ELEMENTS OF GENERAL SCIENCE 



FIG. 121. Lower jaw and teeth of a cat 



then enter the circulatory system and by circulation may pass 

 to the various parts of the body. From the smallest parts of 

 the circulatory system food may pass out into living cells, and 



by assimilation it may be- 

 come a part of the living 

 protoplasm of the body. 



277. Mastication : teeth. 

 In all of the higher animals 

 and man the mouth is a 

 special organ of mastication. 

 It is provided with bones, 

 teeth, and muscles so ar- 

 ranged as to make it possible to masticate the food. Some of 

 the teeth are broad -crowned, so that they may serve as crushers ; 

 others are sharp and may pierce or tear the food substances 

 (fig. 121). Man has both of these kinds of teeth, but some ani- 

 mals that live upon plant food directly (known as herb eaters, 

 or herbivorous animals) have only the broad-crowned teeth. 

 The teeth of the flesh eaters, or carnivorous animals, are all (or 

 nearly all) cutting or tear- 

 ing teeth. It is evident that 

 man's teeth enable him to 

 eat a greater variety of food 

 than do tliose of some of the 

 lower animals. It will be 

 found extremely interest- 

 ing to examine the struc- 

 ture of the teeth to see of 

 what parts they consist and 

 how they are held in place 

 in the bones with which 

 they are connected. 



In form the teeth of man exhibit considerable variation 

 (fig. 122). In the front of the mouth on each jaw there are 

 two pairs of flattened teeth with cutting edges. These are 



FIG. 122. The human lower jaw and 

 teeth 



