52 



ANIMAL LOCOMOTION. 



as the majority of quadrupeds. The increase in the number^ 

 of angles increases the power which an animal has of shorten- 

 ing and elongating its extremities, and the levers which the 

 extremities form. To increase the length of a lever is to 

 increase its power at one end, and the distance through which 

 it moves at the other ; hence the faculty of bounding or leap- 

 ing possessed in such perfection by many quadrupeds.-"- If 

 the wing be considered as a lever, a small degree of motion at 

 its root produces an extensive sweep at its tip. It is thus 

 that the wing is enabled to work up and utilize the thin 

 medium of the air as a buoying medium. 



Another drawback to great speed in man is his erect posi- 

 tion. Part of the power which should move the limbs is 

 dedicated to supporting the trunk. For the same reason the 

 bones of the legs, instead of being obliquely inclined to each 

 other, as in the quadruped and bird, are arranged in a nearly 

 vertical spiral line. This arrangement increases the angle 

 formed by any two bones, and, as a consequence, decreases 

 the speed of the limbs, as explained. A similar disposition of 

 the bones is found in the anterior extremities of the elephant, 

 where the superincumbent weight is great, and the speed, 

 comparatively speaking, not remarkable. The bones of the 

 human leg are beautifully adapted to sustain the weight of 

 the body and neutralize shock.^ Thus the femur or thigh 

 bone is furnished at its upper extremity with a ball-and-socket 

 joint which unites it to the cup-shaped depression (acetabu- 

 lum) in the ilium (hip bone). It is supplied with a neck 

 which carries the body or shaft of the bone in an oblique 

 direction from the ilium, the shaft being arched forward and 

 twisted upon itself to form an elongated cylindrical screw. 

 The lower extremity of the femur is furnished with spirals 

 articular surfaces accurately adapted to the upper extremities 

 of the bones of the leg, viz. the tibia and fibula, and to the 

 patella. The bones of the leg (tibia and fibula) are spirally 



1 The posterior extremities in both the lion and tiger are longer, and the 

 bones inclined more obliquely to each other than the anterior, giving them 

 greater power and elasticity in springing." 



2 " The pelvis receives the whole weight of the trunk and superposed 

 organs, and transmits it to the heads of the femurs." 



