86 



THE HUMAN BODY. 



fall forward on the chest. But (Fig. 39) muscles, 1, in 

 front of the ankle-joint, and others, I, behind it, both 

 contracting at the same time, keep the 

 ff \ joint from yielding ; similarly muscles 

 1 ^ ^ (2) in front of the knee and hip-joints 



are opposed by others (II) behind 

 them, and when we stand both con- 

 tract to a certain extent and keep those 

 joints rigid; and the muscles (III), 

 which run from the pelvis to the back 

 of the head similarly pull against 

 others, 3 and 4, which run from the 

 pelvis to the lower end of the breast- 

 bone, and from the upper end of the 

 breastbone to the anterior part of the 

 skull, and their balanced contraction 

 keeps the head erect. Since the degree 

 to which each muscle concerned con- 

 tracts when we stand must be accu- 

 rately adjusted to the contraction of 

 its antagonist on the opposite side of 

 the joint, we may easily comprehend 

 why it takes us some time to learn to 

 stand, and why a stunned man, whose 

 FIG. -Diagram iiius- muscles have lost guidance from the 



\i 



uSS nervous system, falls. 



Locomotion includes all movements 

 of the body in space, dependent on 

 its own unaided muscular efforts, 

 such as walking, running, leaping, and swimming. 



Explain how the different joints concerned are "braced" in stand 

 ing. Why does it take a child some time to learn to stand? 

 What is meant in physiology by locomotion? 



