THE ALIMENTARY CANAL AND ITS APPENDAGES 445 



are four or five projecting tubercles, which roughen them and 

 make them better adapted to crush the food. Each has usually 

 several roots. The milk-teeth differ only in subsidiary points from 

 those of the same names in the permanent set. 



The Structure of a Tooth. If a tooth be broken open, a cavity 

 extending through both crown and root will be found in it. This 

 is filled during life with a soft vascular pulp, and hence is known 

 as the "pulp-cavity" (c, Fig. 126). The hard parts of the tooth 

 disposed around the pulp-cavity consist of three different tissues. 

 Of these one immediately surrounds the cavity and makes up most 

 of the bulk of the tooth; it is dentine (2, 

 Fig. 126); covering the dentine on the 

 crown is the enamel (1, Fig. 126) and on 

 the root, the cement (3, Fig. 126). 



The pulp-cavity opens below by a narrow 

 aperture at the tip of the root, or at the tip 

 of each if the tooth have more than one. 

 The pulp consists mainly of connective 

 tissue, but its surface next the dentine is 

 covered by a layer of columnar cells. 

 Through the opening on the root blood- 

 vessels and nerves enter the pulp. 



The dentine (ivory) yields on analysis the 

 same materials as bone but is somewhat 

 harder, earthy matters constituting 72 per 

 cent of it as against 66 per cent in bone. FIG. 126,-Section through 

 Under the microscope it is recognized by a premoiar tooth of the 



_ _ ^ . ... ^ cat still embedded in its 



the fine dentinal tubules which, radiating socket, i, enamel; 2, den- 

 from the pulp-cavity, perforate it through- ^ m : . |; thTbone tf the 

 out, finally ending in minute branches which J^?* J aw; c > the pulp ~ 

 open into irregular cavities, the interglobular 

 spaces, which lie just beneath the enamel or cement. At their 

 widest ends, close to the pulp-cavity, the dentinal tubules are only 

 about 0.005 millimeter (* 5 Vir f an inch) in diameter. The cement 

 is much like bone in structure and composition. It is thickest at 

 the tip of the root and thins away towards the cervix. Enamel is 

 the hardest tissue in the Body, yielding on analysis only from 2 

 per cent to 3 per cent of organic matter, the rest being mainly 

 calcium phosphate and carbonate. Its histological elements are 



