D 
then, the middle sacs become filled with air. The pulmonary 
diaphragm is at this time in a state of contraction, and its tense 
aponeurosis, which is attached to the inferior surface of the lungs 
by a fine cellular tissue, causes the lungs to expand to such an 
extent as they are able, for they are not very elastic. In so 
expanding they receive a part of the inspired air, which how- 
ever does not remain stationary in them, but passes on with a 
gentle motion towards the middle air-cells, in which the pres- 
sure has been reduced by the expansion of the walls of the 
thorax. A much greater amount of air however reaches the 
middle air-cells by the more direct, and more open course of the 
larger bronchi, that open into them; and a third portion passes 
into them, at the same time, from those extreme cells that are 
nearest to them, and which are contracting, as we know, at this 
time. Inspiration being completed, expiration commences, and 
endures for a longer time than inspiration. The pulmonary 
diaphragm relaxes, and in consequence of the partial closure 
thereby of certain openings, the middle air-sacs slowly empty 
themselves by openings in part differing from those by which 
they were filled; so that the air passing through the smaller 
bronchi and the substance of the lungs, goes principally out by 
the trachea, but partly into the extreme air-sacs, which at this 
time are dilating. Hematosis is going on therefore during ex- 
piration as well as during inspiration; so that birds are truly 
animals having a double respiration, though not in the sense in 
which Cuvier so defined them’. 
It is exceedingly instructive to observe the way in which 
the organization of birds has been modified to meet the require- 
ments of their marvellous powers of rapid and long-sustained 
flight. Muscles capable of very powerful and rapid action are 
necessary to support the bird in the light and yielding air. This 
again requires very ample provision for respiration. The amount 
of muscular force must also be proportioned to the weight of the 
1 Cuvier, Réegne Animal, 1817, t. 1. p. 290, 
