DIGESTIVE SYSTEM. 165 



cement corpuscle analogous to a bone corpuscle. 

 Such an inclusion is the exception in the life history 

 of these cells. 



3. The osteoblasts are also connective-tissue cells, 

 but cover the bony wall of the alveoli. They lie 

 between the fibers and their function is the pro- 

 duction of bone which anchors the supporting fibers 

 to the alveolar wall. These cells are analogous to 

 the osteoblasts of bone. 



4. The osteoclasts are large, bone-destroying, 

 multinucleated cells that are often called giant cells. 

 They may also act upon and absorb the cementum 

 and dentin. They are not constantly present in the 

 peridental membrane but appear whenever cal- 

 cified tissue is to be destroyed. They apply them- 

 selves to the surface to be absorbed and by their 

 physiological action excavate cavities in which they 

 lie, known as Howship's lacunce. The latter may 

 later fill in with new cementum or bone, thus leaving 

 a permanent record of the process of absorption and 

 repair. The absorption of the roots of deciduous 

 teeth results from the physiological action of the 

 osteoclasts. If for any cause, such as bacterial in- 

 vasion, the osteoclasts fail to appear, the root of 

 the deciduous tooth does not absorb but remains as 

 a permanent obstruction to the developing new tooth. 



5. Epithelial cells, believed by some to be remains 

 of the enamel organ, envelop the surface of the roots 

 and are found in both young and old teeth. Their 

 function is not known. They appear as cords of 

 epithelial cells that anastomose freely to form an en- 

 veloping network, which nowhere seems to unite 



