7 6 HISTOLOGY. 



DENTAL SAC, CEMENT AND ALVEOLAR PERIOSTEUM. 



The papilla and enamel organ together are surrounded by loose 

 mesenchyma extending to the oral epithelium and to the bone trabeculae 

 of the developing jaws, as shown in Fig. 87. The portion of mesenchyma 

 between the trabeculae and the teeth forms the so-called dental sacs. 

 Toward tfce enamel organ the sac is a vascular and very loose tissue (Fig. 

 86) which may form elevations between projections of the outer enamel 

 layer. The peripheral part of the sac is much denser. After birth, 



Cross section of the 

 orbicularis oris muscle. 



Labial gland. 



Dental ridge. 



Enamel 

 organ. 



Enamel. 



Dentine. 

 Pulp. 

 Bone. 



FIG. 87. VERTICAL SECTION THROUGH THE LIP AND JAW OF A HUMAN FETUS 

 OP Six AND A HALF MONTHS. X 9. 



but before the eruption of the teeth, the sac surrounding the root produces 

 the cement or substantia ossea. This is a layer of bone, containing typical 

 lacunae and canaliculi and penetrated by many uncalcified connective tissue 

 fibers (Sharpey's fibers). These may be so numerous as to suggest the 

 columnar appearance of enamel. Their direction is generally radial. 

 Lamellae in the cement are parallel with the surface of the root. Haversian 

 canals are absent except in the outer part of the cement of old teeth. 



As the tooth grows and fills the alveolar socket in the jaw bone, the 

 dental sac is reduced to a vascular fibrous layer, continuous with the 

 connective tissue of the gums at the neck of the tooth. Elastic fibers are 



