198 



COMPARATIVE ANATOMY 



of the root, although teeth erupt before the root has completed its growth. 

 The eruption of the deciduous teeth begins during the seventh month 

 after birth, and is usually completed by the end of the second year. Of 



Dental sac. 



Outer layer. Inner layer. 



Outer enamel cells. 



Enamel pulp. 



Inner enamel cells. 



Dentine 



Odontoblasts 



Dental papilla 

 (future pulp). 



Blood vessel 

 Bony trabecula of the lower jaw 



Fig. 151. — Longitudinal section of a deciduous tooth of a newborn dog. The white 

 spaces between the inner enamel cells and the enamel are artificial, and due to shrinkage. 

 X42. (From Bremer's "Textbook of Histology.") 



the permanent set, the first to erupt are the first molars which appear 

 during the sixth year. The last to erupt are the third molars, which 

 frequently become impacted in the jaw-bone so that eruption is impossible. 



