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 largtr 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 

 it is expelled again, its full influence upon the blood is secured. 



327. Birds, like Eeptiles, 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 MAMMALS, 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 imperfect diaphragm is found in 

 some Birds, which approach most nearly to Mammals in their 

 general structure.) The lungs are suspended, as it were, in 



