THE DENTAL TISSUES. 59 



structural characteristics of a tooth so treated are main- 

 tained, the dental cartilage forming a tough, flexible, and 

 elastic semi-transparent mass. 



In the matrix of a perfect tooth no trace of cellular 

 structure can be detected ; it is uniform and perfectly 

 transparent. 



The Deutinal Tubes. As has been already mentioned, 

 the matrix is everywhere permeated by tubes, the precise 

 direction of which varies in different parts of the tooth, 

 so that the following description of their course must be 

 taken as merely in a general way descriptive, and not as 

 of universal or precise application. Each tube starts by an 

 open circular mouth upon the surface of the pulp cavity ; 

 thence it runs outwards, in a direction generally per- 

 pendicular to the surface, towards the periphery of the 

 dentine, which, however, it does not reach, as it becomes 

 smaller, and breaks up into branches at a little distance 

 beneath the surface of the dentine. 



Near to the pulp they are so closely packed that there is 

 little room between them for the matrix, while near to the 

 outside of the tooth they are more widely separated : their 

 diameter is also greater near to the pulp cavity. 



The dentinal tubes de not pursue a perfectly straight 

 course, but describe curves both on a larger and a smaller 

 scale. The longer curves are less abrupt than the others, 

 and are termed the " primary curvatures ; " they are often 

 compared to the letter f, to which they bear a certain 

 amount of resemblance ; the primary curves are more 

 pronounced in the crown than in the root. 



The secondary curvatures are very much more numerous 

 and are smaller ; the actual course of the dentinal tube is, 

 in many places at all events, an elongated spiral, as may 

 be very well seen in thick sections transverse to the tubes ; 

 by alterations in the focus of the microscope the appearance 

 of the tube making a spiral turn is made very striking. 



