GREGORY, QUADRUPEDAL LOCOMOTION 373 



is provided against not only by the ligaments, but also by the metapodial 

 keels, by the grooved trochlea of the astragalus, by the cnemial crest of 

 the tibia, etc. 



Spiral configuration of limb bones and of articular surfaces. — Good- 

 sir, Pettigrew 19 and others have shown that the articular surfaces of the 

 elbow, ankle and calcaneo-astragalar joints are spirally warped surfaces 

 which act after the manner of screws. The limbs as a whole also are 

 twisted levers with the ridges and muscles arranged spirally. "This 

 arrangement/' says Pettigrew, "enables the higher animals to apply their 

 traveling surfaces to the media on which they are destined to operate at 

 any degree of obliquity so as to obtain a maximum of support or propul- 

 sion with a minimum of slip. If the traveling surfaces of animals did 1 

 not form screws structurally and functionally, they could neither seize- 

 nor let go the fulcra on which they act with the requisite rapidity to* 

 secure speed, particularly in water and air." 



Lost motion. — Lost motion through backward slipping of the foot 

 upon the ground is provided against in the horse by the form and details 

 of the hoof, and in the elephant by the plantar pads. 



Pendulum action of the limbs. — The brothers Weber held that in rapid 

 locomotion the limbs swing freely as pendula, but Marey and later in- 

 vestigators, according to Luciani, 20 hold that the natural swing of the leg 

 is very largely damped and controlled by the flexor muscles. In favor of 

 the view that there is some measure of analogy to the pendulum, we ob- 

 serve that in the horse, the center of gravity of the limb, corresponding 

 to the "bob" of a pendulum, is relatively proximal in position, and this; 

 is associated with rapid oscillation of the limb, whereas in the elephant, 

 the center of gravity of the limb is farther down the shaft, and here we 

 have a slower oscillation of the limb. It will be observed that while the 

 body is moving forward, the propelling limb is moving backward, and its 

 own backward momentum, due to weight alone and to the pull of the ex- 

 tensors, must be overcome by the forward pull of the flexor muscles and 

 by the forward pull of the body as a whole. Hence the heavier the limb, 

 the greater the force expended in overcoming and reversing the mo- 

 mentum of each limb at the end of each stride. 



SPEED 



The speed of a quadruped or biped in motion is measured by the 

 product of the length of the stride into the rapidity of the stride. 



19 Animal Locomotion, pp. 23-24, 28, 29. 1874. 



20 Op. cit., p. 126. 



