anatomical  crown  are  frequently  buried  in  gingival enamel  may  be  attributed  to  the  variation  in  the tissue. As a person gets older, it becomes common for a tooth’s enamel to be completely exposed above the gingiva  and  to  have  root  surface  showing  (gingival recession). The term clinical crown is applied to the part of the crown exposed (visible) in the mouth. thickness,  translucent  proprieties,  and  the  quality  of the crystal structure and surface stains of enamel. Enamel  (fig.  4-6)  is  the  calcified  substance  that covers the entire anatomic crown of the tooth and protects the dentin. It is the hardest tissue in the human The Root body  and  consists  of  approximately  96%  inorganic The root of a tooth is embedded in alveolar bone and is covered by cementurn. The tooth may have a single root or it may have two or three roots. When teeth have more than one root, the region where the roots separate is called the furcation. When a tooth has two roots, the root portion is said to be bifurcated. When it has three roots, the root portion is said to be trifurcated (fig. 4-5). If a tooth has four or more roots, it is said to be multirooted. The tip of each root is called apex. On the apex of each root, there is a small opening that allows for the passage of blood vessels and nerves into  the  tooth.  This  opening  is  called  the  apical foramen. The Cervix The cervix or cervical line (fig. 4-4) is a slight indentation that encircles the tooth and marks the junction of the crown with the root. The cementum joins the enamel at the cervix of the tooth. The point at which they join is called the cementoenamel junction or cervical line. minerals,  1%  organic  materials,  and  3%  water. Calcium  and  phosphorus  (as  hydroxyapatite)  are  its main  inorganic  components.  Enamel  can  endure crushing  pressure  of  approximately  100,000  pounds per  square  inch.  A  layering  of  the  dentin  and periodontium,  coupled  with  the  hardness  of  the enamel,  produces  a  cushioning  effect  of  the  tooth’s different structures enabling it to endure the pressures of  mastication.  Structurally,  enamel  is  composed  of millions of enamel rods or prisms. Each rod begins at the dentinoenamel junction (junction between the enamel and dentin) and extends to the outer surface of the  crown.  Enamel  is  formed  by  epithelial  cells (ameloblasts)  that  lose  their  functional  ability  when the crown of the tooth has been completed. Therefore, enamel,  after  formation,  has  no  power  of  further growth or repair. DENTIN Dentin (fig. 4-6) is the light yellow substance that is more radiolucent than enamel and is very porous; it constitutes the largest portion of the tooth. The pulp chamber is located on the internal surface of the dentin walls. Dentin is harder than bone but softer than enamel.  Dentin  consists  of  approximately  70% inorganic matter and 30% organic matter and water. Calcium  and  phosphorus  are  its  chief  inorganic components. TISSUES OF THE TEETH This section describes the histologic structures of enamel, dentin, cementum, and the dental pulp. Figure 4-6 illustrates the tissues of the teeth. ENAMEL Enamel is translucent and can vary in color from yellowish to grayish white. The different colors of Figure 4-5.—Bifurcated and trifurcated roots. Dentin is a living tissue and must be protected during  operative  or  prosthetic  procedures  from dehydration (drying) and thermal shock. The dentin is perforated by tubules (similar to tiny straws) that run between  the  cementoenamel  junction  and  the  pulp. Cell processes from the pulp reach part way into the tubules like fingers.   These cell processes create new dentin and mineralize it. Dentin transmits pain stimuli by the way of dentinal fibers. Because dentin is a living tissue, it has the ability for constant growth and repair  that  reacts  to  physiologic  (functional)  and pathologic (disease) stimuli. 4-4