366 
s 
nation of the oxygen with the carbon is effected. 
Lastly, we may observe, that, in animals which live 
in water, the respiratory organ may be considered as 
external, and as being constantly moistened by thex 
mass of surrounding fluid, while in those which 
breathe in air, this organ may be regarded as inter- 
nal, and the portion of air that comes into contact 
with it as separated from the general mass, by which 
means the organ is protected from the effects of too 
rapid evaporation, and is always preserved in a moist 
and secreting state, a provision which the condition 
of aquatic animals did not render necessary. 
678. To the mode in which we have supposed the 
carbon to be afforded by the lungs in respiration, it 
has been objected by Dr Bostock, that " it does not 
explain how the regular supply of this substance is, 
at each successive respiration, brought to the lungs 
in a state proper to be discharged *." We certainly 
admit, as this respectable writer states, that the car- 
bon, in common with every other ingredient of the 
body, is derived primarily through the organs of di- 
gestion ; but we do not suppose it to be excreted in 
its first transmission through the lungs. We regard 
carbon as a constituent part of the animal fluids, and 
consider it to be excreted in the lungs as long as the 
blood continues to move. Hence it follows that the 
excretory function in the lungs is only the immediate 
source of the carbon that is supplied to act upon the 
air; and that its remoter source must be sought in 
that function which, by supplying materials to the 
* Edinburgh Medical Journal; NO. xiv. p. 160. 
