52 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



close connection with the dentine a specialized layer of cells called odonto- 

 blasts, which are elongated columnar cells with a large nucleus at the taper- 

 ing 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. 



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 



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. 



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 reti- 

 culum 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 give 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- 

 cavity. The relation of these processes to the tubules in which they lie is 



