1 88 ANIMAL BIOLOGY 



there the food passes into the intestine where digestion is 

 completed and the digested materials absorbed. 



Not only do birds have well-developed lungs, but there 

 are in most species, extensive air sacs connected with the 

 lungs and extending into various parts of the body. In 

 most birds many of the bones are hollow and contain pro- 

 longations of these air sacs. Air may therefore be carried to 

 different parts of the body in a way that suggests a com- 

 parison with its distribution through the tracheal tubes of 

 insects. A sparrow with a broken wing may even take 

 air into its lungs through the hollow of its wing bones. 



Birds are warm-blooded animals, and they have a rapid 

 respiration which is greatly facilitated by the large sur- 

 face afforded by the lungs and air sacs. They have a four- 

 chambered heart and a complete double circulation such 

 as occurs in ourselves. 



The skeleton and muscular system of birds have become 

 highly modified in relation to flight. To progress rapidly 

 through the air means that there must be a large amount 

 of muscle for moving the wings and accordingly we find 

 the pectoral muscles, those extending from the breast bone, 

 or sternum, to the wings, enormously developed. And 

 to give adequate attachment for these muscles the sternum 

 is not only of large size, but in all except a very few kinds 

 of birds, it is furnished below with a large median ridge, 

 or keel. The bones of the wings conform to the same gen- 

 eral plan of structure as do those of the legs. In the outer 

 part of the wing, or what corresponds to the hand in our- 

 selves, some bones of the digits have been lost and others 

 are fused together so that the fundamental plan is some- 

 what obscured. - Such changes may be regarded as a 

 natural consequence of modifying a fore leg so as to adapt 

 it to the new function of flying. 



All species of birds lay eggs, and with rare exceptions 



