TEETH. 



stellate cells containing large nuclei. They may be spread in a single thin 

 layer, or heaped together, making an epithelium of three or four layers. 

 The synovial membrane may be thrown into coarse folds (plicae) or into 

 slender projections often microscopic (villi). The synovial villi, Fig. 73, 

 are variously shaped but are usually finger-like; they ordinarily contain 

 blood vessels and impart a reddish velvety appearance to the membrane. 

 The large folds of embryonic tissue projecting into the joint, but always 

 covered with the mesenchymal epithelium, may become dense fibrous 

 articular discs such as are interposed 

 in the sternoclavicular and mandib- 

 ular joints, or they may form the 

 iibrous cartilage-like menisci of the 

 knee joint. Nerves and blood ves- 

 sels are absent from the discs, men- 

 isci, and labra glenoidalia. 



Synovia [synovial fluid] is g_j.' , 

 water, the remainder being salts, 

 proteids, and mucoid substances, 

 together with fat drops and frag- 

 ments of cells shed from the mem- 

 brane. 



Enamel. - 



Dentine. 



Root. 



Cement. 



TEETH. 



A tooth consists of three parts, 

 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 foramen apicis dentis. The 

 foramen is shown, but is not labelled, in fig. 74. The solid portion of 

 the tooth consists of three calcified substances, the dentine or ivory (sub- 

 stantia eburnea), the enamel (substantia adamantina), and the cement 

 (substantia ossea). Of these the dentine is the most abundant. It forms 

 a broad layer around the dental cavity and" root canal and is interrupted 



Crown. 



Xeck. 



FIG. 74. LONGITUDINAL GROUND SECTION OF A 

 HUMAN INCISOR TOOTH. X 4. 



