TEETH (AS. ttip,Goth. tun)) us, OHG.
zan, Ger. Zahn, tooth; connected with 01r, dOt, Bret., Welsh dant, Lith. duntis, Lat. dens, Gk. 65oGs, odors, bouiv, odoa, Skt. danta, tooth, probably originally the pres. part., with loss of the initial vowel,of Skt. ad. Gk. Neu', eilein, Lat. edere, to eat, OChurch Slay. yami, Lith. edmi, edi, I eat, OIr. ith, Goth. itan, OHG. ezzan, Ger. essen, AS. etan, Eng. eat). Calcareous organs imbedded in the skin within or near the mouth in most verte brates, acting primarily for the prehension and trituration of food. Some teeth, such as the tusks of the wild boar or of the elephant, have secondarily acquired another function, namely, that of offensive and defensive organs, especially with reference to combats between rival males.
Teeth are dermal structures, homologous with the plaeoid scales or denticles which cover the entire body of fishes (q.v.). At the anterior end, say of the body of a dogfish. the skin turns in to form the lining or mucous membrane of the mouth and with it the scales are carried. In fishes these dermal denticles appear in the skin very early both in their ontogenetic and phylo genetic history, but do not occur in Amphioxus or in eyclostomes. They first appear in selaeldans in the form of small dentieles or spines arranged in longitudinal and transverse rows, each at tached to a basal plate, and directed caudad. Both ectoderm and mesoderm take part in the formation of these thermal denticles, as in the case of the teeth of the higher vertebrates. The name 'tooth-germ' is given to the tissue that is destined to develop into the teeth. The meso derm forms a papilla which secretes dentine in its outermost layer. Within the dentine there is a cavity filled with mesodermal tissue and blood vessels. From this cavity fine tubules pass into the dentine. Outside the dentine is a layer of enamel secreted by ectodermal cells. The first impulse toward tooth-formation seems to reside in the derma. As the dermal papilla grows it comes to he covered by a layer of large columnar cells from the epidermis. The enamel is secreted from the lower surface of these epidermal cells. As the dermal cells multiply, they give rise to an elongated papilla which projects backward. The deep-lying connective-tissue layers ed the derma at the same 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 bony ganoids, a hard protective skele ton is thus produced. In the higher vertebrates this bony 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 fishes the teeth are usually abundant, and are either spiny or blunt, and may be set close together like a cobble-stone pavement (see CES TRACIoNT) 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, mandible, vomer, and palatine bones, and occa sionally on the parasphenoid. They occur in additional locations in the larva 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 Trion3rx. , 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 eup-shaped sockets of the bone, the 'al veoli.' This condition exists in the so-called 'theendont' reptiles. The 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 differentiated 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, possessed teeth, which were either imbedded in grooves (Hesperornis) or in definite alveoli (Ichthyornis).