343 PHILOSOPHY OF ZOOLOGY. 
constitution of the white blood of the inferior animals. In 
nearly all the experiments which have been performed on 
the blood, that fluid has been obtained from man, or the 
more common domestic quadrupeds. 
We come now to consider the changes produced on the 
blood by the aerating organs. 
We have already stated the necessity of a constant sup- 
ply of atmospheric air to the continuance of the life of or- 
ganised bemgs. We are here to consider this supply in 
connection with the circulation of the vital fiuid. 
The aerating organs of animals may be divided into two 
kinds, Lungs (Pulmones), and Gills (Bronchiz), both des- 
tined to accomplish the same end. The lungs are suited 
for bringing free air into contact with the blood, and there- 
fore belong to those animals which have their residence on 
the land. The gills are calculated to separate air from wa- 
ter, with which it is always united, and bring it im contact - 
with the blood, and belong therefore to those animals which 
reside in the sea or in fresh water. It is to be observed, 
however, that many animals which reside in the water, breathe 
by means of lungs, and are obliged, at intervals, to come 
to the surface to respire, such as whales; but there are 
no animals which reside on the land and are furnished 
with gills which are obliged to return to the water to re- 
spire. 
Whether the aerating organs be lungs or gills, it appears 
to be the object of nature in their construction to expose a 
large surface to the contact of the air. This object is ac- 
complished by their division into numerous cells and leat- 
like processes, or by their extension on the walls of cavities, 
or the surface of pectinated ridges. The blood brought 
to these organs by the pulmonic vessels, 1s there distribu- 
ted by their terminating branches. Although still retained 
in vessels, it can nevertheless be easily acted upon by the 
