THE TEETH 165 



the external surface of the tooth. The canals of largest diameter are 

 found next the pulp-cavity, and they become smaller as they branch. 

 The structure forming the walls of these tubules is somewhat denser 

 than the intermediate portion, which is sometimes called the intertubu- 

 lar substance of the dentin ; but in some portions of the tooth, the 

 tubules are so abundant that their walls touch each other. Near the 

 origin and near the peripheral ends of the dentinal tubules, are some- 

 times found solid globular masses of dentin, called dentin-globules, ' 

 which irregularly bound triangular or stellate cavities of variable size 

 (interglobular spaces). Sometimes these cavities form regular zones 

 near the peripheral ends of the tubules. The dentin is sometimes 

 marked by concentric lines, indicating a lamellated arrangement. In the 

 natural condition, the dentinal tubules are filled with a clear liquid, which 

 penetrates from the vascular structures contained in the pulp-cavity. 



Cement. Covering the dentin of the root, is a thin layer of true 

 bony structure, called cement, or crusta petrosa. This is thickest at 

 the summit and at the deeper portions of the root, where it sometimes is 

 lamellated, and it becomes thinner near the neck. It finally becomes 

 continuous with the enamel of the crown, so that the dentin is every- 

 where completely covered. The roots of the teeth, covered with the 

 cement, are firmly embedded in the alveolar cavities of the jaws. The 

 alveoli are lined with what is called dental periosteum, which is reflected 

 over the roots as far as the neck, when it becomes continuous with the 

 fibrous tissue of the gums. 



Pulp-cavity. In the interior of each tooth, extending from the 

 apex of the root or roots into the crown, is the pulp-cavity, which con- 

 tains minute bloodvessels and nervous filaments, held together by 

 longitudinal fibres of connective tissue. This is the only portion of the 

 tooth endowed with sensibility. The bloodvessels and nerves penetrate 

 by a foramen at the extremity of each root. 



The dentin and enamel of the teeth must be regarded as perfected 

 structures ; for when the permanent teeth are lost, they are not repro- 

 duced ; and when these parts are invaded by wear or by decay, they are 

 not restored. 



The thirty-two permanent teeth are classified as follows : - 



1. Eight incisors, four in each jaw, called the central and lateral 

 incisors. 



2. Four canines, or cuspidati, two in each jaw, just back of the 

 incisors. The upper canines are sometimes called the eye-teeth, and 

 the lower canines, the stomach-teeth. 



3. Eight bicuspids the small, or false molars just back of the 

 canines ; four in each jaw. 



