ANIMAL TISSUES. TEETH. 375 



Iii certain forms of disease many of the soft parts of the human 

 body become converted into cartilaginous and bony masses, which have 

 received the name of Enchondrorrta. The microscopical characteristics 

 of this change have been described by the author, in the Transactions 

 of the Pathological Society of London, vol. iv. 



Teeth. The teeth are nearly allied to bone in structure ; and in 

 some of the lower vertebrata there is an actual continuity between the 

 bone of the jaw and the teeth. It is desirable to become acquainted 

 with the structure of teeth under the microscope : they are always in- 

 timately related to the food and habits of the animal, and are there- 

 fore highly interesting to the physiologist; they form for the same 

 reason important guides to the naturalist in the classification of ani- 

 mals ; and their value as zoological characters is enhanced by the faci- 

 lity with which, from their position, they can be examined in living or 

 recent animals. 



Professor Owen has said, " If the microscope be essential to the 

 full and true interpretation of the vegetable remains of a former world, 

 it is not less indispensable to the investigator of the fossilised parts of 

 animals. It has sometimes happened that a few scattered teeth have 

 been the only indications of animal life throughout an extensive stra- 

 tum ; and when these teeth happened not to be characterised by any 

 well-marked peculiarity of external form, there remained no other test 

 by which their nature could be ascertained than that of the microscopic 

 examination of their intimate tissue. By the microscope alone could 

 the existence of Keuper-reptiles in the lower sandstones of the new red 

 system, in Warwickshire, have been placed beyond a doubt. By the 

 microscope, the supposed monarch of the Saurian tribes the so-called 

 Basilosaurus has been deposed, and removed from the head of the 

 reptilium to the bottom of the marnmiferous class. The microscope has 

 degraded the tiaurocephalus from the class of reptiles to that of fishes. 

 It has settled the doubts entertained by some of the highest authorities 

 in paleontology as to the true affinities of the gigantic Megatherium ; 

 and by demonstrating the identity of its dental structure with that of 

 the Sloth, has yielded us an unerring indication of the true nature of 

 its food." 



The teeth of man and of most of the higher animals are composed 

 of three different substances, Dentine (known. as ivory in the tusk of 

 the elephant), Enamel, and Cementum, or crusta-petrosa. These are 

 variously disposed, according to the purpose which the tooth is to serve : 

 in man the whole crown of the tooth is covered with enamel, shown in 

 the dark marginal part of fig. 178; its root or fang is covered with 



