312 THE REASON WHY : 



" To these, an over grown e justice of peace, 



With a clarke like a gizzard thrust under each arm ; 

 And warrants for sippets laid in his own grease, 

 Set o're a chafing dish to be kept warme." B. JOHNSOK. 



to this function are proportionately small. Everything thus 

 combines to render the head light, and consequently a long and 

 slender neck is sufficient for its support, 



Had it been necessary to sustain heavier a head at the extremity 

 of a long neck, great muscular development in this region would 

 have been required, and the weight so much increased, as to have 

 materially diminished the powers of flight. Moreover, the heavy 

 head at the extremity of the lever of the neck would have deranged 

 the centre of gravity, and in this way also have interfered 

 with flight. 



j96. Why does the breastbone form an important part of the 

 organization of a bird? 



Because it imparts solidity to the whole of the framework, and 

 supplies a wide base upon which the muscles of the wings are 

 fastened down and steadied. The breastbone also forms a kind 

 of box, which, during the time the body is stretched out in flight, 

 securely retains and supports the soft interior of the bird. 



The more the movements of the wings excite the great muscles 

 which are spread over the inside of the breastbone, the more do 

 those muscles brace and strengthen the frame of the bird. They 

 bear its weight up to the wings, and the wings again, by their long 

 arched form, lay it upon the air. Thus as the bird flies, it is 

 almost insensible of the fact that its body is heavy. 



997. The breast-bone of a bird secures the whole length of the body, and the great 

 central spire of that bone, called the keel, rises from it, so as to give lodgment and 

 attachment to the great muscles of the wings. It will be easily understood that 

 this keel is more largely developed in birds of passage, since its greater prominence 

 implies strength of wing for long-continued flight. Under the breast-bone, and 

 between the back-bone, is a considerable space, occupied by air-cells. These cells 

 represent a curious provision for the extension of the body of the bird, indepen- 

 dently of weight. The air does not only pass into the lungs of birds, but through 

 them, so as to fill a series of cells, composed of fine membranes, which are inter- 

 woven with all the viscera. The heart is surrounded by such a cell. Two great 

 cells are attached to the liver, and in the same manner all the viscera of the 

 abdomen are interspersed with air-cells, and these all communicate. The air thus 

 admitted into the interior of the body extends even into the bones. By inflating 

 these cell*, birds have the power of itutotttly rendering their bodies specifically 



