-'> ZOOLOGY. 



50. Teeth are composed of various structures. The 

 substance forming' the greater part of the tooth, underneath, 

 is called the ivory or dentine. Tin- dense covering of the 

 corona is called enamel. A third substance is occasionally 

 found towards the extremity of the roots, or evm enveloping 

 the enamel and corona (as in the ox), to which the name of 

 cement or cortical substance has been given. 



The ivory of the tooth is composed of an animal matter 

 analogous to gelatine, of phosphate of lime (in the propor- 

 tion of about 64 to 100 in the adult human tooth), of car- 

 boi late of lime (amounting nearly to T ^ parts), and of a 

 small quantity of phosphate of magnesia. The enamel, which 

 differs somewhat in colour from the dentine or ivory, and 

 which is hard enough to strike fire with Hint, shows on 

 analysis slight traces of an animal substance. The phosphate 

 of lime in its composition amounts to y 9 oths The cortical 

 substance or cement scarcely exists in the human teeth ; but 

 in the teeth of oxen, in which it abounds, it furnishes, by 

 chemical analysis, 42 per 100 of organic matter, 50 per 100 

 of phosphate of lime, and 4 per 100 of carbonate of lime. 



In the ivory of the teeth of man we discover, by means 

 of the microscope, a multitude ofjiexuous branching tubes of 

 extreme tenuity (called Haversian canals), which open or 

 terminate in the central cavity : these tubes contain calcareous 

 matter ; they run towards the surface of the tooth, and their 

 divisions terminate frequently in little cavities bearing a close 

 resemblance to the little cells found in the ordinary osseous 

 tissue. The enamel, examined under the microscope, exhibits 

 a multitude of fibres, or rather hexagonal prisms, in appear- 

 ance crystalline, closely pressed against each other, and 

 directed perpendicularly towards the surface of the tooth. 

 Finally, the cortical substance is characterized by the pre- 

 sence of a great number of osseous cellules, and of irregular 

 calciferous tubes. These tissues are not all met with in the 

 teeth of all animals ; the enamel and cortical substance are fre- 

 quently absent in fishes ; and occasionally the dentine, instead 

 of containing a single medullary cavity, contains several. 



51. The teeth, in some animals, instead of being 

 contained or fixed in the alveolar cavities (sockets), unite 

 by their base with the jawbone or maxilla, becoming, as 

 it were, a part of it : this happens in many fishes, and occa- 

 sionally the teeth, instead of resembling bones, offer merely 

 the consistence of horn. Finally, in the whalebone whale 



