58 W. H. F. ADDISON AND J. L. APPLETON, JR. 
ures only 100 to 1104 in the mid-line, it measures 160 to 180u at 
the region of these angles, and is, therefore, thicker here than 
the enamel of the lower tooth. The increased thickness at 
the angles is principally in the inner plexiform layer, the other 
layer being increased only slightly or not at all. The outer 
fibrous layer is distinctly thicker in the uppers and has a slightly 
wider band of pigment in it superficially. This, no doubt, is 
the basis of the more deeply pigmented appearance of the labial 
surface of the upper as compared with the lower teeth. 
The dentine, unlike the enamel, grows continually thicker 
as one passes towards the outer end of the tooth. At the basal, 
erowing, end it begins as an extremely thin layer. The thick- 
ness at different points is seen in figure 6. As the dentine in- 
creases in thickness, the pulp-chamber is in consequence propor- 
tionately reduced. At the distal end there is no longer any 
pulp-chamber and the site of its previous position has been 
filled in by the formation of a kind of secondary dentine. C. 
Tomes (14) notes that ‘“‘in some rodents the final closure of 
the axial tract takes place almost by a continuance of the forma- 
tion of normal fine-tubed dentine, with very little secondary 
dentine of different structure, while in others there is a large 
area of dentine with vascular tracts in it.’’ In the rat there 
is relatively little of this secondary dentine. It is laid down 
in irregular trabeculae, with the pulp tissue, including blood- 
vessels, at first within it. At the exposed surface, however, 
it forms a continuous granular mass with apparently no soft 
tissues in it (fig. 27). The ordinary dentine of the tooth is 
quite typical in structure, with numerous parallel dentinal 
tubules, each having many fine lateral branches. The tubules 
are slightly sinuous, and the lateral branches anastomose with 
those of neighboring tubules. Sometimes a tubule sends off 
at an acute angle a branch nearly equal in diameter to the con- 
tinuation of the main tubule. This is usually in the dentine 
not covered by enamel. Where these large branches come 
off the diameter of the tubule is greater than elsewhere, measur- 
ing nearly 2u. Elsewhere the diameter varies from 1 to 1.7u. 
Slight differences may be seen between the tubules (a) in the 
dentine covered by enamel, and (b) in the dentine free from 
