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- 

 parent 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 thereby assist in 

 dilating the air-cells*. The bronchia, or divisions 

 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 several 

 large air-cells (ccc), which occupy a consi- 

 derable portion of the body, and which enclose 

 most of the large viscera contained in the ab- 

 domen, such as the liver, the stomach, and the 

 intestinest ; and there are, besides, many lateral 

 cells in immediate communication with the 



* Hunter in the Animal Economy, p. 78. 



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

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

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

 cartney, in Rees's Cyclopsedia. — Art. Bird. 



