Teeth, the organs in vertebrates for the seizure and mastication of food, placed at or near the entrance to the alimentary canal. In adult man there are 32, 16 in each jaw, implanted in sockets, and of an irregular conoid form; in the child, previous to the second dentition, there are only 20. For their development see Dentition. The number of the teeth increases in the lower animals, being greatest in the cetaceans and marsupials among mammals, and also considerable in many reptiles and fishes. The portion of a tooth above the socket is called the crown, the concealed part the root or fang; between these there is a more or less marked constriction or neck. In vertebrate animals the teeth, like the bones, have for their earthy basis phosphate of lime, mingled with some carbonate of lime and a certain proportion of fluoride of calcium. The latter substance is more abundant in the enamel of the teeth than elsewhere, but everywhere phosphate of lime is the main ingredient upon which the teeth depend for their solidity and firmness. - A tooth is composed of three different tissues, dentine, crusta petrosa, and enamel.

The dentine, forming the greater part of the body of the tooth, consists of a firm, transparent, nearly homogeneous substratum, composed of about 72 per cent, of calcareous matter and 28 per cent, of organic substance. It is permeated throughout by minute cylindrical channels, called canaliculi, about 1/12,000 of an inch in diameter, which radiate from a central cavity contained in the tooth, called the pulp cavity, toward the external surface of the dentine. Daring their course the canaliculi branch and divide, often several times in succession, becoming thus very much reduced in size and at the same time increased in number. In the central cavity of the dentine is contained the pulp of the tooth, a soft, vascular, and sensitive papilla, the only portion of the tooth which is supplied with blood vessels and nerves. Undoubtedly the canaliculi of the dentine are either channels for the absorption of nutritious fluids from the pulp, or are filled with soft filaments composed of organic material, by which this absorption is accomplished. The crusta petrosa is a thin layer of bony tissue attached to the outside of the dentine in the fang of the tooth, and serving to connect it, by means of its periosteum, more firmly to the socket.

It differs but little from compact bony tissue elsewhere, except that it contains no blood vessels, and is distinguished only by the presence of the irregularly shaped bone corpuscles, which are connected by their radiating filaments with the extremities of the canaliculi of the dentine. The enamel, which covers the surface of the crown of the tooth, is much the hardest of its tissues, containing often over 95 per cent, of calcareous matter. It appears to consist of superimposed layers of calcified epithelium, and is well adapted, by its extreme solidity and almost crystalline texture, to endure the attrition of foreign substances without disintegration. - Three kinds of teeth are distinguishable in mammals, viz., incisors, canines, and molars. The incisors are in the front and median portion of the jaws, and have a simple flattened root and a thin cutting edge, suitable for dividing and collecting food, as in the jaws of the beaver and squirrel and in the lower jaw of the ox. The canines, four in number, are next to the incisors, separated from them by an interval, except in man; the crown is conical, and the root long and simple.

They are the so-called eye and stomach teeth in man, and form a striking characteristic and formidable weapons in the carnivora; they are best adapted for securing and tearing living prey. The molar teeth are the most posterior, and have flattened and tuberculous crowns suited for grinding down vegetable food; they are most developed in herbivorous animals; the roots in man are often much bifurcated, rendering extraction difficult. - Teeth arc so intimately related to the food and habits of animals, so easily examined, and of such indestructible materials, that they are of the first importance in the classification of animals, both living and fossil. When fully formed they are subject to decay, but have no inherent power of reparation; they may increase by abnormal growth of the crusta petrosa, their most highly organized constituent. For the diseases and the mode of treatment of the teeth, see Dentistry. - In fishes the teeth vary from none in the sturgeon and lopho-branchs to countless numbers in the pike and the siluroids.

They are usually conical, but sometimes flattened or pavement-like, villi-form, serrated, and cutting; they may be situated on any of the bones of the oral cavity, on the tongue, and in the pharynx; in most cases they are firmly united to the jaws by continuous ossification, but in some are movable; they are composed of dentine and its modifications, enamel occurring in only a few cases, like the parrot fish (scarus); and they are frequently shed and renewed, the germs being developed from the free surface of the buccal membrane. Among reptiles, the whole order of chelonians (tortoises and turtles), and also the toad family among batrachians, are without teeth. In the others these organs are usually simple, and adapted for seizing and holding but not chewing their food; the number is never so small nor so large as in fishes, and is rarely characteristic of species. They are generally conical, sharp, and smooth, and may be placed on any of the bones entering into the structure of the mouth; the base, never branches into diverging fangs, and in most is anchylosed in various ways to the bone which bears them, as noticed under the different families; dentine and ce- I ment arc always present, and sometimes enamel, as in the saurian crown.

Among mammals, sonic of the edentates, as ant-eaters and pangolins, have no teeth; in the others they | are implanted in sockets, and the molars have I two or more roots when they have a limited growth; they arc confined to the superior, inferior, and intermaxillary hones, a single row in each. Mammals have been divided by Owen into monophyodonts, or those which generate a single set of teeth, and diphyodonts, or those which generate two sets of teeth; the former include the monotremes, edentates, and carnivorous cetaceans, and the latter all the other orders. - For full details on this subject the reader is referred to the following writings of Prof. Richard Owen: "Odontography" (London, 1840-'45); article " Teeth " in vol. iv. of the "Cyclopaedia of Anatomy and Physiology " (1852); and "The Principal Forms of the Skeleton and Teeth," in vol. i. of Orr's "Circle of Sciences" (London; reprinted in Philadelphia, 1854).