166 



THE MICROSCOPE 



renovation exactly the same as their modern representatives, 

 though they may widely differ in many other respects. 



Taking as an example the huge, extinct Megatherium and the 

 modern diminutive representative, the sloth (see PI. 17, Figs, i, 2). 

 The teeth of the modern two-toed sloth differ, in presenting a 

 greater inequality of size than those of the Megatherium, but 

 almost all the other dental characters are the same. The teeth of 

 the Megatherium may be described (see Fig. 3) as a central axis 

 of vaso-dentine, surrounded by a thin layer of hard, or unvascular 

 dentine, which is coated by cement. The vaso-dentine is tra- 

 versed throughout by medullary canals, measuring i — 1,500th of 

 an inch in diameter, continued from the pulp cavity, and anasto- 

 mosing in pairs by a loop, the convexity of which is turned to- 

 wards the origin of the tubes of the hard dentine. The cement is 

 characterised by the size, number, and regularity of the vascular 

 canals which traverse it, running parallel to each other, and anas- 

 tomose in loops, the convexity of which is directed towards the 

 hard dentine. 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. 



The minuter tubes, which pervade every part of the tooth, 

 characterising, by their difference of length and course the three 

 constituent substances, form one continuous and freely intercom- 

 municating system of strengthening and reparative vessels, by 

 which the plasma of the blood was distributed throughout the 

 entire tooth, for its nutrition and maintenance in a healthy state. 

 The oblique direction of the vessels of the vaso-dentine has a use, 

 probably, in thus maintaining the nutrition of the hard dentine at 

 the tip of the tooth, although the vaso-dentine at its level has been 

 worn away.*" 



Scales and carapaces of many reptiles are often suflficiently 

 well preserved to show their structure, and the curious bodies 

 found in the coal measures, and supposed to be modified ossicles 

 of the ventral armour of some genus of Labyrinthodonts, await a 

 microscopic examination, which will probably help in determining 

 their origin. Scales of many fishes require magnifying and careful 



* Owen's Anatomy of the Vertebrates, Vol. III., pp. 274-5. 



