VI THE TEETH 233 



and the alveolar walls, and plays the part of periosteum 

 to both. 



The tissue which forms the chief constituent of a tooth 

 is termed dentine (Fig. 70, A, B, d). It is a dense 

 calcified substance containing less animal matter than 

 bone, and further differing from it in possessing no 

 lacunae, or proper canaliculi. Instead of these it presents 

 innumerable, minute, parallel, wavy tubules (Fig. 71, d), 

 which give off lateral branches. The wider inner ends of 

 these tubules may measure 4/x or 9/m (sy^jg inch) ; they 

 open into the pulp cavity, while the narrower outer 

 terminations ramify at the surface of the dentine, and 

 may even extend into the enamel or cement (Fig. 71). 



The greater part of the crown and almost the whole of 

 the fangs consist of dentine. But the summit of the 

 crown is invested by a thick layer of a much denser tissue, 

 which contains only 2 per cent, of animal matter, and is 

 the hardest substance in the body ; so hard that it will 

 strike fire with steel. This is called enamel (Fig. 70, 

 A, B, a). It becomes thinner on the sides of the crown 

 and gradually dies out on the neck. Examined micro- 

 scopically, the enamel is seen to consist of six-sided 

 prismatic fibres (Fig. 71, A, B) set closely side by side, 

 nearly at right angles to the surface of the dentine. 

 These fibres measure not more than 3/i to 5/i (gg^oo ^ 

 xoVo inch) in transverse diameter and present transverse 

 striations. 



The third tissue found in teeth is a thin layer of true 

 bone, generally devoid of Haversian canals, which invests 

 the outer surface of the fangs and thins out on the neck. 

 This is termed cement (Fig. 70, A, c ; and Fig. 71, C). 



The dental pulp is chiefly composed of delicate con- 

 Uictive tissue. It is abundantly supplied with vessels and 

 nerves, which enter it through the small opening at the 

 extremity of the fang. The nerves are mainly sensory 

 branches derived from the fifth pair of cranial nerves. 



The superficial part of the pulp, which is everywhere in 



