38 VERTEBRATA. 



But in no animals are the muscles of the limbs more 

 firm and compact in texture ; in none are they larger ; 

 in none have they broader surfaces of attachment 

 than in Birds. Thus, the fore limbs, lengthened 

 and developed into vigorous and muscular wings, 

 are fitted to take a strong hold of the air in their 

 successive leaps forward, while the head with the 

 pointed beak, cleaves the air, and the tail as a rudder 

 steers the little aerial vessel, and gives a precise di- 

 rection to the flight. But towering, as many Birds 

 do, above the clouds, when the atmosphere is rare 

 and cold, they require a clothing more fitted to re- 

 tain the animal heat than hair, or even fur. This 

 is given in their feathers, which are of all substances 

 the least conductors of heat, and more particularly 

 in the unwebbed down which lies beneath them. 

 The thick, firm, but soft feathers on the belly of 

 a duck, or other waterfowl, are a beautiful example 

 of the protection afforded against cold, in circum- 

 stances where the animal is peculiarly liable to its 

 influence. 



The long quill-feathers of the wings are so placed 

 as to present a stiff broad surface, with which to 

 strike the air, far more completely adapted for that 

 purpose than the stretched membrane of the Bat. 

 The strongest are attached to the hand, one finger 

 of which alone is developed, though the others can 

 be traced; and the thumb, carrying a little plume 

 of feathers, is distinct and visible. The quills of 

 the hand and wrist are called primaries, those which 

 proceed from the fore-arm are secondaries, and those 



