244 SOLID PARTS OF ANIMALS. 



each jaw ; two next each incisor. Their roots, as the name im- 

 plies, terminate near their extremities in two points, and there 

 is a groove from the neck of the tooth to its bicuspid termina- 

 tion. The cutting extremities of the crown present two tuber- 

 cles, one external, the other internal. 



The grinding teeth or larger molars are six in number in each 

 jaw, and are farthest back of all the teeth. These teeth in the 

 upper jaw have usually three roots, and in the under jaw two. 

 The upper surface of the crown is flat, but has four tubercles ar- 

 ranged crosswise, in order to triturate the food. 



Every tooth is divided by anatomists into the root, the neck, and 

 the crown. The root, or the part of the tooth contained within 

 the alveolus, is similar in its nature to common bone ; the neck is 

 the part of the tooth intermediate between the root and crown, or 

 the portion just in contact with the gums. The crown is the 

 part of the tooth projecting into the mouth, and fully in view. 

 The central portion of it is bone, but exteriorly it is encased by 

 a layer of white and very hard laminated substance, called enamel. 

 This layer is thick on the upper and lateral parts of the crown, 

 but becomes thinner as it approaches the neck, and disappears 

 altogether in the root. 



The teeth of the inferior animals differ in their form and struc- 

 ture from those of man. But a description of them belongs to the 

 comparative anatomist. They are composed of bone and enamel. 



The tusks of the elephant have received the name of ivory. 

 In consequence of its hardness and compact texture, it is suscep- 

 tible of a fine polish, and is on that account applied to a great va- 

 riety of purposes. It is liable, especially East Indian ivory, to 

 become yellow. The tusks of some other animals, as the hip- 

 popotamus and walrus, consist also of ivory. Even human teeth 

 contained a portion of ivory. The enamel differs from ivory in 

 containing very little cartilage, while about a third part of the 

 weight of ivory consists of cartilaginous matter. 



A tooth consists essentially of four parts. 



1. The pulp within the cavity of the tooth. It is from it that 

 the whole tooth originates. In process of time this pulp is fre- 

 quently converted into bone by the deposition of calcareous salts. 



2. The ivory. This constitutes almost the whole of the tooth. 

 It resembles bone in its composition ; but differs from common 

 bone in being harder and denser. 



