. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 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 ki
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 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 b
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