402 THE MECHANICAL FUNCTIONS. 



The actions of the tail, which operates as a rudder, are 

 useful chiefly in directing the flight. When the tail is short, 

 this office is supplied by the legs, which are in that case 

 generally very long; and being raised high and extended 

 backwards in a straight line, are of considerable assistance 

 in the steerage of the animal. In many birds, as in the 

 wood-pecker, the tail is much employed as a support to the 

 body in climbing trees. The caudal vertebrae are often nu- 

 merous, but are short and compressed together; they are re- 

 markable for the great development of their transverse pro- 

 cesses, and for having spinous processes both on their lower 

 and upper sides. The last vertebra, instead of being cylin- 

 drical, has a broad carinated spine for the insertion of large 

 feathers. 



Birds could not, of course, be always on the wing; for a 

 great expenditure of muscular power is constantly going on 

 while they support themselves in the air. Occasional rest 

 is necessary to them as well as to other animals, and means 

 are accordingly provided by nature for their mechanical 

 support and progressive motion while on land. 



The anterior extremities having been exclusively appro- 

 priated to flight, and constructed with reference to the pro- 

 perties of the atmosphere, the offices of sustaining and of 

 moving the body along the ground must be intrusted wholly 

 to the hind limbs. The centre of gravity, before sustained 

 by the wings, must now be brought over the new basis of 

 support formed by the feet; or rather, as it is placed far for- 

 wards, the feet must be considerably advanced so as to be 

 brought underneath that centre. But as the bones of the 

 posterior extremity have their origin from the remote part 

 of the pelvis, which is elongated backwards, at a considera- 

 ble distance from the wings, it became necessary to lengthen 

 some of their parts, and to bend their joints at very acute 

 angles. We accordingly find that while nature, in the for- 

 mation of the limb, has preserved an accordance with the 

 vertebrated type, both as to the number of pieces which 

 compose it, and as to their relative situations, she has devi- 



