DIGESTIVE SYSTEM. 161 



teeth the cementum is thin and in teeth of old per- 

 sons the cementum is thick. This continuous 

 growth is necessary in order to establish attachment 

 for new fibers of the peridental membrane and to con- 

 form to the natural growth of the jaw. 



Between the lamellae, particularly in the apical 

 portion of the root where the cementum is thick, 

 numerous lacunae are present, resembling those of 

 bone. Canaliculi radiate from these lacunae with 

 less regularity, however, than in the case of bone. 

 They may be confined to one side of a lacuna, usually 

 the side toward the surface. 



Local thickenings of the cementum, called hyper- 

 trophies, are common. These enlargements involve 

 one or more of the lamellae and were formerly called 

 exostoses, or cementostoses. 



Peridental Membrane. This is an organic tissue 

 that surrounds the root of teeth and occupies the 

 space between the cementum and the bony wall 

 of the alveoli. Its chief function is to anchor a tooth 

 to the jaw and give support to the gingivus. Its 

 chief constituent is white connective-tissue fibers in- 

 terspersed with a variety of cells, blood-vessels, 

 lymphatics and nerves. 



The fibrous tissue may be divided into two classes : 

 coarse, radiating fibers that form the principal bulk 

 and perform the principal function of anchorage, and 

 a secondary fine variety that interlace with these and 

 unite largely with the anastomosing blood-vessels of 

 the tissue. The principal fibers connect, on the one 

 hand, with the cementum which they enter in bun- 

 dles to form the fibers of Sharpey, and on the other 



