DIGESTIVE SYSTEM. 157 



inite period after the eruption of a tooth. The proc- 

 ess is one of apposition, thickening the dentin at the 

 expense of the pulp. Finally this growth ceases. 

 Irritation of the pulp, or the pulp of some tooth on 

 the same side, may lead to the formation of second- 

 ary dentin. The latter is an imperfect structure. 

 The tubules are smaller and less numerous, while the 

 matrix is less compact and shows a deficiency of 



Fig. 115. Section of dentin in the crown cut in length of the tubules 

 (Noyes). 



inorganic salts. Several deposits of secondary 

 dentin may thus be produced. 



The Pulp. The pulp occupies the center of the 

 tooth. It consists of connective-tissue cells, con- 

 nective-tissue fibrils, a semifluid interfibrillar ground 

 substance, nerve plexus largely non-medullated, 

 blood and lymph vessels. We may recognize three 

 kinds or layers of cells, the most important being the 



