. Animal physiology. Physiology, Comparative; Physiology, Comparative. STRUCTURE AND DEVELOPMENT OF TEETH. 145 its root or fang is penetrated by a small nerve and artery, which are distributed to the membrane that lines the cavity ; and it is to the action of air upon the former, when the cavity is laid open by decay, that the pain of tooth-ache is chiefly due. The reme- dies which are most effectual in removing this pain, such as kreosote, nitric acid, or a heated wire, are those which destroy the vital power of the nerve. 177. But there are teeth, in many animals, which never cease to grow,
. Animal physiology. Physiology, Comparative; Physiology, Comparative. STRUCTURE AND DEVELOPMENT OF TEETH. 145 its root or fang is penetrated by a small nerve and artery, which are distributed to the membrane that lines the cavity ; and it is to the action of air upon the former, when the cavity is laid open by decay, that the pain of tooth-ache is chiefly due. The reme- dies which are most effectual in removing this pain, such as kreosote, nitric acid, or a heated wire, are those which destroy the vital power of the nerve. 177. But there are teeth, in many animals, which never cease to grow, and in which the central cavity is always filled with pulp. Such have no proper root; for additional mat- ter is being continually formed at their base, and thus the Whole tooth is Fl<5- and Teeth of Rabbit. pushed upwards. This is the case with the Elephant's tusks ; and also with the large teeth that occupy the front of the jaw in Rabbits, Squirrels, Rats, and other gnawing animals (Fig. 91). The upper edges of these teeth are being constantly worn away by use ; but they are kept up to their proper level by the growth of the tooth from below. But it sometimes happens that one of these teeth is broken off; and the one opposite to it in the other jaw is then thrown into disuse. It continues, however, to grow up from below ; but, not being worn down at the top, its length increases greatly, so that it may become a source of great incon- venience to the animal. 178. The teeth are generally made up of two or more distinct substances, differing in structure and properties. That which usually forms their principal part, is termed the ivory ; but the summit or crown of the tooth is generally clothed with a much harder substance, which is termed enamel; and its fang is covered with a substance closely resembling bone, and termed the cortical substance. Although the ivory, enamel, and cortical substance occupy these different positions in most teeth, they are all mixed toget
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