at the same time, by the action of the respiratory or- 
gan, is presented to the same surface, where its oxy- 
gen combines with the exhaled carbon, and both 
~x>pass off together in the form of carbonic acid gas. 
This chemical union> especially in the higher ani- 
mals, is increased and promoted by the great extent 
of surface of the respiratory organ ; by the apparent 
attraction or adhesion subsisting between this surface 
and the air ; by whatever accelerates the motion of 
the blood, and increases the exhalation of carbonic 
matter ; and, lastly, by the constant supplies of fresh 
air furnished by the action of 'the respiratory or- 
gans. ^ 
677. In many animals, however, which belong 
to the inferiqr classes, and in the whole class of fish- 
es, as well as in aquatic plants, the air is not present- 
ed to the respiratory organ in an elastic state, but 
through the medium of solution in water. We have 
seen that vegetable and animal substances possess the 
power of separating air from water ; and a similar 
separation we must suppose to be made by the bodies, 
and especially by the respiratory organs, of aquatic 
animals. By the exertion of this power, the oxygen 
gas, contained in water, is brought into contact with 
the carbon as it exhales from the animal system, and 
a chemical combination, with the usual phenomena, 
takes place. In aquatic animals, no cells or recep- 
tacles for containing air are provided, so that, as in 
some animals of the vermes class, the union takes 
place at the surface of the blood-vessels ; but in ter- 
restrial animals, a cellular structure for receiving the 
air is interposed, at the surface of which the combi- 
