98 MICROSCOPIC ANATOMY OF THE TEETH 



the Kangaroo rats, treated with silver nitrate by the process 

 of Ramon y Cajal, the matrix of the enamel is uniformly 

 stained of a yellowish brown, the tubes are very deeply 

 stained and vary greatly in diameter, and very fine cross - 

 branches pass horizontally at right angles to the tubes ; 

 these are also seen in some sections of the teeth of Macropus, 

 and their origin is a little difficult to understand. Similar 

 cross -branches are described by Sir John Tomes in the enamel 

 of the rodents. This uniform staining of the matrix is not 

 seen in other mammalian enamels, and would appear to 

 indicate that in these animals it is not so completely 

 calcified as in higher forms . Another appearance in the incisor 

 teeth of Bettongia has a very strong bearing upon the dis- 

 puted question of the origin of the tubes in marsupial enamel. 

 In about the middle third of the length of the long lower 

 incisor of Bettongia, many of the tubes from the dentine, after 

 passing half-way across the width of the enamel, terminate 

 in bulb-like closed ends ; these terminations being directed 

 towards the enamel surface and not towards the dentine 

 would appear to indicate that they are true dentinal tubes, 

 and not tubes of enamel origin which have become con- 

 nected with them, according to the view of the nature of 

 the tubes in marsupial enamel held by C. S. Tomes (figs. 38 

 and 55). 



In the Cuscus (Phalanger orientalis) the tube system is 

 very much reduced, but similar bulb -like terminations are 

 seen, many of which are quite near the dentine and others 

 farther within the enamel substance. These bulb-like 

 endings have a great similarity to the so-called spindles seen 

 in human teeth (see p. 78), and probably have a similar 

 origin in areas of imperfect calcification of the interprismatic 

 substance. A comparison of figs. 37 and 56 will demonstrate 

 the great similarity between the spindles in human enamel 

 and these expanded terminations of the tubes in marsupials. 

 In the Phalanger (fig. 56 A) the tubes from the dentine 

 are in many places seen to enter these spaces, and after 

 traversing them to pass out on their distal ends and terminate 

 deeper in the enamel. This is also occasionally seen in the 

 similar spaces in human teeth, but tubes so prolonged do 

 not pass in so deeply as in marsupials (fig. 56 B). 



