THE ALIMENTARY CANAL AND ITS APPENDAGES. 331 



is compressed laterally, and has usually a groove partially 

 subdividing it into two. At its tip the separation is often 

 complete. The molar teeth or grinders (Fig. 109) have large 

 crowns with broad surfaces, on which are four or five project- 

 ing tubercles, which roughen them and make them better 

 adapted to crush the food. Each has usually several fangs. 

 The mil fc -teeth only differ 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 fang will be found 

 in it. This is filled during life with a soft vascular pulp, and 

 hence is known as the "pulp-cavity" (<", Fig. 110). 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. 110); covering the dentine on the crown is 

 the enamel (1, Fig. 110) and, on the fang, the cement 

 (3, Fig. 110). 



The pulp-cavity opens below by a narrow aperture at the 

 tip of the fang, 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 fang 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 CG per cent in bone. Under the 

 microscope it is recognized by the fine dentinal tiibules 

 which, radiating from the pulp-cavity, perforate it through- 

 out, finally ending in minute branches which 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 ( T -Vo ^ an inch) in diameter. The cement is 

 much like bone in structure and composition, possessing 

 lacunae and canaliculi, but rarely any Haversian canals. It is 

 thickest at the tip of the fang and thins away towards the 

 cervix. Enamel is the hardest tissue in the Body, yielding 

 on analysis only from two per cent to three per cent of 

 organic matter, the rest being mainly calcium phosphate and 

 carbonate. Its histological elements are minute hexagonal 

 prisms, closely packed, and set on vertically to the surface of 



