Page:Collier's New Encyclopedia v. 09.djvu/498

LEFT TOOTH 436 TOOTH mon in fisheg. In all fishes the teeth are shed and renewed, not once only, as in mammals, but frequently during the whole course of their lives. Tortoises and turtles, toads, and certain extinct saurians are toothless. Frogs have teeth in the upper, but not in the lower jaw. Newts and salamanders have teeth in both jaws and upon the palate; and teeth are found on the palate as well as on the jaws of most serpents. In reptiles, as a general rule, the base of the tooth is anchylosed to the bone which sup- ports it. The completion of a tooth is soon followed by preparation for its re- moval and succession, the faculty of de- veloping new tooth germs being appar- ently unlimited in this class. The extinct Odontornithes are the only birds with teeth. Of mammals there are a few genera and species devoid of teeth. The 11-12 SECOND TEETH The figures refer to years after birth true ant-eaters, the pangolins, and the echidna are strictly toothless. It is only in the mammals that we have a well- marked division of the teeth into the four kinds of incisors, canines, premo- lars, and molars. The teeth are so admirably adapted for the special purposes which they are called on to fulfill that it is generally easy, from a careful examination of them, to say to what class of animals they belong, and to draw various conclusions regarding the habits and structure of the class gener- ally. Thus, in carnivorous animals the molars are not grinding teeth, but pre- sent sharp cutting edges, and those of the upper and lower jaw overlap each other, resembling a pair of scissors in their action. In insectivorous animals the molars have a tuberculater surface, with conical points and depressions, so arranged as to lock into each other. In frugivorous animals, living on soft fruits, these teeth are provided with rounded tubercles, while in herbivorous animals they have a broad, rough surface, re- sembling a millstone. Diseases of the Teeth. — Decay (caries) is by far the most common of the diseases which affect the teeth, and consists in a gradual and progressive disintegration of the tooth substance. The exciting cause of caries has been proved to be due to the action of micro-organisms producing lactic acid. Decay is rarely met with on smooth surfaces exposed to the friction of food and the direct washings of the saliva. It usually begins in some pit or groove in the enamel or between the teeth, such points forming a lodgment for the de- velopment of the organisms. Once the enamel has been penetrated the decay proceeds more rapidly, spreading later- ally beneath the as yet healthy enamel and toward the pulp. Caries is most common in early life, by far the greater number of cavities making their appear- ance between the ages of 6 and 18. Pain may be felt soon after the enamel has been penetrated, or may be delayed until the nerve (pulp) has become almost or quite exposed. Ultimately the pulp be- comes exposed when the pain increases and may become very violent, especially if the nerve be pressed on by food forced into the cavity by mastication. Should this take place the pulp becomes acutely infiamed and soon dies, when the pain may either cease or go on till an alveolar abscess is formed. Periostitis and Alveolar Abscess. — Per- iostitis is an infiammation of the mem- brane (periosteum) which covers the roots of the teeth and lines their sockets. It may be either general or local. When general the majority of all of the teeth are commonly involved. When the disease is local — confined to one or two teeth — it may result from a blow or some such injury, or it may proceed from an in- flamed pulp ; but by far the most common cause is the presence of a dead nerve, the poisonous products of which are liable at any time to cause violent inflammation at the end of the root. Alveolar abscess may be defined as a suppuration around the root or roots of a tooth. It is of two varieties, acute and chronic. Its causes are those of peri- ostitis, which precedes it, the continuous and throbbing nature of the pain indicat- ing the formation of matter (pus) within the surrounding bone. Exostosis is characterized by an in- crease in the thickness of the cementum, the external of the two hard tissues forming the roots of the teeth. Its forms vary from a small nodule or patch to a quantity sufficient to invest the entire