58 



ANIMAL LOCOMOTION. 



articulations (hip-joints). The trunk also rotates in a forward 

 direction on the foot which is placed upon the ground for the 

 time being. The rotation begins at the heel and terminates 

 at the toes. So long as the rotation continues, the body rises. 

 When the rotation ceases and one foot is placed flat upon the 

 ground, the body falls. The elevation and rotation of the 

 body in a forward direction enables the foot which is off 

 the ground for the time being to swing forward pendulum 

 fashion ; the swinging foot, when it can oscillate no further 

 in a forward direction, reversing its course and retrograd- 

 ing to a slight extent, at which juncture it is deposited on 

 the ground, as explained. The retrogression of the swinging 

 foot is accompanied by a slight retrogression on the part of 

 the body, which tends at this particular instant to regain a 

 vertical position. From this it follows that in slow walking 

 the trunk and the swinging foot advance together through a 

 considerable space, and retire through a smaller space ; that 

 when the body is swinging it rotates upon the ilio-femoral 

 articulations (hip-joints) as an axis ; and that when the leg 

 is not swinging, but fixed by its foot upon the ground, the 

 trunk rotates upon the foot as an axis. These movements 

 are correlated and complementary in their nature, and are 

 calculated to relieve the muscles of the legs and trunk en- 

 gaged in locomotion from excessive wear and tear. 



Similar movements occur in the arms, which, as has been 

 explained, are articulated to the shoulders by ball-and-socket 

 joints (fig. 26, X x\ p. 55). The right leg and left arm advance 

 together to make one step, and so of the left leg and right 

 arm. When the right leg advances the right arm retires, and 

 vice versd. When the left leg advances the left arm retires, 

 and the converse. There is therefore a complementary swing- 

 ing of the limbs on each side of the body, the leg swinging 

 always in an opposite direction to the arm on the same side. 

 There is, moreover, a diagonal set of movements, also com- 

 plementary in character : the right leg and left arm advancing 

 t;ogether to form one step ; the left leg and right arm advanc- 

 ing together to form the next. The diagonal movements 

 beget a lateral twisting of the trunk and limbs ; the oscilla- 

 tion of the trunk upon the limbs or feet, and the oscillation 



