234 THE VITAL FUNCTIONS. 



assistance to the diaphragm, when, from any cause, a diffi- 

 culty arises in dilating the chest. 



In Birds the mechanism of respiration proceeds upon a 

 different plan, of which an idea may be derived from the 

 view given of the lungs of the Ostrich, at L. L., Fig. 377. 

 The construction of the lungs of birds is such as not to admit 

 of any change in their dimensions; for they are very compact 

 in their texture, and are so closely braced to the ribs, and up- 

 per parts of the chest, by firm membranes, as to preclude all 

 possibility 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 transparent membrane which 

 covers the lower surface of the lungs. They have the ef- 

 fect of lessening the concavity of the lungs towards the abdo- 

 men 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 ad- 

 mitted, through these apertures into several large air-cells 

 .(c c c,) which occupy a considerable portion of the body, 

 and which enclose most of the large viscera contained in the 

 abdomen, such as the liver, the stomach, and the intestines;f 

 and there are, besides, many lateral cells in immediate com- 

 munication with the lungs, and extending all down the sides 

 of the body. Numerous air-cells also exist between the 

 muscles, and underneath the skin; and the air penetrates even 

 into the interior of the bones themselves, filling the spaces 

 usually 9ccupied by the marrow, and thus contributing ma- 



* Hunter in the Animal Economy, p. 78. 



f 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. Macartney, in Rees's Cyclopaedia. Art, Bird* 



