At every step, the knees are stretched out; the legs kept as straight
as possible; the feet almost graze the ground; the tread is neither
with the mere balls of the toes, nor with the whole sole of the foot;
and the spring is made rapidly from one foot to the other, so that they
pass each other with great velocity--(Plate IX. fig. 2).
But the abdominal members are not the only ones in motion, although
it is in them that the greatest developement takes place. Throughout
the whole time of running, a strong and permanent contraction of
the muscles of the shoulder, arm, and forearm takes place: this,
though very violent, is less for the purpose of aiding motion than
of preserving the immobility of the thorax, which is pressed upon
the whole thoracic member, whose flexors and adductors are strongly
contracted. The degree of velocity, however, must be proportioned to
the length of the steps. Too slow and long, as well as too quick and
short, steps, may be equally injurious.
[Illustration: _Plate IX_
_Page 38._