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 

 drawn 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. But 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 ? 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- 



