BIRDS. 409 



§ 2. Ulrds. 



It is in Birds alone that we find the most perfect 

 adaptation of strncture to the purposes of rapid 

 and extensive flight : in them the frame of the 

 skeleton, the figure, position, and structure of the 

 wings, the size of the muscles, the peculiar nature 

 of their irritability, and even the outward form of 

 the body have all a direct and beautiful relation to 

 the properties of the element in which Nature has 

 intended them to move. In their formation a new, 

 and in as far as relates to the organs of progressive 

 motion, a more developed type is adopted ; still 

 preserving a conformity with the general plan of 

 the vertebral organization, and with the general 

 laws of its developement. 



The skeleton of birds has the same constituent 

 parts as that of other vertebrated classes: the bones 

 of the anterior extremity, though destined exclu- 

 sively to support the wing, retain the same divi- 

 sions, and are composed of the usual elements; and 

 the general form of the body is that best calculated 

 to glide through the air with the least resistance. 

 As birds swallow their food entire, there is no ne- 

 cessity for any part of the bulky apparatus of hard 

 and solid teeth, large muscles and heavy jaws, 

 which are required by most quadrupeds : hence the 

 head admits of being greatly reduced in its dimen- 

 sions ; and the form of the beak, which is drawn to 

 a point, and cuts the opposing air, tends to facili- 

 tate the progress of the bird in its flight. 



In tlie conformation of the body, also, every cir- 



