DENTINE OR IVORY 53 



close connection with the dentine there is a specialized layer of cells called 

 odontoblasts , which are elongated columnar cells with a large nucleus at the 

 tapering ends farthest from the dentine. The cells are all embedded in a 

 mucoid gelatinous matrix. 



The blood vessels and nerves enter the pulp through a small opening 

 at the apical extremity of each root. 



A layer of very hard calcareous matter, the enamel, caps the dentine of 

 the crown; beneath the level of the gum is a layer of true bone, called the 

 cement or crusta petrosa. The enamel and cement are very thin at the neck 

 of the tooth where they come in contact, the cement overlapping the enamel. 

 The enamel becomes thicker toward the crown, and the cement toward 

 the lower end or apex of the root. 



FIG. 61. A. A Longitudinal Section of a Human Molar Tooth, c, Cement; d, 

 dentine; e, enamel; v, pulp cavity (Owen). B. Transverse section. The letters indicate 

 the same as in A. 



Dentine or Ivory. Dentine closely resembles bone in chemical com- 

 position. It contains, however, rather less animal matter. 



Structure. Dentine is finely channelled by a multitude of delicate tubes, 

 which by their inner ends communicate with the pulp cavity, and by their 

 outer extremities come into contact with the under part of the enamel and 

 cement, and sometimes even penetrate them for a greater or less distance, 

 figures 63, 64. The matrix in which these tubes lie is composed of " a retic- 

 ulum of fine fibers of connective tissue modified by calcification, and, 

 where that process is complete, entirely hidden by the densely deposited 

 lime salts" (Mummery). 



The tubules of the dentine contain fine prolongations from the tooth 

 pulp, which gives the dentine a certain faint sensitiveness under ordinary 

 circumstances and, without doubt, have to do also with its nutrition. They 

 are probably processes of the dentine cells or odontoblasts lining the pulp 



