JOINTS 



99 



On microscopic examination the synovial villi are seen to vary greatly 

 in shape. They are covered by a simple or double layer of synovial 

 epithelium, and usually, but not invariably, they contain vessels. The 

 synovia (synovial fluid) consists chiefly of water (94%), the remainder 

 including salts, albumin, mucoid substances, fat droplets and fragments 

 of cells shed from the membrane. 



Enamel. 



Dentine. 



Crown 



TEETH. 



A tooth consists of three parts, crown, neck, and root or roots. The 

 crown is that portion which projects above the gums; the root is the part 

 inserted into the alveolus or socket in 

 the bone of the jaw; and the neck, 

 which is covered by the gums, is the 

 connecting portion between the root 

 and crown. A tooth contains a dental 

 cavity filled with pulp. The cavity is 

 prolonged through the canal of the 

 root to the apex of the root, where it 

 opens to the exterior of the tooth at 

 the Joramen apicis dentis. The fora- 

 men is shown, but is not labelled, in 

 Fig. 88. The solid portion of the 

 tooth consists of three calcified sub- 

 stances, the dentine or ivory (sub- 

 stantia eburnea], the enamel (sub- 

 stantia adamantina), and the centent 

 (substantia ossea) . Of these the den- 

 tine is the most abundant. It forms 

 a broad layer around the dental cav- 

 ity and root canal, and is interrupted 

 only at the foramen. Nowhere does 

 the dentine reach the outer surface 

 of the tooth. In the root it is covered 

 by the cement layer, which increases 

 in thickness from the neck toward 

 the apex; and in the crown it is en- 

 closed by the broad layer of enamel. The enamel, however, becomes 

 thin toward the neck, where it meets and is sometimes overlapped by the 

 cement. The pulp, dentine, and cement are of mesenchymal origin, the 

 dentine and cement being varieties of bone. The enamel is an ectodermal 

 formation, but so intimately associated with the others that it may be 

 described with them. 



Root. 



Cement. 



FIG. 88. LONGITUDINAL GROUND SECTION OF A 

 HUMAN INCISOR TOOTH. X 4. 



