352 
the animal to furnish carbon in that mode and state 
by which this chemical action on the air is accom- 
plished. Consequently we conclude, that the carbon 
supplied by animals, during the exercise of the re- 
spiratory function, is an animal excretion, depend- 
ent, like ofher excretions, on the motion and distri- 
bution of the blood. 
661. We before maintained that this carbon, in li- 
ving animals, was brought into actual contact with 
the air through the medium of the exhalent vessels 
in the lungs, and we remarked (156.) the peculiar 
circumstance of this exhalation being furnished as 
well from the venal, as from the arterial, blood in 
those organs. The following observations of Dr 
Barclay entirely accord with these views. " The 
change produced in the blood by respiration," says 
he, " is probably owing more to exhalation than ab- 
sorption, for no artery exhales so freely as the pul- 
monic. A watery injection thrown into this artery 
with a small force, will flow copiously into the bron- 
chi, and without occasioning any thing like rupture 
in its smaller vessels. But whether the exhalation 
be, or be not, the principal cause of change in the 
blood, pulmonic blood, when exposed to the air 
through the medium of its vessels, is always observ- 
ed to change its colour a great deal faster, while the 
exhalents continue to act with a vital energy, than 
when they act slowly and feebly as inanimate or- 
gans *." 
On Muscular Motion, p. 
