HISTOLOGY. 95 



dental bulbs grow (one for each milk tooth), and sub- 

 sequently become hollow on the under surface and cov- 

 er a conoid upgrowth of connective tissue cells (the 

 papillae). The dental bulbs continue to expand one 

 for each tooth, assuming the form of the crown of the 

 tooth, and the inner layer of the cells become columnar. 

 Each of these columnar cells secretes an enamel prism, 

 and the bulb is now called the enamel organ. "While 

 this has been in progress the outer cells of the papillae 

 have elongated to form the odontoblasts which deposit 

 the dentin. From what has been said, it will be seen 

 that the enamel is deposited outwards and the dentin 

 inwards. The remains of the papillae become the pulp 

 of the tooth. The permanent teeth are formed simi- 

 larly. The cementum is deposited later by the alveolar 

 periosteum." Lcroy. 



Exercise No. 50 Examine the L. $. of the tooth 

 supplied. 



Make out the structure enamel. 

 Exercise No. 51. Examine the T. S. of the tooth. 



ESOPHAGUS. 



The esophagus is composed of four coats, viz. : mu- 

 cous, sub mucous, muscularis mucosae, muscular and 

 fibrous coat. 



