54 



APPLIED PHYSIOLOGY 



The floor is formed by the tongue and the lower 

 jaw. 



69. The jaws. The lower jaw is a semicircular bone, 

 whose hinder extremities are curved upwards. Each tip 

 forms a hinge which turns in a socket just in front of the 

 ear. It carries a semicircle of teeth, which exactly fit 

 against a similar semicircle upon the upper jaw. The 

 lower jaw is moved by powerful muscles in three direc- 

 tions : first, up and down ; second, sidewise ; third, back- 

 ward and forward. 



The upper jaw is a strong bone of irregular shape, firmly 

 fixed to the rest of the skull. Its interior is hollowed out 

 to form a cavity called the antrum, which has a small 

 opening into the nose. The upper ends of the teeth some- 

 times project so far upward as to make slight elevations 

 upon its floor. Sometimes an inflammation or abscess of 



a tooth may extend to the 

 antrum, so that it becomes 

 filled with pus, producing a 

 very serious trouble. 



70. Teeth. The teeth are 

 hard, bony pegs set deeply 

 into the lower jawbone and in 

 the edge of the hard palate. 

 There are sixteen on each 

 jaw. Counting from the mid- 



The teeth at the age of six and 

 one half years. /, the incisors; 

 O, the canine ; M, the molars ; the 



last molar is the first of the permanent die of the front of each jaw, the 



side 



like 



bicuspids; JV, of the second molar; chisels, SO as to bite Or gnaw 

 the sac of the third molar is empty. re ^i r j j 11 j 4.1. 



MARSHALL ff tne f d, and are called the 



incisors. In a squirrel, they 



are long and sharp, so as to gnaw through wood. The 

 third tooth is the canine. It is a round, firmly set tooth, 



