142 



ZOOLOGY. 



&6 number of the toes is reduced to its minimum: in the 



IKTM-. for example, there is hut one toe or linger perfect, two 

 other- bring imperfect ;in<l concealed ( Fig. *')): in others 

 there ;ire two. either alone or with vestiges of one or two 

 other-. and these always short and not very moveable. 



In those also remarkahlc for speed of foot, tin- limbs are 

 long; and this is a necessary result of the mechanical prin- 

 ciples already explained. 



Fig. 87. Skeleton of the Kangaroo. 



291. Leaping. In walking, the weight of the body i 

 sustained by a portion of the locomotory apparatus, whilst its 

 centre of gravity is being pushed forward by the other part 

 of the apparatus, so that man never ceases to touch the soil. 

 In leaping it is otherwise ; the body is thrown into the air. 

 and becomes, as it were, a projectile. To effect this, the 

 articulations are strongly flexed, so that, by a strong and 

 forcible and sudden extension, the body may be forced up- 

 wards from the resisting soil. Between the body and tin- 

 soil there is, in fact, an apparatus representing an elastic 

 spring the joints; on these extending violently and sud- 

 denly one of two things must happen, if the spring be suf- 

 ficiently strong either the soil must yield or the body, and 

 this being generally the only moveahle. gives way ami becomes 

 the projectile. Were the soil to yield under the feet, it is 

 obvious that no leap could take place. With (iuadruprds.it 

 is principally the hinder extremities which act as the spring 

 or force ; and hence, in animals of great >] d. as the ante- 

 lope. lmr>e. &C., thr<r limbs are long, and flexed, and slender. 



