THE PHYSIOLOGY OF THE BLOOD. 
517 
tains hydro -carbon, which, coming in contact with the oxygen of 
the atmosphere, forms the watery vapour and carbonic acid. I 
think the former the most probable. 
That the saline matter in the serum is a great cause of the 
arterial or bright colour of the blood, I believe, is the general 
opinion. This one fact will tend to confirm it — venous blood, 
when carefully separated from its serum, is not brightened by 
oxygen gas ; and arterial blood, when its serum is displaced by 
pure water, becomes as dark as venous. The fact, that acids 
darken the colour of blood, appears a partial proof that venous 
blood owes its dark colour to the presence of carbonic acid. 
With respect to most acids this colour remains; but the carbonic 
acid forms an exception, for on the removal of it the blood 
resumes its bright arterial colour. 
That life is not indispensable to the arterialization of the blood 
is proved by artificial respiration being kept up after death, when 
the same change takes place. Again, the same changes take 
place in the blood, both in the body and out of it, when exposed 
to the same chemical causes; so that it seems legitimate to infer, 
as this process may be solely determined by the laws of chemical 
action, that the vital principle is not essential to the change. 
Dr. Stevens believes the carbonic acid is removed by an 
affinity which it has for the atmosphere. 
Lavoisier is of opinion, that the formation of carbonic acid is 
one source of animal heat. 
The circulation of the blood in the foetus and in the adult, with 
the changes which it undergoes, as well as the mode of nutri- 
tion in the two, greatly differ in many essential points. The 
foetus, for nutrition and growth, depends on the mother, through 
the medium of the placental membranes, and partly on them for 
the purification of the blood. Whereas the adult depends for 
support on food taken into the stomach ; and on the lungs and 
skin principally for the purification of the blood. 
There are various opinions on the means by which the foetus 
is supported. Our much-respected Professor is of opinion, I 
believe, that the vessels of the foetus, and the maternal vessels, 
circulate alongside in the placental membranes ; and in so doing, 
the foetal blood derives oxygen from the blood of the mother : 
but if this were the case, 1 would ask, how is the foetus to be 
formed, as well as supported, and whence are the secretions that 
are found in different parts of it, for the formation of which 
blood, or something similar, is required ? The blood, though not 
so pure, I believe is composed of the same constituents. 
Another more likely theory is, that the blood of the mother is 
deposited by the arteries into the cellular structure of the 
