248 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the teeth, var}^ in their chemical composition, and, as a consequence, in 

 their different degrees of hardness, dependent upon the varying amount 

 of inorganic matter found within them, thus : 



Dentine. Enamel. Cement. 



Organic matter, . . . .28.01 3.59 32.24 



Inorganic matter, . . . 71.99 96.41 67.76 



The sharp angles and prominences on the crown of the compound 

 tooth are formed of enamel, as is the entire free surface of the simple 

 tooth. The deeper hollows in the crown of compound teeth are formed 

 by the wearing of the cement, while the substances varying in hardness 

 between these two are formed by the dentine. The fang of the tooth, 

 where inserted in the alveolus, is further covered by a membranous 

 lining, the periosteum, which is soft and contains vessels and nerves, and 

 which is reflected into the pulp-cavity through the opening in the fang 

 of the tooth. When ossified, this membrane forms osteo-dentine. 



cae 



FIG. 98. HALF OF A FOSSIL, TOOTH OF ELEPHANT (Dens lamellosus). (Nuhn.) . 

 I, the single segments, or secondary teeth ; s, enamel ; z, dentine; cae, cement. 



Teeth are entirely absent in birds, but are generally present in 

 fishes, amphibia, reptiles, and mammalia. In the latter class alone are 

 two sets of teeth met with : the first, the deciduous or milk-teeth, which 

 are only temporary, fall out and give place to the permanent teeth. With 

 the exception of a few fishes, such as the sheep's-head, and certain her- 

 bivorous reptiles, the teeth in fishes, amphibia, and reptiles as a class, are 

 solely prehensile ; they serve simply for seizing and dividing their prey 

 into portions small enough to be swallowed. It is only in the mammals 

 that the teeth serve actually as organs of mastication. 



The teeth of fishes present greater varieties than those found in any 

 other class. They may be almost innumerable or they may be reduced to 

 a single tooth, as in the lepidosiren, which has only a single dental plate 



