56 



SPECIAL HISTOLOGY. 



and lie parallel to the axis of the tooth, others are rounded 

 and pyriforra. Those are most remarkable which have a 

 very elongated form, together with a narrow, canal-like cavity 

 (fig. 185), inasmuch as their resemblance to the dentinal canals 

 is unmistakeable. The canaliculi often resemble feathers and 

 brushes, and unless the lacunae are altogether isolated, connect 

 them with one another, and anastomose with the ends of the 

 dentinal canals. In the thinnest part of the cement, towards 

 the crown, the lacunae are invariably absent ; they are first 

 met with, as a rule, about the middle of the fang, but are here 

 scattered and solitary; towards the extremity their number 

 gradually increases, and they not unfrequently take on a very 

 regular arrangement, as in the external layer of the long 

 bones, lying in series in the layers of the cement, and sending 

 most of their canaliculi inwards and outwards, so as to give rise 

 to an even, fine, transverse striation of the cement. The thicker 

 layers of cement which occur in old teeth, present immense 

 quantities of lacunae, but these are to a great extent irregular, 

 and have mostly the elongated form. Many lacunae are bordered, 



Fig. 193. 



singly or in groups, by a very 

 distinct, clear, yellowish, slightly 

 undulated margin, which par- 

 tially or entirely surrounds them ; 

 it has perhaps some relation to 

 the cells from which the cavities 

 are developed. 



Haversian canals do not occur 

 in young teeth, where the cement 

 has ouly its normal thickness; but 

 they are very common in old 

 teeth, especially molars, and in 

 hyperostoses one, three, or more 

 enter the cement from without, 

 branch out two or three times, 

 and then terminate in blind ex- 

 tremities. Their diameter is too small (0*005 — O'Ol'") to con- 

 tain medulla as well as blood-vessels, and they are commonly 

 like those of the bones, surrounded by a few connective lamellae. 



Fig. 193. Cement and dentine of the root of an old tooth: a, pulp cavity; b, 

 dentine ; c, cement, with lacuna? ; e, Haversian canals. From Man. 



