40 



directed towards, and in most cases is in close contiguity with, the layer of hard 

 dentine. Fine tiihules are sent off, generally at right angles, from the medullary 

 canals, which quickly divide and subdivide, form anastomosing reticulations, and 

 communicate freely with the similar tubules that radiate from the lacuna; or calci- 

 gerous cells, ib. r, r. These cells are dispersed throughout the dentine, and present 

 an oblong form, with the long axis transverse to that of the tooth, measuring smooth 

 of an inch in diameter. The cavity of the cell, which is not quite occupied by their 

 opake contents, is often very clearly demonstrated. The tubes, which radiate from 

 the cells nearest the hard dentine, and from the terminal loops of the vascular canals, 

 intercommunicate freely with the tubes of the hard dentine. The tooth of the Mega- 

 therium thus offers an unequivocal example of a course of nutriment from the den- 

 tine to the cement, and reciprocally. 



In the structure which the fossil teeth of the Megatherium and its extinct con- 

 geners clearly demonstrate, we have striking evidence of the rich organization of 

 those once-deemed extravascular parts, and that they were pervaded by vital activity. 

 All the constituents of the blood freely circulated through the vascular dentine and 

 the cement, and the vessels of each substance intercommunicated by a few canals 

 continued across the hard or unvascular dentine. 



With respect to those minuter tubes, the more important as being more imme- 

 diately engaged in nutrition, which pervade every part of the tooth, characterizing 

 by their difference of length and course the three constituent substances, they form 

 one continuous and freely intercommunicating system of strengthening and repara- 

 tive vessels, by which the plasma of the blood was distributed throughout the entire 

 tooth for its nutrition and maintenance in a healthy state*. 



The grinding surface of the molars of the Megatherium differs, on account of the 

 greater thickness of the cement on their anterior and posterior surfaces, from those 

 of all the smaller Megatherioids, in presenting two transverse ridges; one of the 

 sloping sides of each ridge being formed by the cement, the other by the vascular 

 dentine; whilst the unvascular dentine, as the hardest constituent, forms the sum- 

 mit of the ridge, like the plate of enamel between the dentine and cement in the 

 Elephant's grinder. The great length of the teeth and concomitant depth of the 

 jaws, the close set series of the teeth, and the narrow palate, are also strong features 

 of resemblance between the Megatherium and Elephant in their dental and maxillary 



* The first statement of the continuation of filamentary processes of the pulp into the tissue of the growing 

 tooth was published in the ' Comptes Rendus de l'Acadcmie des Sciences,' Paris, 1839, p. 787, and the earliest 



observation of their continuation into the dentinal tubuli was, I believe, recorded in the following passage: 



" I had the tusk and pulp of the great Elephant at the Zoological Gardens longitudinally divided, soon after 

 the death of that animal in the summer of 1847. Although the pulp could be easily detached from the inner 

 surface of the pulp-cavity, it was not without a certain resistance ; and when the edges of the co-adapted pulp 

 and tooth were examined by a strong lens, the filamentary processes from the outer surface of the pulp could 

 be seen stretching as they were withdrawn from the dentinal tubes before they broke." — Art. Teeth, Cyclo- 

 paedia of Anatomy, vol. iv. p. 929. 



