at  the  cusps,  thinning  to  a  knife  edge  at  the cervical  line.  It  is  formed  only  once  and  cannot regenerate or repair itself. Thus, when enamel is destroyed  by  decay,  operative  dentistry  is  required to  reconstruct  the  tooth.  Enamel  has  no  nerve fibers  and  cannot  register  sensations. .  Dentin  is  the  light  yellow  substance  that makes up the bulk of the tooth. It is softer than enamel   but   harder   than   bone   and   is   located inside the crown under the enamel. The point at which the dentin and the enamel meet is called the dentinoenamel junction. Dentin is also found inside the root of the tooth under the cementum. The   inner   surfaces   of   the   dentin   forms   a hard-walled cavity that contains and protects the pulp. Unlike  enamel,  dentin  continues  to  form throughout   the   life   of   the   tooth.   When   the dental  pulp  is  mildly  stimulated  as  a  result  of caries, cavity preparation, abrasion, attrition, or erosion, a protective layer of secondary dentin is formed  on  the  pulp  wall. Even  though  dentin  is  not  sensitive  to  stimuli, sensation may result when mechanical, thermal, or  chemical  stimuli  are  applied  to  it.  The sensation  comes  not  from  the  dentin  itself  but from  cells  that  extend  into  it.  These  cells  are actually part of the pulp, not the dentin, and they are sensitive to stimuli. l   Cementum   is   a   bonelike   substance, although it is not as hard as bone. It forms a pro- tective layer over the root portion of the dentin. The cementum joins the enamel at the cervix of the  tooth. The main function of cementum is to anchor the  tooth  to  the  socket  by  attaching  to  the principle  fibers  of  the  periodontal  ligament. Cementum  is  formed  continuously  throughout the life of the tooth. Thus, it compensates for the loss of tooth substance due to wear by attaching new  fibers  of  the  periodontal  ligament  to  the  root. l  Pulp  is  soft  tissue  that  fills  the  pulp cavity.  This  tissue  contains  numerous  blood vessels and nerves that enter the tooth through the  apical  foramen.  It  is  enclosed  within  the  hard, unyielding  dentin  walls  of  the  pulp  cavity.  The cavity has two parts: the pulp chamber and the root,  or  pulp  canal.  The  chamber  is  located inside the crown. The canal is located inside the root. An important function of the pulp is to form dentin. It provides the cells from which dentin is formed  and  supplies  the  dentin  with  blood. Pulp  responds  to  external  stimuli,  providing sensation  to  the  tooth.  It  responds  to  irritation either   by   forming   secondary   dentin   or   by becoming  inflamed.  Since  the  walls  of  the  pulp chamber  and  root  canal  permit  no  expansion  of the pulp tissue, any inflammatory swelling of the tissue will compress the blood vessels against the walls.  This  results  in  a  condition  known  as hyperemic pulp, which can lead to necrosis of the pulp tissue. Tissues of the Periodontium The  tissues  that  surround  and  support  the teeth  are  the  cementum,  the  alveolar  process, the  periodontal  ligament,  and  the  gingivae. Collectively,  these  tissues  are  known  as  the periodontium.  Throughout  the  following  dis- cussion,  refer  to  figure  2-5. .  The  alveolar  process  is  the  portion  of  the maxillae and mandible that forms and supports the  sockets  (alveoli)  of  the  teeth. The alveolar process can be divided into two parts:  the  alveolar  bone  proper  and  [he  supporting alveolar bone. The alveolar bone proper is a thin layer  of  bone  that  lines  the  tooth  socket  and attaches  the  principal  fibers  of  the  periodontal ligament.  The  supporting  alveolar  bone  is  the portion of the alveolar process that surrounds the alveolar  bone  proper  and  gives  support  to  the tooth   socket. l  The  periodontal  ligament  consists  of hundreds  of  tissue  fibers  that,  except  at  the  apical foramen, completely surround the tooth root. The ligament acts as a shock absorber, reducing the impact  of  the  teeth  as  they  occlude. l The gingivae are the soft tissues that cover the   alveolar   process   and   surround   the   necks of   the   teeth.   They   consist   of   an   outer   layer of  epitheliums  and  an  inner  layer  of  connective tissue. Healthy  gingivae  are  pink,  firm,  and  resilient. They  have  a  stippled  appearance.  Stippling  refers to the “orange peel” texture of the healthy tissue. Inflammation  causes  a  loss  of  stippling.  When inflamed,  the  gingivae  may  become  sore  and swollen,  and  they  may  bleed. 2-42