ia 
_ some Birds, which approach most nearly to Mammals in their 
RESPIRATION OF BIRDS AND MAMMALS. 279 
Their lungs, however, are not so minutely subdivided as are 
those of Mammals; but the surface over which the air can 
act upon the blood is immensely extended, by a provision 
which is peculiar to this class. The air introduced by the 
windpipe passes not only into the lungs properly so called, 
but into a series of large air-cells, which are disposed in 
various parts of the body, and which even send prolongations 
into the bones, especially in Birds of rapid and powerful 
flight, whose whole skeleton is thus traversed by air. The 
mode in which some of the bronchial tubes, or subdivisions of 
the windpipe, pass from the lungs to these air-cells, is shown 
in fig. 161. Now, by this arrangement, a much larger quan- 
tity of air is taken-in at once, and a much more extensive sur- 
face is exposed to its action, than could otherwise be provided 
for ; and as the air which is received into the air-cells has to 
pass through the lungs, not only when it is taken-in, but when 
itis expelled again, its full influence upon the blood is secured. 
327. Birds, like Reptiles, are destitute of the peculiar 
apparatus by which Mammals are enabled to fill their lungs 
with air; but it is introduced without any effort on their 
parts. For the cavity of their trunk is almost surrounded by 
the ribs and breast-bone ; and the elasticity of the former 
keeps it generally in a state corresponding to that of our own 
lungs when we have taken-in a full breath. Thus the state 
of fullness is natural to the lungs and air-cells of Birds. 
When the animal wishes to renew their contents, however, it 
compresses the walls of the trunk, so as to diminish its cavity 
and to force out some of the air contained in the lungs, &c. ; 
and when the pressure is removed, the cavity returns to its 
previous size by the elasticity of its walls, and a fresh supply 
of air is drawn into the lungs. The air-sacs answer the same 
purpose in Birds as in Insects, diminishing the specific gravity 
of the body, by increasing its size without adding to its weight, 
and thus rendering it more buoyant. 
328. In Man and other Mammats, the lungs are confined 
to the upper portion of the cavity of the trunk, termed 
the thorax ; which is separated from the abdomen by the 
diaphragm, a muscular partition, whose action in respiration 
is very important. (An imperfeet diaphragm is found in 
_ general structure.) The lungs are suspended, as it were, in 
