GENERAL STRUCTURE OF BIRDS. 
93 
nest. The nutritive functions are performed with extra¬ 
ordinary activity in Birds, that the means may be supplied 
for the maintenance of their locomotive activity. Their blood 
is particularly rich in red particles, and its heat is usually 
considerably above that of Mammals. Its circulation is very 
energetically carried on; and although the lungs themselves 
are constructed upon a type inferior to that of Mammals, 
and the mechanism of respiration is less complete, yet, by an 
extension of the respiratory organs through the whole fabric, 
the aeration of the blood is carried on with unequalled 
energy (§ 326). 
80. The arrangement of the organs contained in the cavity 
of the trunk of Birds differs from that which has been 
described in Mammals, chiefly 
in this,—that there is usually 
no diaphragm to separate the 
chest from the abdomen, and 
that although the lungs them¬ 
selves are confined to the upper 
part of this cavity, they are con¬ 
nected with a series of air-sacs 
which are distributed through 
the whole of it. In the accom¬ 
panying figure, which repre¬ 
sents the internal organs of the 
Ostrich, the heart is seen at a, 
the stomach at b, and the in¬ 
testinal tube at c. The windpipe, 
d, opens into the lungs, e, which 
are themselves small, and are 
attached to the ribs, instead of 
lying freely in the cavity of the Fig. si.— lungs of the Ostrich. 
chest; but the space they would 
otherwise have occupied is filled 
up by the large air-cells, /,/, 
which communicate freely with 
the lungs and with each other, and which even occupy a large 
part of the cavity of the abdomen, as seen in the figure. 
81. In the class of Beptiles we find a variety of form so 
remarkable, that, if we were influenced by this alone, we 
should scarcely regard the animals it contains as belonging to 
the heart; b, the stomach ; c c, the 
intestines; d, the trachea; e, the 
lungs; ///, air-cells,in which are also 
seen the tubes by which these air- 
cells communicate with the lungs. 
