76 



ADVANCED PHYSIOLOGY 



Enamel 



position, far back near the hinge of the lower jaw, gives 

 great leverage; and being farthest from the mouth opening 

 and nearest the largest part of the cheek, room for their 

 grinding function is insured. The remarkable fitness of each 

 structure for its work is a striking fact which may be no- 

 ticed in all parts of the body. All of these are permanent teeth. 

 All except the molars are preceded in childhood by baby teeth 

 which are lost in early years. Permanent teeth may appear as 

 early as six years of age, and since this is so, particular pains 

 should be taken that they are properly cared for from tho time 

 of their first "cutting through." 



If a tooth is cut open, it proves to be made of four kinds 

 of material. The outside layer (see Fig. 37), an extremely 



hard deposit of calcium phosphate, 

 is called the enamel. This is thick- 

 est on the exposed surface of the 

 tooth, or crown, and diminishes until, 

 as the tooth enters the gum, it gives 

 way entirely to a softer substance, 

 the cement. This cement covers 

 the roots and connects the teeth 

 firmly with their sockets in the jaw 

 bone. It is this substance which 

 yields when a tooth is extracted. 



Inside the enamel and cement is a 

 uniform layer, the dentine. This, 

 too, is calcium phosphate in compo- 

 sition, but is less hard than the 

 enamel. Inside the dentine is a space occupying the 

 central part of the tooth, and extending down into the 

 tips of the roots. This central space is occupied by a soft, 

 pasty mass, like the marrow of a bone, filled with fatty and 

 connective tissues, blood vessels and nerves. These vessels 

 and nerves enter the tips of the roots, the former bringing 



FIG. 37. A 



THROUGH A 



