190 PHYSICAL EXPRESSION. 



of the head is the result of the action of a weaker 

 piece of nerve-muscular mechanism than flexion ; 

 extension of the hands at wrists and knuckles often 

 occurs with head extension, as in the expression of 

 astonishment. 



Movements of Lower Jaw. The lower j aw is j ointed 

 on to the skull ; in the natural upright position of 

 the body gravity tends to make the jaw fall. Two 

 pairs of muscles, the masseter, and the temporal, 

 raise the jaw, and keep the teeth in contact ; these 

 muscles are both supplied by the motor division of 

 the fifth pair of nerves of the brain. The masseter 

 muscle arising from the cheek bone (the malar and 

 zygoma) is inserted into the angle of the lower jaw ; 

 the temporal muscle arises from the fossa of that 

 name, and is seen swelling out and may be felt as a 

 hard mass in the temple when the jaws are tightly 

 clenched. The jaw is depressed by a group of 

 muscles supplied by the ninth pair of nerves of the 

 brain. The mouth is often opened when the jaw 

 is depressed, always when the jaws are opened 

 widely, but the lips can be parted while the jaws 

 are still closed, showing the teeth. The jaw is 

 depressed and the mouth opened in the expression 

 of astonishment.* 



The jaw often drops somewhat as a passive move- 

 ment, owing to relaxation of the muscles which 

 support it. It is strongly depressed in yawning, 

 occasionally so much so as to cause dislocation; 

 this is owing to strong extensor depression of the 

 bone. 



* Darwin, " Expression of the Emotions," p. 280. 



