6 THE FOOD OF ANIMALS 



or, to speak more learnedly, is digitigrade (fig. 304). The sharp 

 claws which arm these digits are obviously related to the capture 

 of other animals. To be efficient, however, these weapons must 

 be kept sharp, and hence, when not in use, they are drawn back 



or retracted into special sheaths, being 

 smartly thrust out when the animal 

 strikes its prey. They are kept sharp 

 by scratching against the bark of trees 

 or any handy table-leg. Like our own 

 nails, to which indeed they correspond, 

 they grow continuously throughout life. 

 ^T^^ The teeth are still more interest- 



Fig. 304. -Digit of cat ing, and are specialized in a very 



extraordinary manner. To understand 

 the way in which they are adapted to 

 a special purpose, the best plan will perhaps be to first consider 

 the arrangement of our own teeth (see vol. i, p. 35). A com- 

 plete set of human second or permanent teeth consists of thirty- 

 two members, eight cutting or incisor teeth in front, four pointed 

 canines or eye-teeth, and twenty grinders or cheek-teeth. The 

 number and arrangement may conveniently be expressed by 

 what is called a dental formula^ as follows : 



Incisors = - , canines = - , grinders = - ~ (consisting of 



2 2 1 7 \ 



premolars = - - , and molars = - = ) . 

 2-2' 3- 3 / 



Each fraction represents one kind of tooth, the numerator indi- 

 cating those of the upper jaw and the denominator those of the 

 lower. The dashes divide the right-hand teeth from the left- 

 hand ones, the numbers in all cases corresponding, as the teeth 

 are symmetrically disposed. 



The cheek-teeth are placed under two headings, premolars 

 and molars, which are much alike in general characteristics, 

 though in the human subject each premolar has two rounded 

 projections or cusps on its crown, and is therefore often called a 

 bicuspid, while each of the molars has four cusps. The essential 

 difference, however, between these two kinds of teeth is that 

 premolars have predecessors among the temporary or milk teeth, 

 while the molars have not. 



