RESPIRATION IN BIRDS. 329 



by firm membranes, as to preclude all possi- 

 bility of. motion. They in part, indeed, project 

 behind the intervals between the ribs, so that their 

 whole mass is not altogether contained within 

 the thoracic cavity. There is no large muscular 

 diaphragm by which any change in the capacity 

 of the chest could be effected, but merely a few 

 narrow slips of muscles, arising from the inner 

 sides of the ribs, and inserted into the thin trans- 

 paren membrane which covers the lower surface 

 of the lungs. They have the effect of lessening 

 the concavity of the lungs towards the abdomen, 

 at the time of inspiration ; and they thereby assist 

 in dilating the air-cells*. The bronchia, or divi- 

 sions of the trachea (t), after opening, as usual, 

 into the pulmonary air-cells, do not terminate 

 there, but pass on to the surface of the lungs, 

 where they open by numerous apertures. The 

 air is admitted, through these apertures, into seve- 

 ral large air-cells (c c c), which occupy a consi- 

 derable portion of the body, and which enclose 

 most of the large viscera contained in the abdo- 

 men, such as the liver, the stomach, and the in- 

 testines! ; and there are, besides, many lateral 

 cells in immediate communication with the 



* Hunter on the Animal Economy, p. 78. 



t It was asserted by the Parisian Academicians, that the air 

 gets admission into the cavity of the pericardium, in which the 

 heart is lodged. This error was first pointed out by Dr. Ma- 

 cartney. (See Rees's Cyclopeedia. — Art. Bird.) 



