296 THE VITAL FUNCTIONS. 



their texture, and are so closely braced to the ribs, 

 and upper 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 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 bron- 

 chia, or divisions of the trachea (r), after opening 

 as usual into the pulmonary air-cells, do not ter- 

 minate 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 (c c c), which occupy a con- 

 siderable 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 ;t and there are, besides, many lateral 

 cells in immediate communication 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 ; fill- 



* 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. That this was an error was fir^it pointed out by Dr. Ma- 

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



