66 MICROSCOPIC OBJECTS. 



In some specimens prepared by Mr. Pritchard, a thin 

 vertical section has been made of a tooth in situ, thus 

 exhibiting a section of the jaw with its cavities for the 

 nerves and vessels, as also the manner in which the alveolar 

 process which forms the socket is constructed. This 

 process, when a tooth is extracted, is absorbed. Each 

 slide should (when the size of the specimen will permit) 

 contain both a transverse and a vertical section of a 

 similar tooth ; otherwise we cannot obtain a correct idea 

 of their true structure. In very large teeth, as the 

 grinders of the elephant, one quarter of a transverse 

 section is sufficient for microscopic examination. It has 

 been long a question among physiologists, whether a tooth 

 in man when once perfected, grows or is renewed. Some 

 recent experiments warrant the assertion, that even a 

 fractured or decayed tooth may be restored. 



The magnifying powers best suited for a general 

 examination of teeth is 40 diameters. For viewing the 

 canals 100 to 150 diameters will be required. Under 

 the polarising microscope many teeth are highly interest- 

 ing objects, especially large teeth, as those of the sea- 

 horse, transverse sections of which sometimes exhibit 

 the black cross. 



Whalebone. — This substance, which grows in the form 

 of long plates from the interior of the upper jaw of the 

 whale instead of teeth, serves to intercept the small prey 

 on which that mammal feeds. To examine its structure 

 under the microscope, it is necessary to obtain a very 

 thin shce cut across the fibres, and to mount it between 



