84 THE STRUCTURE AND LIFE OF BIRDS chap. 



large air chambers under the skin, and when these are 

 filled it floats like a bladder on the surface. 



The problems connected with the lungs and their 

 extensions are many and difficult, and I shall devote 

 to them a separate division of this chapter (see p. 105). 



The Process of Breathing. 



It will be well first to say something about the 

 mechanism of breathing in man, and then show how 

 different it is in birds. A man creates the vacuum 

 within him which the air rushes in to fill partly by 

 means of the diaphragm, partly by means of the ribs. 

 The diaphragm is a partition which separates the 

 cavity which contains the heart and lungs from that 

 which contains the intestines. Muscles descend from 

 it to the ribs and stronger ones to the spinal column. 

 When these muscles contract, the lung chamber is 

 enlarged, a vacuum is created, the air rushes in and dis- 

 tends the lungs. Diaphragmal breathing is impossible 

 to a bird since it has no fully developed diaphragm. 

 Indeed the oblique septum, to which the name of 

 diaphragm is often given is apparently so different in 

 its nature and situation that it has been doubted 

 whether we can regard it as the same organ as the 

 diaphragm of mammals. Its arrangement is very 

 complicated. One part lies on the under surface of 

 the lungs and under the cervical air-sacks which, thus, 

 are in a chamber by themselves. The other, the 

 entirely membranous and oblique part, at its anterior 

 end connects with the former along the line of the 

 backbone ; further back it springs from the pelvis. 



