A STUDY OF BLOOD MANUFACTURE 79 



dental formula. It consists of a series of fractional represen- 

 tations, the numerators of which represent the teeth in the 

 upper jaw, while the denominators show the corresponding 

 teeth of the lower jaw. The incisors, canines, premolars 

 (bicuspids), and molars are indicated by their initial letters. 

 The dental formula of man is, therefore, 



2 + 2 1 + 1 2+2 



which means that the adult human being should have 8 

 incisors, 4 canines, 8 bicuspids, and 12 molars. 



Milk Teeth. During early childhood there appears a first 

 set of milk teeth, which later are loosened and displaced by 

 the growth of the permanent set that we have just described. 

 There are but twenty teeth in the milk set, and the formula 

 is as follows : 



2 + 2 1 + 1 2 + 2_ 



Bicuspids are therefore wanting, and the milk molars occupy 

 the position in each half jaw which later is filled by the two 

 bicuspids of the permanent set; hence, the molars of the per- 

 manent set develop in a region of the jaw that bears no teeth 

 during childhood. The teeth appear gradually, the lower 

 incisors usually being the first to push through the gums at 

 about the sixth month. The third permanent molars of each 

 half jaw often appear as late as the twentieth year; they 

 are called the wisdom teeth. 1 



Structure of Teeth. The exposed portion of a tooth is 

 called its crown. It is covered with a layer of e-nam'el, 

 which is the hardest tissue in the body. The root or fang of 



1 The roots of the milk teeth are gradually absorbed and finally the 

 teeth loosen and come out. This process is called " shedding" and is 

 very slow, occupying, in some cases, a year or more. The milk teeth 

 are replaced by the permanent teeth, which appear usually after the 

 milk teeth are shed. The following table, compiled from Bromell's 



