THE TEETH 



329 



low magnification, and is especially conspicuous in reflected light; it is 

 due to the difference with which the groups of transverse and longitudi- 

 nally cut enamel prisms reflect the rays of light. The dark bands are 

 known as the radial lines, or the prism stripes of Schreger. 



Ground sections of dried tooth show also brownish lines in the enamel 

 having an arrangement approximately parallel with 

 the surface of the tooth. These contour lines of 

 Eetzius are explained by von Ebner as the result of 

 :air-filled fissures in the dried enamel. They are also 

 ;said to be the result of the wavy direction of the 

 <enamel prisms. They are most probably caused by 

 imperfect calcification, marking growth stages in the 

 ^development of the enamel. 



Cementum. The dental cement, or crusta pe- 

 trosa, is a thin layer of bony tissue which invests the 

 root of the tooth. It forms a very thin layer at the 

 neck of the tooth, but gradually increases in thick- 

 ness as it approaches the tip of the fang. 



The cementum consists of parallel layers of bony 

 lamellae between which many lacunae with their bone 

 corpuscles are included. Bone canaliculi radiate 

 from the lacunae and frequently open into the inter- 

 globular spaces of the granular layer. There are no 

 Haversian systems in the cementum., but the thicker 

 portions are frequently penetrated by vascular canals 

 which, like Volkmamrs canals, are not accompanied 

 by concentric lamellae. The cementum is firmly 

 united to the granular layer of the dentin, the matrix 

 of the two tissues being continuous. 



The cementum is invested with a periosteal coat, 

 the periodontium, pericementum, alveolar periosteum, 

 or root (peridental) membrane, of dense fibrous tissue which, at the neck 

 of the tooth, unites with the dense connective tissue of the gum to form 

 an annular thickening of very dense fibrous tissue which encircles the 

 tooth and is known as the circular dental ligament. The root membrane 

 contains no elastic fibers, but sends considerable numbers of slender 

 white fibrous bands (Sliarpey's -filers} into the cementum. These bands 

 effect a firm anchorage of the tooth to the alveolar wall. They are anal- 

 ogous to the perforating fibers of Sharpey which bind the periosteum to, 

 the osseous lamellae. 



FIG. 309. A GROUP 

 OFENAMEL 

 PRISMS CUT LON- 

 GITUDINALLY 

 FROM THE INCI- 

 SOR TOOTH OF 

 THE RAT, SHOW- 

 ING THEIR IR- 



REGULARLY 



BEADED CHARAC- 

 TER AND THE 

 CROSS STRIA- 

 TIONS. 



Shorter and slen- 

 derer prisms are 

 interpolated periph- 

 erally. X 375. 



