xl INTRODUCTION. 
nature, consisting of a portion of the impure blood just 
returned from thence, with that which has been aérated 
in the lungs. It is clear that the blood is by this me- 
chanism but partially changed by the action of oxygen ; 
in other words, that the quantity of respiration, speaking 
with reference to the physiological meaning of the term, is 
comparatively small. Hence arises the circumstance that 
these animals have what is called cold blood ; for as it is 
from respiration that the blood derives its heat, and the 
temperature of the body is thereby sustained in animals 
which have more perfect respiration, it follows that where 
this function is but imperfectly performed, the animal 
heat, muscular force, and all the other functions dependent 
upon respiration, will be diminished. In the fishes the 
same effect is produced by different means. These are 
likewise cold-blooded animals ; and in these, also, this pecu- 
liarity arises from the small quantity of respiration which 
they enjoy ; but the phenomenon which in the reptiles is 
produced by the transmission of a part only of the blood 
through the respiratory organ at each contraction of the 
heart, is in the fishes effected by the different medium 
through which the oxygen is presented to the blood; for 
im the latter, although the whole of the blood is sent 
through the branchia, or gills, the quantity of oxygen, 
held in solution in the water which bathes them, is so 
small as to effect but a partial change in the condition of 
the blood. In one case the circulation is complete, but 
the respiration is aquatic, and therefore imperfect ; in the 
other, the application of the air to the blood is immediate, 
but the circulation is incomplete. In the Mammalia, on 
the contrary, where the heart is double, and the circula- 
tion consequently complete, combined with atmospheric 
respiration, the animal heat is considerable; and in birds 
