488 THE MECHANICAL FUNCTIONS. 



not being expended in the mere effort of giving 

 expansion to the several textures, and of swelling 

 the bulk of the frame, sometimes to inordinate 

 dimensions, are employed rather in reducing 

 the elements of the organization into compact 

 forms, and in concentrating their energies, so as 

 ultimately to attain the extent of power and 

 harmony of action, which are displayed in the 

 higher orders of warm-blooded quadrupeds. 



It is to these favoured tribes that we must 

 look for examples of the most complete develope- 

 ment of the skeleton, and the most advantageous 

 disposition of mechanic force. We have seen 

 that reptiles, from the comparative shortness of 

 their limbs, and the torpidity of their muscular 

 powers, are but ill adapted for rapid progression. 

 In all the more perfectly formed quadrupeds of 

 the class mammalia, the trunk of the body, being 

 raised high upon the limbs, possesses great range 

 of motion, and can traverse with fewer steps a 

 given space. 



The office of the limbs, as far as they are con- 

 cerned in progressive motion, is two-fold. They 

 have, first, to sustain the weight of the body, 

 which they must do by acting in opposition to 

 the force of gravity ; and they must, secondly, 

 give the body an impulse forwards. Let us con- 

 sider more particularly the relations which the 

 structures bear to each of these two functions. 



The limbs of quadrupeds constitute four 



