296 
and the variation in appearance, which the blood ex- 
hibited, would correspond with the degree in which 
that change took place. An exhalation of fluid is 
constantly going on from all the moistened surfaces 
of the body, and may easily be conceived to have is- 
sued in a greater or less degree, from the recently 
exposed veins of the rabbits. According, therefore, 
to the activity with which this process proceeded, the 
colour of the blood would be more or less rapidly 
changed. In the same manner, Dr Barclay observes, 
that the blood in the lungs, when exposed to the air 
through the medium of its vessels, is always obser- 
ved to change its colour a great deal faster, when the 
exhalents continue to act with a vital energy, than 
when they act slowly and feebly, as inanimate or- 
gans *; and we have elsewhere remarked (143.), 
that, when the exhalent function has entirely ceased, 
no change is induced on the air, and no apparent 
change seems then to be effected in the blood. 
591. It is not, however, necessary that the car- 
bonic matter, which combines with oxygen, should 
be afforded by a living action, else carbonic acid 
could not be formed by dead moistened bladders, 
nor by the blood after its removal from the body. 
In substances deprived of life, therefore, the carbon 
may rather be said to escape by evaporation, than by 
exhalation, which, physiologically speaking, is a liv- 
ing action ; but in whicheve*r way this matter come 
into contact with oxygen gas, carbonic acid is form- 
* On Muscular Motion, p. 524. 
