THE STRUCTURE OF THE TEETH. 491 



unequal height at different parts of the jaw, or are interrupted by 

 larger or smaller intervals, or diastema. 



The temporary teeth, though of course, in each case, of smaller size, 

 have forms like those of the permanent teeth of the same name. The 

 crowns of the incisors are chisel-shaped, those of the canines pointed, 

 and those of the molars square, and provided with several cusps. The 

 first upper molar, the largest of all, has three cusps, and the second 

 four ; the first lower molar four, and the second five. The fangs of 

 the temporary incisors and canines, are single ; those of the lower 

 molars are two in number ; those of the upper, three. In both jaws, 

 they are more divergent than those of the permanent teeth. 



The hard mass of a tooth is hollowed out, so as to form a cavity, 

 called the pulp cavity, because, during life, it contains a soft substance 

 named the pulp. This pulp cavity, Fig. 84, i f , ra', varies in shape with 

 that of the tooth ; it occupies the base of the crown, and is prolonged 

 down each fang, in the form of a small canal, which opens at the point. 

 The pulp consists of areolar tissue, supplied with vessels and nerves, 

 which enter at the minute opening at the point of the fang; it is the 

 remains of the vascular and nervous papilla, upon which the tooth is 

 originally formed. 



The hard portion of the tooth surrounding the pulp, is composed of 

 three substances ; viz., the tooth substance, ivory, or dentine, the 

 enamel, and the crusta petrosa, or cement (see Fig. 84). 



The dentine forms the greater part of the tooth, immediately sur- 

 rounds the pulp cavity, and corresponds, in form, with the tooth itself. 

 Its hardness is owing to the large quantity of earthy matter which it 

 contains, its chemical composition being 72 parts of earthy to 28 of 

 animal matter; whilst ordinary bone shows a proportion of 66 J to 33J. 

 The earthy salts contain 66.7 of phosphate of lime, 8.3 of carbonate 

 of lime, 1.8 of phosphate of magnesia and other salts, and some traces 

 of fluoride of calcium. The animal substance is converted into gelatin 

 on being boiled. 



The dentine consists of microscopic tubes, called the dental tubuli, 

 which have hard walls, and are embedded in an intermediate hard sub- 

 stance. These tubuli, originally described by Leeuwenhoek, commence 

 by minute orifices on the walls of the pulp cavity, and proceed out- 

 wards in a slightly wavy course, close together ; they soon divide di- 

 chotomously, and reach the superficial portion of the dentine, near the 

 surface of which they terminate in fine branches, in loops, or in minute 

 dilatations from which still finer branches proceed, or else in minute 

 dentinal cells. The diameter of the inner or larger ends of the tubes, 

 is about the 7 ^^th of an inch ; their terminations are immeasurably 

 fine. These tubuli might be compared to extremely minute Haversian 

 canals, their finest terminal ramifications to the canaliculi, and the 

 minute dentinal cells to the corpuscles or lacunae of bone (p. 46). 

 The dentine is, indeed, regarded as modified bone. In Man, the den- 

 tinal cells are few in number, and very minute, so that their simi- 

 larity to the lacunae of bone is not so striking as it is in the teeth 

 of the horse and other animals, in which they are larger and more 

 numerous. In the recent state, the dental tubuli are occupied by 



