2 
and principal of these was originated by Camper, who in 1771 
discovered accidentally that the hollow bones of birds commu- 
nicate with the great air-sacs; and conceived that the air con- 
tained in these, being warmed, lessened the specific gravity of 
the bird, and so rendered it fitter for flight. Since the time of 
Camper this view has been almost universally adopted. Thus 
Owen in’a work just published adopts it’. In disproof of this 
view, the lecturer gave a calculation to shew that a pigeon weigh- 
ing 10 ounces, or 4375 grains, would have its weight in air 
diminished by only a fraction of a grain in consequence of the 
rarefaction of the air in its air-sacs and hollow bones from the 
warmth of the body; so that the floating power resulting from 
such rarefaction would be almost inappreciable. We cannot 
then for a moment suppose that the extensive and complicated 
system of air-vessels has for a principal object a saving in weight 
so trifling: not to mention that this system is found in all birds, 
even such as do not fly. 
Another theory that has been very generally adopted was 
propounded and developed by Cuvier’, who assigned to the 
air-cells an office directly supplementary to that of the lungs; 
so that the air having passed through the lungs and reno- 
vated the blood in the pulmonary capillaries, is brought once 
again into contact with the blood in the systemic capillaries 
in every part of the body, and again exchanges oxygen for 
carbonic acid with the vital fluid. This theory was founded 
on a great anatomical mistake, into which, strange to say, 
many eminent anatomists have fallen, viz. that the air passes 
from the air-sacs into the cavities of the peritoneum, peri- 
cardium, and pleure, and even extends itself between the 
muscles, and beneath the skin: whereas it is in reality confined 
1 Owen, On the Anatomy of Vertebrates, Vol. 11. 1866, p. 216; also Art. Aves 
in Todd’s Cyclop. 
2 Anatomie Comparée, 1805, t. IV. p. 330, and Owen, 1. c. cum multis aliis 
auctoribus. 
