210 



HUMAN PHYSIOLOGY 



CHAP. 



metatarsal, and phalanges together constitute the lever. 



The phalanges constitute the fulcrum ; the muscles of 



the calf, attached to the heel 

 bone by the tendon of Achil- 

 les, furnish the power ; and the 

 weight of the body resting upon 

 the ankle is the load. This illus- 

 trates a lever of the second 

 class. In a similar way every 

 muscular movement can be 

 shown to fulfill the conditions 

 of one or more of these levers. 

 147. Standing. When lying 

 on the back perfectly quiet, with 

 every muscle relaxed, as in sound 

 sleep, the skeletal muscles are 

 doing no work. An infant can- 

 not stand at birth, and yet its 

 muscles may be used in kicking 

 and throwing the arms about. 

 It is only after the bones have 

 gained sufficient strength, and 

 the muscles have come under 

 the control of the will, that the 

 child can assume the erect pos- 

 ture. 



Standing involves the use of 

 many muscles and nerves. But 

 after we have learned to stand, 

 these nerves stimulate the vari- 



FlG. 113. Showing the at- 

 tachment of the most im- 

 portant muscles used in 

 standing. The muscles at 

 I, II, and III tend to keep 

 the body from falling for- 

 ward; and those at i, 2, 3, 

 4, and 5 tend to keep it 

 from falling backward. The 

 arrows indicate the direc- 

 tion in which the muscles 

 pull. (Huxley.) 



