DIGESTION AND ABSORPTION 



333 



Then come two teeth on each side, called premolars. Lastly, the 

 flat-topped molars, or grinding teeth. Food is caught between ir- 

 regular projections on the surface of the molars and crushed to a 

 pulpy mass. 



Laboratory Exercise. — Procure from the dentist examples of each kind of 

 teeth. Identify and draw in your notebook one of each of the four classes. 



Dental Formula of Man. — It is possible, as we have seen, to classify 

 mammals partially on the basis of the kind and number of teeth they 

 possess. The number of these teeth may be graphically shown by means 

 of what is called a dental formula. In a dental formula, the teeth of the 

 upper jaw (the right and left sides separately) form the numerator of the 

 fraction; those of the lower jaw form the denominator. This dental 

 formula of man is graphically shown as follows : — 



incisors 



2 + 2 . 

 2 + 2 ' 



premolars = — ^^ : 

 ^ 2 + 2 ' 



camnes = 



molars = 



1 + 1 . 

 1 + 1' 

 3+3 . 

 3 + 3 ' 



total, 32. 



Man differs from other vertebrate animals in that when young the child 

 has a set of teeth which later fall out and are replaced by the thirty-two 

 teeth known as the permanent set. 



The first set, known as the milk teeth, consists of twenty teeth arranged 

 as follows : — 



mcisors 



2+2 . 1+1 1 2+2 



= : canines = : molars = — ' — ; 



2 + 2 1 + 1 2 + 2 



total, 20. 



The permanent teeth appear to push out the milk teeth ; this is indeed 

 the case, as the beginnings of the permanent teeth are found very early in 

 life under the milk teeth. The so-called wisdom teeth (four molars) do 

 not appear until the eighteenth to the twenty-first year of life. 



Internal Structure of a Tooth. — If a tooth is cut 

 lengthwise, it is found to be hollow; this cavity, called 

 the pulp cavity, corresponds to the cavity containing 

 marrow in bones. In life it contains living material — 

 the blood vessels, nerves, and cells which build up the 

 bony part of the tooth. The bulk of the hard part of 

 the tooth consists of a limy material called dentine. Out- 

 side of this is a very hard substance called enamel; this 

 substance, the hardest in all the body, is thickest on the 

 exposed surface or crown of the tooth. What is the use 

 of this hard layer? Why is it so placed? Each tooth 

 is held in its place in the jawbone by a thin layer of 

 bony substance called cement. 



Section of a tooth; 

 a, enamel; b, 

 dentine; c. pulp 

 cavity contain- 

 ing blood ves- 

 sels and nerves; 

 d, cement. 



