200 STUDIES IN PHYSIOLOGY 



to stand erect, yet, if we stop to think of it, in this ap- 

 parently simple process a great many muscles must act 

 together at the same instant. That this cooperation of the 

 muscles is due to the control exercised by nerve tissue is 

 proved by the fact that faintness or any sudden shock to 

 the nervous system, destroys for the time being the power 

 of standing. Not only must the muscles of a small child 

 be developed before it can support itself on its feet, but the 

 brain cells and nerves also must be educated. The ten- 

 dons of the muscles, we have found, run over the joints, and 

 the muscles are arranged in pairs. In standing, the various 

 joints (ankle, knee, thigh, trunk, and neck) are kept rigid 

 by the combined pull of both extensors and flexors. Small 

 wonder, then, that it takes a child a year or two to learn to 

 stand, for more than a dozen sets of muscles must be taught 

 to work harmoniously (see Fig. 93). 



Walking. When standing, one makes the muscles of one's 

 limbs contract at one and the same time. Walking, on the 

 other hand, involves the successive action of the various 

 flexors and extensors. If we are standing with both feet 

 together, and put the right foot forward, the motion is 

 accomplished by the flexor muscles in front of the hip joint. 

 We then touch the right heel to the ground, and later the 

 whole sole of the foot. Meanwhile the body has fallen 

 forward so its weight comes to rest on the right leg. A 

 forward push is given with the toes of the left foot by the 

 contraction of the big extensor muscle at the back of the calf 

 of the leg. Walking may, therefore, be described as a series 

 of falls in a forward direction, in which the balance is re- 

 stored by thrusting out the other, foot. Not only are the leg 

 muscles used in this form of locomotion, but also many other 

 muscles are brought into play. Thus it is easier to walk if 

 the arms are allowed to swing. The whole body sways more 

 or less from side to side as well as backward and forward, 

 and this involves motion between the vertebrae. 



Running. When one is walking, one foot or the other is 



