THE TEETH IN REPTILES. 577 



pendent of sex, but characterizes groups of animals. Thus in the ox tribe, 

 although the temporary incisors appear above the gum in both jaws, the per- 

 manent incisors are not developed in the upper jaw, but remain in a rudimen- 

 tary condition within the bone. 



The four canine teeth also exist, in a rudimentary state, in all young 

 Ruminants, though they never rise above the gum. In both jaws of the young 

 whalebone whale, rudiments of teeth exist, which are never further developed. 



Birds. In Birds the horny coating of the edentulous jaws is developed in 

 successive laminae, from the tegumentary membrane covering those bones. In 

 the parrots this horny coat is thick, and is formed and supported upon pa- 

 pillse. The absence of teeth in birds is associated with the existence of a mus- 

 cular stomach or gizzard. 



Reptiles. Of these animals the Saurians exhibit the most perfect dentition, 

 then the Ophidians, whilst the Chelonia are edentulous, their jaws being 

 covered with a thick and dense horn, variously modelled, so as to act in bruis- 

 ing or dividing the food, the jaws of the vegetable feeders being thick, and 

 those of the carnivorous species sharp on their edges. 



In the Reptiles which possess teeth, the number varies, but in existing spe- 

 cies, it is never very small, being thirty in certain monitor lizards, and twenty- 

 nine, the lowest known number in the Ophidian amphisbsena. The number is 

 not so determinate, nor are individual teeth so specially characterized, as in 

 the Mammalia. In the crocodiles, and in many lizards, the teeth are limited 

 to the jaw-bones ; but they exist also on the pterygoid bones in the roof of the 

 mouth, in the iguana, and on the palatine and pterygoid bones, in most Ophi- 

 dia. In many of the latter, teeth are absent from the intermaxillary bones. 

 The jaw-teeth form single arches, excepting only in the csecilia or blind-worm, 

 in which the lower front teeth are arranged in a double row. 



The typical form of the reptilian tooth is conical, but in a few species this 

 is departed from. These conical teeth vary greatly in size, from the minute 

 teeth of the blind-worm, to the powerful canine-like teeth of the crocodile. 

 They are sometimes cylindrical, but more frequently compressed, or much 

 flattened and blade-like, having sharply trenchant, or even serrated, margins. 

 The surface is either smooth and polished, or longitudinally striated. In the 

 iguanas, the crowns of the teeth are widely expanded, and their sides and mar- 

 gins curiously notched. The teeth are relatively longest in Serpents, and in 

 the case of the poison-teeth or fangs, present a remarkable structure. These 

 poison-fangs are strongly recurved, and contain a canal, opening at both ends 

 on the anterior or convex aspect of the tooth, above, close to the gum, and 

 below, a short distance from the point of the tooth. The secretion of the 

 poison-gland, found at the side of the head, is conveyed by a duct, to the 

 opening of the poison-canal near the base of the tooth. Into this the poison 

 is forced by muscles which tighten the gland capsule and compress the gland ; 

 and thence it is conveyed through the opening, in front of the point of the 

 tooth, into any wound. The poison-fangs are anchylosed, or fixed by osseous 

 union, to the superior maxillary bones ; but since, in the poisonous serpents, 

 these bones are movable, the poison-teeth can either, as when at rest, lie flat 

 upon the gum, or they can be brought into a vertical position, in the act of 

 striking. 



The teeth of Reptiles have a short undivided root, which is, for the most 

 part, anchylosed to the bone on which it rests. In the crocodiles, however, 

 the teeth are separate, and are lodged in deep sockets ; in the black alligator, 

 the front teeth are embedded in sockets, whilst the hinder ones are fitted into 

 a continuous groove. In the serpents and geckos, the anchylosed teeth are 

 fixed to the sides of shallow sockets, but in the chameleons and most lizards, 

 to the inner surface of a single alveolar plate. 



Reptilian teeth always contain dentine and cement, sometimes also enamel 

 and true bone. In most Saurians, the enamel exists as a thin coating over 

 the crowns. The presence in certain teeth, of bone, besides the cement cover- 

 ing the dentine, depends on the conversion of the base of the pulp into bone, 

 as the tooth becomes anchylosed to the jaw. The microscopic structure of the 

 dentine differs slightly from that of the dentine in Mammalia, its substance 

 being traversed by canals communicating with the pulp-cavity. In the iguana 



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