50 



CONNECTIVE TISSUE. 



the same manner. The bone becomes of greater circumference by 

 the deposits made by the periosteum externally, and the medul- 

 lary canal is made larger by the absorption of a portion of its walls. 

 In the repair of bones, as after fractures, the periosteum performs 



the same office as in 

 the original formation 

 of bone. 



Dentin. The con- 

 sideration of this sub- 

 stance calls for a de- 

 scription of the teeth, 

 of which it forms an 

 important part. 



A tooth (Fig. 40) is 



Enamel. 



/ 



jS$r~ Pulp-cavity. 



Dentin. 



"T/ Cementum. 



divided anatomically 

 into the crown, the vis- 

 ible portion, which pro- 

 jects above the gum ; 

 the root, the portion 

 out of sight within the 

 alveolus or socket ; and 

 the neck, the constricted 

 portion joining the 

 crown and the root. 

 In the center of the 

 crown and extending 

 into the roots is the 

 pulp-chamber, the 

 openings of which, at 

 the tip of the roots, 

 are apical foramina, 

 through which pass 

 b 1 o o d-v e s s e 1 s and 

 nerves into the pulp- 

 chamber, which con- 

 tains dental pulp. 

 This latter is com- 

 posed of a gelatinous 

 connective tissue with 

 branched cells, to- 

 gether with the blood- 



_ 



FIG. 40. Scheme of a longitudinal section 

 through a human tooth ; in the enamel are seen 

 the "lines of Retzius" (Bohm and Davidoff). 



vessels and nerves just 



mentioned; lymphatic vessels are absent. Some of the cells 

 are m contact with the dentin of the tooth, and having been 

 concerned in its formation are called dentin-forming cells or 

 odontoblasts. 



The solid part of a tooth, excluding the pulp-chamber and its 



