236 



THE ORGANS 



tially parallel. In their passage through the dentine the dentinal 

 canals describe two series of curves, known as primary and secondary 

 curves. The primary curves take the form of an elongated S from 

 pulp to enamel or cementum, the secondary are twistings of the canals 

 (drawn out corkscrew). These twistings are very fine as many as 

 200 turns having been described in the length of a canal. In their 

 passage through the dentine the main canals gradually grow smaller 

 until their diameter is from 0.5 to in. They give off minute side 

 branches from 0.3 to o.6/( in diameter, which leave the main tubules 



KH 



Dk 



Fig. 140. — From Longitudinal Section through Root of Human Molar Tooth 

 (X200) (Sobotta), showing junction of dentine and cementum. C, Cementum; D, 

 dentine; A', Tomes' granular layer; Dk, dentinal canals; A'//, lacunse of cementum. 



at almost right angles, but soon turn slightly outward. They anasto- 

 mose with similar branches from other canals. This anastomosis takes 

 place not only between branches of adjacent canals, but also between 

 branches of canals some distance apart. The main canals terminate 

 either in blind extremities, or form loops by anastomosing with neigh- 

 boring tubules. Some of the tubules have quite extensive terminal 

 branchings, other tubules have only two or three end branches. A 

 few tubules run sHghtly beyond the limits of the dentine into the 

 enamel. The arrangement of the dentinal canals and their branches 

 differs in different parts of the tooth. In the crown there arc few 

 large branches and the main canals show distinct primary and second- 

 ary curves, most of them ending blindly in brush-like branchings 



