ANIMALS. 419 



his discourse, however various the subject, the 

 mouth takes correspondent situations ; and deaf 

 men have been often found to see the force of 

 those reasonings which they could not hear, un- 

 derstanding every word as it was spoken. 



" The under jaw in man possesses a great va- 

 riety of motions, while the upper has been thought, 

 by many, io be quite immoveable.* However, 

 that it moves in man, a very easy experiment will 

 suffice to convince us. If we keep the head fixed, 

 with any thing between our teeth, the edge of 

 a table for instance, and then open our mouths, 

 we shall find that both jaws recede from it at the 

 same time ; the upper jaw rises, the lower falls, 

 and the table remains untouched between them. 

 The upper jaw has motion as well as the under ; 

 and, what is remarkable, it has its proper muscles 

 behind the head, for thus raising and depressing 

 it. Whenever, therefore, we eat, both jaws move 

 at the same time, though very unequally ; for the 

 whole head moving with the upper jaw, of which 

 it makes a part, its motions are thus less obser- 

 vable/* In the human embryo, the under jaw is 

 very much advanced before the upper. " In the 

 adult, it hangs a good deal more backward : and 

 those whose upper and under row of teeth are 

 equally prominent, and strike directly against 

 each other, are what the painters call- under- 

 hung ; and they consider this as a great defect 



* M. Button is of this opinion. He says, that the upper jaw is ira- 

 moveable in all animals. However, the parrot is an obvious exception ; and 

 so is man himself, as shown above. 



