250 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



maxillary and free mandibular bones, or the lower and upper jaws. In 

 some fishes, as in the carp, all the teetli are in the back of the mouth, 

 while in most fishes there are teeth not only on the maxillary and man- 

 dibular bones, but also on the bones around the middle part of the mouth; 

 even, sometimes, being found on the median line of the palate. In cer- 

 tain cartilaginous fishes the teeth depart from the usual rule, in conse- 

 quence of which they are mostly inclosed in the bone on which they rest, 

 but are attached by ligaments, so as to allow the teeth to be bent back- 

 ward in the mouth by pressure. In most fishes the enamel and cement 

 substances are absent, the body of the teeth being composed simply of 

 dentine, which on its external surface is more compact than when found 

 in mammals. The teeth of fishes are, as a rule, replaced several times 

 during life, especially in the cartilaginous fishes. 



In amphibia, fine, prehensile teeth are found on the upper jaw and 

 palate-bones of the frogs and salamanders; more seldom on the lower jaw 

 also. In toads only palatal teeth are present. 



In reptiles the jaws may be either covered with a thick, dense horn, 

 which assists in dividing the food, or they may exhibit the most perfect 

 dentition, as in the saurians. 



The number of teeth is always very large, and while in crocodiles and 

 many lizards they are limited to the jaw-bones, they also exist on the ptery- 

 goid or palatine bones, and on the roof of the mouth of most ophidians. 

 The typical form of the teeth of reptiles is conical, and they vary greatly 

 in size, from the minute tooth of the blind-worm to the powerful canine- 

 like teeth of the crocodile. They are sometimes cylindrical, but maj' be 

 flattened, or even have serrated margins. Their surface is smooth, or is 

 notched. In serpents thejr are relatively longest, and present a remark- 

 able structure in the case of the poison-teeth or fangs, which are strongly 

 curved and contain a canal opening at both ends of the tooth : on the 

 anterior or convex aspect of the teeth above, close to the gums, and 

 below on the concave surface, a short distance from the point of the 

 tooth. These teetli are usually confined to the upper jaw, and the canal 

 serves to convey the secretion of the poison-glands, by the duct, to the 

 substances in which the tooth is imbedded, the poison being forced out 

 by muscles which join the gland-capsule and compress the gland. The 

 poison-fangs are fixed to the superior maxillary bones, but, since these 

 in poisonous serpents are movable, the teeth can when at rest either lie 

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

 act of striking. Reptilian teeth always contain dentine and cement, and 

 sometimes, also, enamel and true bone, the dentine differing slightly from 

 that of mammals, its substance being traversed by canals which commu- 

 nicate with the pulp-cavity. As the teeth of the reptile wear away they 

 fall out, and are replaced by an almost unlimited succession of new ones. 



