ADDITIONAL FACTS ABOUT BIRDS. 115 



that it is the downward stroke of the wing, caused by 

 the contraction of the pectoralis major, which raises 

 and supports the bird in the air, while the action of 

 the pectoralis minor does nothing but bring the wing 

 into position for another downward stroke. 



The strong, deep keel on the sternum furnishes a 

 place for the attachment of these muscles. In kinds 

 that do not fly, such as the ostrich, the sternum is not 

 keeled, and the pectoralis muscles are not strongly 

 developed. ' 



No living bird has any teeth. The jaws are 

 encased in a horny beak. Some fossil birds, 

 among the earliest birds that lived, had teeth, and 

 rudiments of teeth have been found in some embry- 

 onic birds. 



The heart is four-chambered, and the circulation 

 is double and complete. The aorta, instead of sending 

 off two branches, right and left, as in the frog, 

 bends to the I'ight. That is, it retains only the right 

 branch. The bodily temperature is constant, and 

 about 104 degrees F. ; so the bird is said to be warm 

 blooded. 



The breathing is by lungs, which are attached to 

 the back. There is no diaphragm, but the movement 

 of the ribs and abdominal walls serves to enlarge the 

 cavity of the chest and so causes the air to rush into 

 the lungs. 



Besides the lungs, the pigeon has a series of acces- 

 sory air sacs which serve not only to furnish more air 

 to the body, but to make it lighter in- relation to its 

 size. 



Everything about the bird seems to have reference 

 to its power of flight. Flying is the characteristic 

 motion of birds, and the body is built with especial 

 reference to that fact. The long bones are hollow and 

 without marrow. The bones of the skull are filled 

 with air spaces. Feathers are the lightest possible 



