Dr. Dr osier on the Functions of the Air -celts in Birds. 231 



that the air-cells are a sort of second respiratory apparatus, so that 

 birds may be described, as they were by Cuvier, as animals having 

 a double respiration. In disproof of these views, it was shown that 

 a pigeon weighing 10 ounces, or 4375 grains, would have its weight 

 in air diminished by less than one grain in consequence of the rare- 

 faction of the air in its air- sacs and hollow bones ; so that the floating- 

 power resulting from such rarefaction would be almost inappreciable. 

 Again, the air-cells are bounded by delicate membranes, in which 

 the blood-vessels are very minute and sparsely scattered. Hence 

 very little blood is offered for oxidation in them. 



Some of the earlier observers, as Harvey and Perrault, in the 

 middle of the seventeenth century had correctly described the air- 

 cells of birds as sacs that enclose and confine the air received from 

 the openings, on the inferior surface of the lungs, in which the bronchi 

 terminate. Later observers, however, have generally fallen into the 

 error that the air passes from the air-sacs into the cavities of the pe- 

 ritoneum and the pericardium, and even extends itself between the 

 muscles, and beneath the skin in some cases ; and notwithstanding 

 that Guillot and Sappey have shown that the air does not pass out 

 of the air-sacs, such errors are repeated even at the present day. 

 The lungs of birds are not very elastic, are fixed to the ribs at the 

 upper part of the thorax by close cellular tissue, and bound down by 

 an aponeurosis formed by the tendons of the pulmonary diaphragm ; 

 so that they cannot draw in much air by expansion. They are 

 moreover small, and are penetrated by the principal bronchi, which 

 open upon their surfaces. Such lungs are quite incapable of acting 

 in inspiration in the same manner as the lungs of reptiles and mam- 

 mals. Capacious membranous bags are therefore provided to receive 

 the inspired air, the volume of which is much greater in the case of 

 birds than in the case of mammals. But the larger quantity of air 

 inspired would be of little use if it were merely drawn into the air- 

 sacs to be simply expelled again ; for the greater part of the inspired 

 air does not pass through the lungs, but direct through certain large 

 bronchial tubes into the air-sacs situated within the thorax. There 

 are another set of air-sacs situated without the thorax — namely, two 

 very large sacs in the abdomen, and several others anterior to the 

 thorax. When the thoracic air-sacs expand, the others contract, 

 and vice versd. The alternate expansion and contraction of the two 

 sets of air-cells causes currents of air to play continually through the 

 spongy tissue of the lungs peculiar to birds, and to pass between the 

 almost naked capillaries, first described by Mr. Rainey (in 1848) as 

 forming the only walls of the areolar spaces that answer to the air- 

 cells of the mammalian lung. The air-spaces between the capillaries 

 are, according to Mr. Rainey's measurements, only $aVot h °f an 

 inch, and the quantity of air in them must soon be deprived of oxygen 

 and saturated with carbonic acid. Hence the necessity of its con- 

 tinual change. This change is effected by constant streams of air 

 that fan the capillaries in passing from one set of air-sacs to the 

 other. The intricate courses which the air takes in passing in and 



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