400 THE HUMAN BODY 



are four or five projecting tubercles, which roughen them and 

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

 several fangs. 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 fang will be found in it. This 

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

 as the "pulp-cavity" (c, Fig. 127). 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. 127); covering the 

 dentine on the crown is the enamel (I, Fig. 127) and, on the fang, 

 the cement (3, Fig. 127). 



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 66 per cent in bone. Under the microscope it 

 is recognized by the fine dentinal tubules which, radiating from the 

 pulp-cavity, perforate it throughout, 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 (^V<r f an m ch) in diameter. The cement is 

 much like bone in structure and composition. 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 the subjacent dentine. It is thickest over the free 

 end of the crown, until worn away by use. Covering the enamel in 

 unworn teeth is a thin structureless horny layer, the enamel cuticle. 



The Tongue (Fig. 128) is a muscular organ covered by mucous 

 membrane, extremely mobile, and endowed not only with a deli- 

 cate tactile sensibility but with the terminal organs of the special 



