INSECTIVORES. 421 



the fangs ; at which part it is commonly more highly organized, tra- 

 versed by medullary canals which generally present concentric walls, 

 and thus assumes the character of true bone : in the BoricidcE the 

 cement is frequently continued into the substance of the jaw itself. 

 The small proportion of dentine in comparison with the thick layer 

 of enamel has been already alluded to in the Shrews, yet the 

 dentinal tubuli are, at their commencement, very little inferior in 

 diameter to those of the human incisors ; the trunks, as Retzius has 

 observed, are unusually short, and are resolved into radiated peni- 

 cilli of undulating branches, which quickly subdivide, interlace, 

 and anastomose together near the boundary line between the dentine 

 and enamel. In most of the Insectivora, the secondary branches 

 of the calcigerous tubes are very conspicuous, especially in the 

 dentine forming the fangs. The dentinal cells are rarely well 

 defined ; in the large canines of the Centetes they are sub-hexagonal, 

 and about gwo^h of an inch in diameter ; but diminish in size towards 

 the periphery of the dentine. 



The calcigerous tubes in the anterior incisor of the Hedgehog 

 {Erinaceus europceus) present, in a longitudinal slice of the tooth, 

 the usual disposition, vertical in the middle of the crown, diverging 

 obliquely at the sides, and gradually becoming transverse at the base, 

 which course they maintain throughout the long fang to its slightly 

 enlarged extremity, where the tubuli become irregularly and strongly 

 curved, crossing each other, and forming a net- work. In the incisor 

 here described the end of the fang was closed by cement, through 

 which a single vascular canal passed to the pulp-cavity : this 

 cavity was widely open in the rest of the fang, and extended more 

 than half-way up the enamelled crown. 



The dentinal tubes at the middle of the crown ascend without 

 any primary curvature ; they dichotomize earlier and more frequently 

 than usual, and the secondary undulations of these sub-divisions 

 of the main-tubes are irregular. The oblique tubes at the sides of 

 the crown form a short but strong curve at their commencement, 

 concave towards the summit of the tooth, then proceed in a straight 

 and parallel course obliquely upwards, and bend gently down near 

 the enamel, forming at the boundary line a rich reticulation by their 



