272 PHYSIOLOGY AND HEALTH. 



upper. The same action may not only be very distinct- 

 ly felt, but even seen, on the temple, in the muscle that 

 closes the jaw. The upper end of this muscle is fixed upon 

 the bone of the temple, and passes down behind the project- 

 ing arch of the cheek-bone to the upper side of the lower 

 jaw. When it contracts and draws up the lower jaw, it 

 swells out on the temple so prominently that its increase and 

 diminution are seen every time we move the jaw, either in 

 mastication of food or talking. 



628. The muscles have only a power of contraction. 

 They have no active power of forcible expansion. They 

 can draw the bones together, but they cannot push them 

 apart. The muscles that bend the wrist or the ankle can- 

 not straighten it out again. But nature has made beautiful 

 provision for this, by affixing to every joint two or more sets 

 of muscles for the various kinds and directions of motion 

 required. Upon the hinge-joints there are two sets, one 

 to bend, the other to straighten them. Thus the muscle on 

 the front of the upper arm bends the elbow, and the muscle 

 on the back of the upper arm, straightens it. The same ar- 

 rangement is found in the muscles that move the wrist, the 

 fingers, the knee, the ankle, and the toes. 



629. It is interesting to see this antagonism of muscles, 

 and their alternate working in the movements of the fingers. 

 If we clasp the right fore-arrn, at its largest part, three inches 

 below the elbow, with the fingers and thumb of the left hand, 

 and then drum rapidly with the fingers of the right hand, we 

 shall feel the swelling and decline of the muscles on the 

 opposite sides of the arm, alternating with each other, and 

 precisely corresponding with the motions of the fingers. 

 While the fingers are bending, the inside muscles swell and 

 the outside muscles decline; and, while the fingers are 

 straightening, the outside muscles swell and the inside mus- 

 cles decline. The same alternation may be felt in the 

 movements of any other joint. 



630. The hinge-joints want two sets of muscles only 

 one to bend, the other to straighten But some other joints 



