TEETH, hard structures developed in the mouth and adjacent parts of vertebrated ani mals and concerned in the obtaining and masti cation of food and secondarily in a variety of other functions. aThey present,' says Owen, "many varieties as to number, size, form, struc ture, position and mode of attachment, but are principally adapted for seizing, tearing, divid ing, pounding or grinding the food. In some species they are modified to serve as formidable weapons of offense and defense; in others, as aids in locomotion, means of anchorage, instru ments for uprooting or cutting down trees or for transport and working of building materials.
- They are characteristic of age and sex; and in man they have secondary relations, subservient to beauty and to speech. Teeth are always inti mately related to the food and habits of the animal and are, therefore, highly interesting to the physiologist; they form, for the same reason, important guides to the naturalist in the classification of animals.' For further infor mation as to the development of the varied forms of teeth in relation to use, see KINE TOGENESIS.
Teeth are a production of skin, and homolo gous with the scales of fishes, and various other hardenings of the surface; in some of the fishes and lower invertebrates there is an insensible gradation from one to the other. In one large class, the birds (q.v.), they are now wholly absent, their functions being performed by the horny covering of the jaws (bill) or by the giz zard, or both; but in the earliest extinct birds they were present in both jaws and had a close resemblance to those of reptiles. Turtles, also, and many amphibians are toothless; and in some of the inferior mammals teeth are present only as embryonic rudiments, which disappear before or soon after birth. Many small, sharp hardened structures in the worms, echinoderms, mollusks, insects and other invertebrate animals, which are more or less concerned in biting, are popularly spoken of as teeth, but, strictly speak ing, should be otherwise designated. In general the present article will treat of the teeth as found in the mouth of man and the higher vertebrates, where they arise from the gum or covering of the jaw-bones, each rooted in a socket or sockets of its own formed by the alveolar processes of the maxillary bone.
Structure of Teeth.-- A tooth begins, early in embryonic life, by the development from the mucous membrane of the gum of a group of modified epithelial cells which dip down into the substance of the gum and form an organ, the germ of the tooth, which will furnish the enamel needed. Below that there next develops
a mass of special tissue which takes the shape of the future tooth. In due time it begins to calcify upon the surface, and this process pro ceeds, downward and inward, until all of the substance of the papilla has been changed into solid dentine except a central cavity which re mains filled with growing tissue (pulp), sup plied with blood and nerves, This dentine is the principal constituent of the greater number of teeth, and is seen to best advantage in the massive ivory of the tusks of the elephant and walrus, and consists of an organic basis richly impregnated with mineral (chiefly phosphate of lime) disposed in the form of minute tubes, each open at its inner end and occupied by a fibril of living nutritive tissue connected with the pulp. In the ordinary case these dentinal tubes radiate from the pulp cavity in a slightly wavy course to the outer surface. ((The hard substance of the tooth is thus arranged in hol low columns, perpendicular to the plane of pres sure, and a certain elasticity results from these curves; they are upright where the grinding surface of the crown receives the appuise of the opposing tooth, and are horizontal where they have to resist the pressure of contirotr, teeth. The tubuli also receive the plasma transuded from the remains of the vascular pulp, which circulates by anastomosing branches of the tubuli through the dentine, maintaining a sufficient though languid vitality of the sys tem. The delicate nerve branches on the pulp's surface convey sensations of impressions affect ing the dentine —every one has experienced the acute sensations when decay has affected the dentine or when mechanical or c.hemical stimuli have °set the teeth on edge." When a part of the primitive vascular pulp from which the dentine is developed remains per manently uncalcified, red blood is carried by °vascular canals° into the substance of the tissue. Such dentine is called vaso-dentine and is often combined with true dentine in the same tooth, as, for example, in the large incisors of certain rodents, the tusks of the elephant, and the molars of the extinct megatheriutn. When the celhilar basis is arranged in concentric layers around the vascular canals, and con tains °radiated cells? like those of bone, this is termed osteo-dentine, and resembles true bone very closely.