THE TEETH. 



83 



birds have not only muscular giz- 

 zards, but by swallowing stones, 

 furnish themselves with temporary 

 stomachic teeth of a most efficient 

 description. 



Even among teeth, properly so 

 called, namely, structures impreg- 

 nated with mineral matter, there is 

 great variety. In. fishes and reptiles, 

 they may be jointed to bones by 

 means of ligament, or welded to them 

 immovably, and may be attached to 

 numerous bones abutting on the oral 

 cavity; indeed, true teeth occur 

 (Synodontis) lying in the integument 

 unconnected with any bone. In cro- 

 codiles, the teeth are in large sockets. 

 In mammals, they are confined to 

 the inferior, superior, and inter-max- 

 illary bones, and are fastened in 

 closely fitting sockets, so that, when 

 full grown, they cannot drop out, 

 even when the bones have been, 

 macerated. 



A tooth, such as may be obtained 

 from the human subject, consists of 

 a crown projecting above the gum, 

 and a roob imbedded in a socket of 

 the jaw, and the place where these 

 meet is called the neck. The root 

 may consist of one fang or several ; 

 and at the extremity of each fang 

 is a little opening leading into a 

 cavity in the interior of the tooth, 

 which is filled with a soft and 

 sensitive substance called the pulp, 

 while the blood-vessels and nerve of 

 the pulp pass through the little 

 opening. The main mass of the 

 hard substance of the tooth is COKL- 



Fig. 43. INCISOTI TOOTH, 

 vertical section, a, pulp 

 cavity ; b, 6, dentine 

 showing the general 

 curves of the tubes and 

 three contour lines cross- 

 ing them; c, enamel, with 

 coloured bands crossing 

 its prisms; d, crusta pe- 

 trosa. 



