CHANGES IN BLOOD BY RESPIRATION. 
261 
acid. These changes will take place out of the living body as 
well as in it; provided that the blood can be exposed as com¬ 
pletely to the influence of the atmosphere. When blood is 
dr^wn from a vein into a basin or cup, the dark hue of the 
surface is usually seen to undergo a rapid alteration, so as to pre¬ 
sent the arterial tint; but this is confined to the upper surface, 
because it alone is exposed to the influence of the atmosphere. 
The alteration takes place still more rapidly and completely 
if the blood be exposed to pure oxygen gas ; but even then it 
is almost confined to the surface. It is not prevented, even 
though the direct communication between the blood and the 
gas be cut off by a membranous partition, as it is in the living 
animal; for if the blood be tied up in a bladder, the gas has 
still the power of penetrating to it, and of effecting the change 
in it; and the change is manifested, not only by the alteration 
in the aspect of the blood, but by the disappearance of a 
certain quantity of oxygen, and its replacement by carbonic 
acid. Now if, by spreading out the blood in a very thin layer, 
we expose a much larger surface to the air, or if, by frequently 
shaking it, we continually change its surface, we render the 
change more complete. Eut still it is accomplished far less 
effectually than it is in the lungs or gills of a living animal; 
for when it is passing through their capillaries, it is divided 
into an immense number of very minute streams, each of 
which is completely exposed to the influence of the air, and 
the combined surface of which is very great. 
304. The question next arises, what becomes of the Oxygen 
which disappears, and what is the origin of the Carbonic acid 
which is thus produced by respiration 'l This question will 
now be considered. 
305. When charcoal is burned in a vessel filled with air, 
the oxygen disappears, and is replaced by an equal bulk of 
carbonic acid: at the same time there occurs a consider¬ 
able disengagement of heat. During respiration, the same 
phenomena occur : there is always an evident relation between 
the quantity of oxygen employed by an animal, and the 
amount of carbonic acid it produces (the latter being usually 
somewhat less than the former); and, as we shall see hereafter 
(Chap, ix.), there is always a greater or less amount of heat 
produced. There exists, then, a great analogy between the 
principal phenomena of respiration, and those of the combus- 
