i66 



THE DISSECTION OF PIG EMBRYOS FOR STUDY 



teeth grow and fill the alveoli the dental sac becomes a thin vascular layer con- 

 tinuous externally with the alveolar periosteum, internally with the periosteum of 

 the cement layer of the tooth. 



Dental sac 



Inner layer 



Outer enamel cells 

 Enamel pulp 



"Atamel cells 



Enamel 



Dentine 



Odontoblasts / 



Epithelial sheath 



Dental papilla (future pulp) 



Blood-vessel 



Bony trabecula of the lower 



jaw 



Fig. 159. — Longitudinal section of a deciduous- tooth of a newborn dog. X 42. The white 

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

 and Stohr). 



When the crown of the tooth is fully developed the enamel organ disinte- 

 grates, and as the roots of the teeth continue to grow their crowns approach the 

 surface and break through the gums. The periods of eruption of the various 



