Tlie lungs extend on each sitle much fartlier than in 

 quadrupeds, are firmer in their texture, and have larger 

 cells. On their dorsal aspect, they are applied close to the 

 ribs, and fill up hollows between them. On their central 

 aspect, they are covered by a delicate pleura, having nu- 

 merous perforations. These apertures lead to air-cells 

 which occupy the cavity of the thorax, and the sides of the 

 abdomen. The cells communicate freely with one an- 

 other, and with other cavities situated underneath the skin, 

 in the middle of the bones, and extending even to the quills. 

 These cells, wherever placed, have still a direct communi- 

 cation with the lungs, and receive and part with the air 

 through the windpipe *. The walls of the cells are occa- 

 sionally fm-nished with muscular threads. 



^ arious conjectures have been offered by different obser- 

 vers, concerning the use of these air-cells. Few seem to con- 

 sider that they are subservient to the aeration of the blood 

 Some regard them as aiding the voice, while others 'conclude 

 that they serve to vary the density of the animal, and en- 

 able it to accommodate itself to the different actions of fly- 

 ing, swimming, or diving. 



The kidneys form a row of irregular lobes on each side 

 of the lumbar yertebrne, and fill up cavities in the haunch 



structure of the asperas arteriae, or windpipes of several birds, and in the 

 land-tortoise." By Dr Parsons, Phil. Trans. 1766, p. 204.—" An Essay 

 on the Tracheae or Windpipes of various kinds of birds." By Dr Latham, 

 Lin. Trans, iv. p. 90. and the Suppt. Mont. Ornith. Diet, at the end. 



• The intimate connection subsisting between the lungs and the air cells, 

 is easily exhibited by making an incision into the abdomen of a fowl, arid 

 then obstructing the windpipe by a ligature. Respiration will be carried 

 on through the incision. We have seen, in the heron, respiration perform- 

 ed during a whole day through the broken humerus. See " An account of 

 certain Receptacles of air in Birds, which communicate with the LungsJ* 

 &c., by John Hunter, Phil. Trans. 177i. p. ?0v>. 



VOL. II. P 



