274 ANNALS NEW YORK ACADEMY OF SCIENCES 



The diverse adaptations in the limbs, considered as compound levers, 

 are related to either or both of the factors, "length of stride" and "rapid- 

 ity of stride." 



LENGTH OF STRIDE 



Length of limb. — Length of limb is the first factor of length of stride. 

 It is generally proportional to height at the shoulders and hips. Length 

 of limb has been attained in cursorial animals by lengthening the lower 

 and middle segments of the limb, in graviportal animals by lengthening 

 especially the proximal segments of the limb. 



Angle of stride. — -Angle of stride is the second factor. It is measured 

 by the arc described by the lower end of the femur or humerus in swing- 

 ing from the position of extreme extension to that of extreme flexion. A 

 wide angle of stride not only lengthens the stride, but also enables each 

 limb, first, to be placed in turn below or beyond the center of gravity in 

 order to secure more continuous support for the center of gravity, and, 

 secondly, it enables the propelling limb to exert its propulsive effort for 

 ■a relatively long period. 



Acceleration increment of stride due to ballistic power of limbs. — Those 

 portions of the stride that are due simply to the length of the limb and to 

 the angle of the stride might, if determined, be illustrated by moving the 

 inert limbs of a dead animal suspended in the air. In the slow walk of 

 a biped, the successive positions of the legs might for our purposes be rep- 

 resented by a series of inverted Y's ( A A A A A A ) with the lower ends 

 touching. Each f\ represents a single step and two successive A *s repre- 

 sent a stride. In the rapidly moving animal, however, the stride receives 

 a very considerable increment, due to the impetus imparted by the pro- 

 pelling limb and to the forward motion of the body as a whole, which 

 carries the forwardly moving foot to a position far in advance of its own 

 unaided reach. This "acceleration increment," as it may be called, in- 

 creases with the velocity of the movement and .is proportional to what 

 we may designate the ballistic power of the limb. This ballistic power 

 may be defined as excess propulsive power over and above that which 

 is necessary to move the limbs as stilts and to support the weight of the 

 Ibody; it is expended in lengthening the stride. Ballistic power and 

 the acceleration increment of the stride are measured by the length of 

 time at least three of the feet are off the ground together during a single 

 stride of a quadruped running at full speed. In Fig. 1, representing an 

 elephant in rapid motion (ambling), it will be observed that three of the 

 feet are never off the ground at the same instant; whereas Stillman's 

 Pigs. 2-10 show a galloping horse in which at least three of the feet are 



