440 THE MECHANICAL FUNCTIONS. 



upon its victim wdth an impetus which nothing can 

 resist. 



The speed with which a quadruped is capable of 

 advancing depends more on the disposition of the 

 muscles and the extent of the articulations, and 

 more especially on the power of the extensors of 

 the hind extremities, than on the form of the body. 

 Great length and muscularity in the hind legs are 

 generally attended with considerable power of leap- 

 ing. This is exemplified in the Jerboa and the Kau- 

 guroo, animals, which, from the disproportionate 

 shortness of their fore legs, are totally incapacitated 

 from walking ; and for the same reason, they can- 

 not run with any degree of swiftness. It is only in 

 climbing up a steep acclivity that the jerboa is 

 enabled to employ all its limbs : in a descent, on the 

 contrary, it uses only its fore legs, the hinder being 

 dragged after them. But, when pursued, these ani- 

 mals are capable, for a long continuance, of taking 

 leaps of nine feet distance, and of repeating these 

 leaps so quickly, that the Cossacks, though mounted 

 on the swiftest horses, are unable to overtake them. 



The Kanguroo, in almost all his movements, 

 brings into action his powerful tail, which is fur- 

 nished with very strong muscles, and may be con- 

 sidered as constituting a fifth limb. It is of great 

 assistance to the animal in taking leaps ; and, 

 during its repose, contributes, together with the 

 hind feet, to support the weight of the body, as on 

 a tripod, and to leave at liberty the fore legs, which 

 may then be employed as arms. 



The J^areand the Rabbit furnish other instances 

 of an extraordinary length of the hinder legs de- 

 priving the animal of the power of walking, and 



