126 THE HUMAN BODY 



bone (tibia). Others passing before and behind the knee-joint 

 keep it from yielding; and so at the hip-joints: the others again, 

 lying in the walls of the abdomen and along the vertebral column, 

 keep the latter rigid and erect on the pelvis; and finally the skull 

 is kept in position by muscles passing from the sternum and 

 vertebral column to it, in front of and behind the occipital con- 

 dyles. 



Posture the Task of the Extensor Muscles. In the mainte- 

 nance of posture the muscles which bear the strain are in general 

 the extensors, since posture requires that the joints shall be kept 

 from bending. So far as the flexors co-operate they do so by pre- 

 venting overextension, a part which calls for relatively little exer- 

 tion. The degree of pull manifested by the extensor muscles in 

 the maintenance of posture is slight but steady. In the course 

 of a day we may become aware of postural fatigue, showing that 

 muscular activity has been present, although we may not have 

 been conscious of definite volition. This mild degree of sustained 

 contraction, which differs strikingly from the ordinary rapid and 

 extensive contractions of skeletal muscle, is known as tonus. In 

 the maintenance of posture it is chiefly extensor tonus. In what 

 respects tonus is comparable with ordinary contraction and in 

 what respects different, is not yet known. 



Locomotion includes all the motions of the whole Body in 

 space, dependent on its own muscular efforts: such as walking, 

 running, leaping and swimming. 



Walking. In walking the Body never entirely quits the ground, 

 the heel of the advanced foot touching the ground in each step 

 before the toe of the rear foot leaves it. The advanced limb sup- 

 ports the Body, and the foot in the rear at the commencement of 

 each step propels it. 



Suppose a man standing with his heels together to commence 

 to walk, stepping out with the left foot: the whole Body is at first 

 inclined forwards, the movement taking place mainly at the 

 ankle-joints. By this means the center of gravity would be 

 thrown in front of the base formed by the feet and a fall on the 

 face result, were not simultaneously the left foot slightly raised 

 by bending the knee and then swung forwards, the toes just clear 

 of the ground, and, in good walking the sole nearly parallel to it. 

 When the step is completed the left knee is straightened and the 



