PROFESSOR OWEN ON THE MEGATHERIUM. 
583 
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 
organization. In both these gigantic phyllophagous quadrupeds provision has like- 
wise been made for the maintenance of the grinding machinery in an effective state ; 
but the fertility of the Creative resources is well displayed by the different modes in 
which this provision has been effected : in the Elephant, it is by the formation of 
new teeth to supply the place of the old when worn out ; in the Megatherium, by the 
constant repair of the teeth in use, to the base of which new matter is added in pro- 
portion as the old is worn away from the crown. Thus the extinct Megatherium had 
* The first statement of the continuation of filamentaiy processes of the pulp into the tissue of the growing 
tooth was published in the ‘ Comptes Rendus de I’Academie des Sciences,’ Paris, 1839, p. 787, and the earliest 
observation of their continuation into the dentinal tuhuli 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 filamentaiy 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- 
psedia of Anatomy, vol. iv. p. 929. 
