54 



PHYSIOLOGY. 



by fine tubes called dentinal tubules. The inner ends of these tubules 

 open into the pulp-cavity, whence they radiate in every part of the 

 dentine toward the surface of the tooth. They have a direction gen- 

 erally parallel, with a wavy, undulating course. In the pathway 

 toward the periphery they subdivide into several parallel branches 

 which anastomose with each other. The average diameter of the 

 tubule is 1 / 450 o i ncn - Near the end of the tubule the arrangement is 

 in globular spaces which communicate with each other and are known 

 as the interglobular spaces of Purkinje, or Tomes granular sheath. 



C * D 



Fig. 9. Portion of a Longitudinal Section of the Root of a Human 

 Molar Tooth. X 200. iSoBOTTA.) 



The figure shows the structure of the boundary between dentine and cemen- 

 tum. In the cementum distinct bone spaces with bone canaliculi are seen. 

 The dentinal tubules here show especially numerous divisions and lateral 

 branches. The granular layer shows small, irregular interglobular spaces. C. 

 Cementum. D, Dentine. Dk, Dentinal tubules. K, Granular layer (small 

 interglobular spaces). KH, Bone spaces of the cementum. 



ENAMEL. The hardest of all organized substances is known as 

 enamel. It is a bluish-white material capping the crown of the tooth. 

 It is thickest on the triturating surface of the tooth. Chemically it 

 consists of 3. parts of organic matter and 97 of earthy matters, prin- 

 cipally calcium phosphate. Under the microscope the enamel appears 

 in the form of hexagonal columns about 1 / 500 o i ncn i n diameter. 



THE CEMENT, OR CRUSTA PETROSA. This substance covers the 

 fang of the tooth, gradually becoming thicker toward its extremity. 



