7819.] and on the Blood in general. 267 
body; no gas, except oxygen, or those compounds containing it 
in a free state, gives to blood that fine florid colour which it 
possesses in the arteries, and which appears essential to render 
it capable of performing its important offices. Some act upon 
it chemically and decompose it ; while others, without exerting 
any very evident chemical action, appear nevertheless to render 
it of a darker colour than venous blood itself. A question has 
arisen among physiologists, whether the nitrogen entering into 
the composition of atmospheric air be absorbed, or otherwise 
altered in respiration, and consequently whether it be of any 
further use in that function than merely acting as a diluent to 
the oxygen. The most common opinion at present is, that it is 
not absorbed in respiration. Some physiologists, however, are 
of a different opinion, and maintam that under certain circum+ 
stances, it is absorbed in considerable quantity. The matter, 
therefore, at present may be considered as sub judice.* 
Thirdly, We come to inquire whether the changes which take 
place in respiration differ in degree in different classes of animals 
compared with one another, or in different individuals of the same 
class. On this part of the subject, good experiments are much 
wanting. With respect to the first consideration, we can only 
speak generally. None of the more perfect animals are capable 
of existing, even for a few minutes, without oxygen ; while many 
of the inferior ones can exist for a considerable time upon very 
little. Birds, from the magnitude of their lungs, and some other 
circumstances of their economy, are supposed, generally speak- 
mg, to require most oxygen, and next to them the mammalia ; 
but I am not aware that any comparative experiments have been 
made upon the subject which we can rely upon. In both these 
classes of animals, however, the difference between the venous 
and arterial blood in point of colour is very striking. 
Fishes, from the circumstances of their situation, must con- 
sume much less oxygen than either of the above classes; and 
frogs, toads, and other animals of this class have been found te 
live much longer in a given quantity of air than birds or small | 
quadrupeds of an equal size. The differences also in point of 
colour between the venous and arterial blood of those. animals 
which require little oxygen, are stated to be very triflimg, and 
almost imperceptible. Vauquelin found that insects of the grass- 
hopper tribe generally died before the whole oxygen of the 
vessel in which they were confined was consumed ; while other 
insects, as the bee, are stated to consume the last particle of this 
gas. From Vauquelin’s experiments also this appears to be the 
case with snails and other molusca, to such a degree indeed, that 
this chemist even recommends their use as an eudiometer, or 
means of separating the whole of the oxygen from a mixture 
* ¥or the best observations on the effects of the different gases in respiration, see 
Davy’s Researches on Nitrous Oxide. 
