314 Miscellaneous. 
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 mammalianlung. ‘The air-spaces between the capillaries 
are, according to Mr. Rainey’s measurements, only >; th of 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 
out of the air-cells and bronchial tubes of various orders is difficult 
to describe, especially without diagrams. 
The respiration of birds, even when in repose, has been shown to 
be much more active than that of mammals. But in order that 
birds may be equal to the enormous exertion required of them for 
sustaining themselves in the air for considerable periods of time, very 
ample provision must be made for respiration. If therefore the lungs 
were constructed after the mammalian type, they would require to 
be very large, and powerful muscles must have been provided for the 
