78 



STUDIES IN PHYSIOLOGY 



Kinds of Teeth. 1 In the complete set of an adult there 

 are thirty-two teeth. They may be divided into four groups, 

 each kind of tooth having a definite type of structure that 

 adapts it for a special use. In front there are eight teeth 

 with chisel-shaped edges. The four upper teeth work upon 

 the corresponding teeth of the lower set something like the 

 blades of a pair of scissors ; these eight teeth have, there- 

 fore, received the name in-cis'ors (Latin inci'sum, from inci'do, 

 inci'dere = to cut into). Just behind the incisors on either 

 side of the jaw is a tooth resembling the sharp-pointed teeth 



in the mouth of 



123 4 a dog ; from this 



fact these four 

 teeth in the hu- 

 man mouth are 

 called canines' 

 (Latin ca'nis = 

 dog). Still far- 

 ther back in 

 each half jaw 

 are two bi-cus'- 

 pids (Latin bi = 

 two 4- cuspis = point), so called because the free end of 

 each has two projections, one of which lies next the cheek, 

 the other toward the interior of the mouth. (In other ani- 

 mals, the teeth corresponding to the bicuspids of man are 

 called pre-mo'lars.) The three back teeth on either side of 

 the upper and lower jaws have broad surfaces, from which 

 project four or five elevations. When the food is caught 

 between these mo'lar teeth (Latin molaris = a, millstone), it 

 is ground into a pulpy condition, and thus is well prepared 

 for mixture with the digestive juices. 



Dental Formula. For convenience in comparing the teeth 

 of different animals we use a form of expression called a 



FIG. 25. Teeth from Half of Upper Jaw. 



1 = incisors. 2 = canines. 



3 = bicuspids or premolars. 4 = molars. 



1 Teeth of various kinds should be procured from a dentist, and 

 should be cleaned by boiling in a solution of caustic soda. 



