PHYSIOLOGY OF MOVEMENT. 741 



the shoulder, through the action of the extensor pedis, which, with the 

 metacarpal extensors, converts the angle of the pastern-joints into 

 :i straight line, the axis of the phalanges forming an angle anteriorly 

 with the axis of the metacarpus. Simultaneously with this movement 

 the knee becomes extended, the axis of the metacarpus forming a 

 straight line with that of the radius, this movement also being accom- 

 plished by the contraction of the metacarpal extensors. Finally, the fore- 

 arm returns to its extended position in a line with the upper arm, partly 

 through the action of the ligaments of the elbow-joint and partly from 

 the contraction of the five olecranon muscles. During this forward 

 motion of the foot in extension the humerus leaves its position of flexion, 

 and the foot striking the ground, the fore limb again becomes vertical, 

 and by the action of the pectoral latissimus dorsi muscles the body weight 

 is again transferred to it. The lever motion of the muscles producing 

 these movements is seen in Figs. 301 and 302. Instead, however, of the 

 simple motions of flexion and extension above described, which 

 occur when one forefoot is simply raised from the ground and again 

 returned to the same spot, the flexed forearm and' carpus may be carried 

 beyond the vertical line through the extension of the humerus by the 

 contraction of the various extensors, aided by the abductors of the 

 humerus, while at the same time the posterior angle of the scapula 

 is drawn backward and downward by the contraction of the rhomboid 

 and trapezius muscles. At first, in the forward motion of the lower end 

 of the humerus flexion is increased, but the farther it advances forward 

 and the more the scapula rotates backward the more the shoulder angle 

 becomes opened, and, under the action of the adductors of the humerus, 

 directed outward and upward, so that the extensors of the lower joints 

 are put on the stretch and their contraction converts the flexion of tjie 

 limb into extension, the fore leg thus describing a pendulum-like 

 motion, and in its extended position is directed forward and downward 

 and strikes the ground in front of its previous position (Fig. 303). Then, 

 from the impulse communicated to the trunk from the hind legs, the 

 weight of the body is gradually transferred to the fore leg, which, 

 the foot remaining on the ground, gradually becomes more and 

 more vertical from the forward motion on it of the trunk (Fig. 304). 

 As, however, the inertia of the moving body carries the shoulder be3 7 ond 

 the vertical, the axis of the fore limb is then directed downward 

 and backward, the assumption of this position being aided by the 

 forward rotation of the scapula, while the extensors of the forearm (the 

 foot being fixed on the ground) not only sustain the elbow- and shoulder- 

 joints, but, by their contractions, give an additional forward impetus to 

 the body. The extreme extended position of the limb puts the flexors 

 of the lower joints on the stretch and so leads to their contraction, and 



