390 ANIMAL PHYSIOLOGY. 
main facts known are that—1. The quantity of this gas is as 
great in serum as in blood, or, at all events, the quantity in 
serum is very large. 2. The greater part may be extracted by 
‘an exhaustion-pump; but a small percentage (2 to 5 volumes 
per cent) does not yield to this method, but is given off when 
an acid is added to the serum. 3. If the entire blood be sub- 
jected to a vacuum, the whole of the CO, is given off. 
From these facts it has been concluded that the greater, part 
of the CO, exists in the plasma, associated probably with sodium 
salts, as sodium bicarbonate, but that the corpuscles in some 
way determine its relations of association and disassociation. 
Some think a good deal of this gas is actually united with the 
red corpuscles. 
We may now inquire into the more intimate nature of respi- 
ration in the blood. From the facts we have stated it is obvi- 
ous that respiration can not be wholly explained by the Henry- 
Dalton law of pressures or any other physical law. It is also 
plain that any explanation which leaves out the principle of 
pressure must be incomplete. 
While there is in oxy-hemoglobin a certain quantity of oxy- 
gen, which is intra-molecular and incapable of removal by re- 
duction of pressure, there is also a portion which is subject to 
this law, though in a peculiar way; nor is the question of 
temperature to be excluded, for experiment shows that less 
oxygen is taken up by blood ata high than at a low tempera- 
ture. 
We have learned that, in ordinary respiration, the propor- 
tion of carbonic dioxide and oxygen in different parts of the 
respiratory tract must vary greatly; the air of necessity being 
much less pure in the alveoli than in the larger bronchi. 
From experiments on blood, venous and arterial, to deter- 
mine the conditions of pressure, temperature, etc., under which 
‘the injurious gas is got rid of and the necessary one absorbed, 
it has been found that the partial pressure of oxygen in the 
‘lungs is sufficient to bring about that surrender of oxygen to 
the blood necessary to keep it all but saturated with this gas 
as it is believed to be; and that, so far as carbonic anhydride 
is concerned, the same law holds—i. e., the partial pressure in 
the blood is ordinarily greater than in the alveoli. 
By means of an apparatus by which one of the smaller 
bronchi may be occluded for a certain period, and also. allow 
of withdrawal of samples of the air in the occluded portion of 
lung from time to time, to ascertain its composition, attempts 
