4 
three causes; first, to the ereater rapidity of the circulation, on’ 
account of which more blood passes through the lungs in a given 
time ; secondly, to the more abundant supply of air drawn in at 
each act of inspiration; thirdly, to the more complete exposure 
of the blood in the pulmonary capillaries of a bird to the action 
of the respired air. It must be observed, that it is necessary 
that these three conditions should all be satisfied at the same 
time, to make the lungs more perfect as respiratory organs. 
For if more blood pass through the lungs in a given time, 
it would be of no avail unless more fresh air were at the same 
time supplied for its aeration. And again, if these two con-_ 
ditions were satisfied, the quicker transit of the blood would 
necessitate the provision of increased facility for interchange of 
the gases by osmose through the walls of the pulmonary capil- 
laries; otherwise the blood would pass on before hematosis was 
sufficiently effected. Now in the bird the first of these condi- 
tions is satisfied by the more rapid action of the heart; the 
second by the greater quantity of air inspired; the third by the 
peculiar structure of the lungs, in which, as was first pointed’ 
out by Mr Rainey, in 1848, the whole circumference of the 
almost naked pulmonary capillaries is exposed to the action of 
the air in the lungs, which passes between and around them, 
in the areole that answer to the cells of the mammalian lung. 
The lungs of birds being very small cannot contain much 
air. When however the ample chest expands, a large quantity of 
air rushes in by the trachea, and passing through the larger 
bronchi, that open into the great air-sacs, fills them. It has long 
been known that the air-sacs do not all expand at the time of in- 
spiration, but only the two pairs that lie at the sides of the thorax, 
and are in contact with the ribs and the intercostal muscles: these 
may be called the middle sacs. The others, namely, the abdominal, 
the cervical, and the anterior thoracic (which may be called the 
extreme sacs) contract at this time. In expiration the action of 
both sets of cells is reversed. During the time of inspiration, 
