TEETH 



III 



upon the outer surface of the dentine. This bone is a part of the tooth and 

 is known as the substantia ossea or cement. It is thinnest at the neck of 

 the tooth, and increases in thickness downward toward the apex of the root, 

 over which it forms a considerable cap (Fig. 88). The deeper part of the 

 root develops after the eruption of the crown. 



The cement contains typical bone cells, enclosed in large lacunae which 

 connect with one another through canaliculi (Fig. 100). The dentinal 

 surface sometimes appears resorbed and the dental canaliculi then end 

 abruptly; occasionally they appear to anastomose with those of the cement. 



Cross section of the 

 orbicularis oris muscle. 



Labial gland. 



Dental lamina. 



.Enamel organ. 



FlG. 101. VERTICAL SECTION THROUGH THE LIP AND JAW OF A HUMAN EM- 

 BRYO OF Six AND A HALF MONTHS. X 9- 



The lamellae of the cement, which are seldom well marked, are concentric- 

 ally placed around the root. In young teeth Haversian canals are absent, 

 but in old teeth they occur in the outer layers near the apex of the root. 

 Connective tissue fibers, comparable with Sharpey's fibers in bone, pass 

 radially through the cement. They cross the dental sac and enter the 

 bone of the alveolus, thus binding the tooth to its socket. 



As the tooth enlarges and fills the socket, the dental sac becomes re- 

 duced to a thin layer consisting of the alveolar periosteum externally and 

 the dental periosteum internally, with vascular connective tissue between. 

 Frequently these are described as a single layer. It may contain fragments 

 of the epithelial sheath. It has few elastic fibers, but is well supplied with 



