1024 



VEKTEBBATEB ANIMALS 



is surrounded by a. system of tubuli (fig. 755), which radiate into 

 the surrounding solid substance. These tubuli, however, do not enter 

 lacunae, nor is there any concentric annular arrangement around the 

 medullary canals ; but each system of tubuli is continued onwards, 

 through its own division of the tooth, the individual tubes sometimes 

 giving off lateral branches, whilst in other instances their trunks 

 bifurcate. This arrangement is peculiarly well displayed, when 

 sections of teeth constructed upon this type are viewed as opaque 

 objects (fig. 756). In the teeth of the higher Yertebrata, however, 

 we usually find the centre excavated into a single cavity (fig. 757), 

 and the remainder destitute of vascular canals ; but there are inter- 

 mediate cases (as in the teeth of the great fossil sloths) in which the 

 inner portion of the dentine is traversed by prolongations of this 

 cavity, conveying blood-vessels, which do not pass into the exterior 



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FIG. 754. Perpendicular section of 

 tooth of Lamna, moderately en- 

 larged, showing network of me- 

 dullary canals. 



Fie. 755. Transverse section of por- 

 tion of tooth ofPristis,more highly 

 magnified, showing orifices of me- 

 dullary canals, with systems of 

 radiating and inosculating tubuli. 



layers. The tubuli of the 'non-vascular' dentine, which exists by 

 itself in the teeth of nearly all mammalia, and which in the elephant 

 is known as ' ivory,' all radiate from the central cavity, and pass 

 towards the surface of the tooth in a nearly parallel course. Their 

 diameter at their largest part averages 1( nVorith of an inch; their 

 smallest branches are immeasurably fine. The tubuli in their course 

 present greater and lesser undulations ; the former are lew in number, 

 but the latter are numerous; and as they occur at the same part of 

 1 he course of several cont iguous 1 ubes t hey give rise to the appearance 

 of lines concentric with the centre of radiation. These , secondary 

 curvatures' probably indicate in dentine, as in the crab's shell, suc- 

 cessive stages of Calcification. The tubuli are occupied, during the 

 life of the tooth, by delicate threads of protoplasmic substance. r\ 

 tending into them from the central pulp. 



T\\o other substances, one of them harder and the other softer 



