TEETH. derma at the sume time secrete salts of lime and thus a sort of connective-tissue bone (the so-called 'tooth-cement') is produced, and by means of it the spine gains a firm support. The basal plates of tooth-cement form a shield of considerable protective value, and in some fishes, such as the bonj' ganoids, a hard protective skele- ton is thus produced. In the higher vertebrates this bon}' covering is lost over most of the body, but persists in the head region, and becomes some of the most important bones of the skull. In fislies the teeth are usually abundant, and are either spiny or blunt, and may be set close together like a cobble-stone pavement (see Ces- TBACIONT) to fit them for grinding up shells. In the swordfish they form the teeth of the sword. Amphibians possess few teeth. They are conical, end in a single or double apex, and are usually present on the premaxilla, maxilla, mandible, vomer, and palatine bones, and occa- sionally on the parasphenoid. They occur in additional locations in the larvae of some forms. Teeth are present in all groups of reptiles ex- cept the Chelonia, and even there exist for a time in the embryonic stage of Trionyx, showing that the toothless state has been secondarily ac- quired by the turtles. The reptilian teeth are usually larger and stronger, and are limited to more special regions, than among lower verte- brates. The teeth of reptiles, as well as those of fishes, are in general 'homodont,' or alike throughout the series. In reptiles the teeth are connected with their support in two ways : either the teeth are firmly fused to the bone, or else they lie in cup-shaped sockets of the bone, the 'al- veoli.' This condition exists in the so-called 'thecodont' reptiles. Tlie teeth, which are fused to the bone may either lie in a furrow at the inner side of the jaw-bone ('pleurodont' type) or else they arise from its free upper edge (in the case of the lower jaw) , and are 'acrodont.' In many kinds of snakes certain of the maxillary teeth of the upper jaw are difl'erentiated into organs of offense and defense. These are the 'fangs' or poison-carrying teeth. (See Snake.) Birds have no teeth. Fossil birds (q.v.), how- ever, p'ossessed teeth, which were either imbedded in grooves (Hesperornis) or in definite alveoli (Ichthyornis) . 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 set. 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 Amphibia 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 85 TEETH. 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, but 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 ease 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 f c ^ pm -| m I := 44. For eutherian mammals: t I c I pm I m I = 44. For man:
- f " T P"* f '" I ^^ 22.
It is often very difficult to determine which teeth are to be 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 hard secretions — the dentine.
