The human tooth fits into the jaw-bone much like a peg into its socket. This manner of union is termed 'gomphosis.' The tooth does not, how ever, occupy the entire socket, for there is be sides a lining of periosteum, which at the root turns upward around the tooth as far as the neck, where it becomes continuous with the gums. This packing around the tooth is some what elastic and allows of slight motion. The elasticity of the packing doubtless diminishes the shock which would be caused in mastication were the teeth rigidly attached in their bony sockets. When the periosteum is inflamed the swelling pushes the tooth outward in the socket and loosens it. The dentine forms the great mass of the tooth, surrounds the pulp cavity, and is covered, in the human tooth by a layer of enamel, the hardest of animal tissues; in other mammals this layer of enamel may cover all or only a part of the exposed portion of the tooth, or may be absent. The condition that more commonly prevails is that the enamel envelops the entire crown of the tooth and stops rather abruptly at about the level of the gum.
In its finer structure dentine is in some cases difficult to distinguish from true bone. Typical dentine, such as exists in human teeth, differs from bone both in structure and in chemical composition, and is known as 'hard' or `unvasen lar' dentine. It is hard, elastic substance with a yellowish white hue, which is made up of a number of minute, wavy, branching tubules, parallel to one another and opening internally into the pulp cavity. These tubuli contain finely cylindrical prolongations (dentinal fibres) from cells in the pulp cavity, intimately connected with the nerves of the pulp. The different kinds of dentine have been classified as (I) hard or unvascnlar dentine; (2) plici-dentine; (3) vaso-dentine; (4) osteo-dentine. The unvascu lar dentine has been described above. In the second class the pulp cavity is irregular in out line, with many tubes and canals traversing the dentine in various directions. some carrying blood vessels. Such a condition obtains in many of the lower vertebrates and in some mammals. The osteo-dentine closely approaches bone in struc ture. In it pulp and calcified tissue are mixed up, for there is no distinct pulp cavity. Such dentine is found in the teeth of many sharks.
The cement forms 'a coating over the roots of the teeth and when unusually thick may even unite the roots of contiguous teeth. In the hu man teeth it extends upward to cover the edge of the enamel. it covers, at first, the entire teeth of certain vertebrates, such as the elephant, and covers the crowns of the teeth of ruminants. It is derived from the tooth-follicle and is closely allied to hone.
The cavity of the tooth is filled with the tooth pulp, Nrhiell is composed of a gelatinous matrix and contains cells, nerves, and blood-vessels. The outermost cells, known as `odontobtasts,' form a layer next the bone. These cells are con nected by processes with one another and with deeper lying cells, and on the periphery the pro cesses extend into the dental tubuli.
The gums are continuous with the mucous membrane of the month, but are much thickened. Within the grans are tendinous faciculi, and others extend up into them from the periosteum.
These give the gums hardness. They are richly supplied with blood-vessels, but have few nerves. The gums are continuous with the periosteum of the alveoli, a connective tissue richly supplied with blood-vessels and nerves.
BIBLIOGRAPHY. Tomes, Manual of Dental AnBibliography. Tomes, Manual of Dental An- atomy, Human and Comparative (London, 1876); Owen, Odontography (London, 1840-45). See INTEGUMENT ; SKELETON ; DENTISTRY.