:;-j<; 



DR W. G. AITCHISON ROBERTSON ON THE RELATION OF 



6. The average width of the intertubular substance of the dentine. 



7. The course and direction of the dentinal tubules, and if branched. 



The following table contains these measurements in the three rabbits : — 

 Measurements of Lower Incisor Teeth in Rabbits. 





Newly-born. 



One Month Old. 



Adult. 



Total length of tooth, 



i inch 0-2 



\ inch - 5 



1J inch 1-12 



Greatest length of pulp cavity, 



i i. 0-17 



M » o-43 



1 „ 10 



Greatest breadth of pulp cavity, 



•rV » °' 033 



A » °-° 4 



tV „ 0-073 



Thickness of dentine at middle of tooth, 



T £ F „ 0-0063 



A » °' 024 



AV >. 0-044 



Greatest thickness of dentine at crown, 



j\ » 0-02 



A „ 0-08 



6 O-l 9 



Diameter of dentinal tubules at origin, 

 Width of intertubular dentine, 



i 0-0000416 



i 0-0000416 



1 0-0000416 



•2 4 000 )' vwwiiu 



Woir » 0-000125 



^Vo „ 0-000165 



24oo0 " " wwllu 



Ww » 0-000165 



Character of dentinal tubules, . 



Run obliquely in 

 straight lines ; no 

 branches; slightly 

 wider near origin. 



Wavy course; not 

 branched. 



Wavy course ; many 

 branches. 



The results of this table may be summarized as follows : — 



1. The fact of the great increase in length of the tooth is evident, it being six times 



longer in the adult than in the newly-born rabbit. 



2. The pulp cavity increases in length in the same proportion. 



3. The width of the pulp cavity increases in a progressive manner. 



4. The thickness of dentine at the middle of the tooth and also at the crown 



increases nearly six times. 



5. The diameter of the dentinal tubules at their proximal end remains the same at 



each stage of growth. They are all slightly larger at their origin and diminish 

 in calibre very gradually as they are traced outwards. 



6. The dentinal tubules become gradually more wavy in their course, and their 



lateral branches become evident in the adult tooth. 

 The odontoblasts we know form a complete lining to the inner surface of the dentine, 

 and thus form, as it were, a bag enclosing the pulp and having its mouth at the inlet of 

 the pulp cavity. Dr Haycraft suggested that the ring of odontoblasts which forms the 

 mouth of this bag might fitly be called the " formative ring," because it is apparently 

 here that new dentine is constantly being formed. The new dentine pushes upwards 

 that previously formed, which carries with it the odontoblasts attached to its inner surface 

 by the dentinal fibres. The odontoblasts which once composed the "formative ring" 



