RESPIRATION IN BIRDS. 15 



is afforded when the mouth is shut, as well as when it is open. 

 The lungs resemble in some respects a pair of bellows, inas- 

 much as they alternately receive and give exit to the atmo- 

 spheric air. But they do not possess this power of themselves ; 

 theyare merely expansile and compressible bags, which, on being 

 widened, are filled with the air, which rushes in by the pressure 

 of the atmosphere, and on being condensed, are emptied by the 

 pressure of a muscular apparatus adapted for that purpose. In 

 the Mammalia, a muscular expansion, named the diaphragm, 

 which separates the cavity of the thorax, containing the lungs 

 and heart, from the cavity of the abdomen, containing the 

 stomach, intestine, liver, and other organs, is a principal agent 

 in respiration, by its alternate contraction and relaxation en- 

 larging or diminishing the thoracic cavity, the walls of which 

 are always in contact with those of the lungs. But besides the 

 diaphragm, the abdominal and costal muscles also operate in 

 producing these eftects. In Birds, however, there is no dia- 

 phragm, properly so called, although it exists in many species 

 in an imperfect state ; and therefore the alternate expansion 

 and contraction of the thorax and abdomen, which form but 

 one cavity, are effected by the muscles attached to the ribs, 

 sternum, and pelvis. The lungs are comparatively small, and 

 not so expansile as in quadrupeds, but they have on their surface 

 apertures communicating with large cells in the interior of the 

 thorax and abdomen, and with others in the neck, and even 

 with the cavities of many of the bones. Thus by inspiration, 

 not only are the lungs filled w4th air, but also the cells dis- 

 posed in various parts of the body ; and it is for this reason that 

 in birds respiration is said to be double. 



The lungs are seen, on opening a bird, to be of a very bright 

 red colour, approaching to light vermilion, and of a spongy tex- 

 ture. They are two in number, of a flattened oblong form, 

 destitute of lobes, extending from the second to the last dorsal 

 vertebra, crossed obliquely by the oesophagus beneath, and in 

 contact with the kidneys behind. They are not suspended 

 freely, as in the Mammalia, being bound down by a fascia in 

 front, and sunk as it w^ere into deep cavities above, between 

 the dorsal portions of the ribs, and in a large hollow on each 



