84 THE STRUCTURE AND LIFE OF BIRDS cuap, 
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 riks. 
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.. 
