36 ANIMALIA VERTEBRATA. 
Subdivision of the Vertebrata into Four Classes. 
We have just seen how far vertebrated animals resemble 
each other; they present, however, four great subdivisions or 
classes, characterised by the kind or power of their motions, 
which depend themselves on the quantity of their respiration, 
inasmuch as it is from this respiration that the muscular fibres 
derive the strength of their irritability. 
The quantity of respiration depends upon two agents: the 
first is the relative amount of blood which is poured into the 
respiratory organ in a given instant of time; the second is the 
relative amount of oxygen which enters into the composition 
of the surrounding fluid. The quantity of the former de- 
pends upon the disposition of the organs of circulation ‘and 
respiration. 
The organs of the circulation may be double, so that all the 
blood which is brought back from the various parts of the 
body by the veins, is forced to circulate through the respi- 
ratory organ, previous to resuming its former course through 
the arteries; or they may be simple, so that a part only of 
the blood is obliged to pass through that organ, the remainder 
returning directly to the body. 
The latter is the case with reptiles. The quantity of their 
respiration, and all their qualities which depend on it, vary 
with the amount of blood thrown into the tS at each pul- 
sation. 
Fishes have a double circulation, but their organ of respi- 
ration is formed to execute its function through the medium 
of water; and their blood is only acted on by the portion of 
oxygen it contains, so that the quantity of their respiration is 
perhaps less than that of reptiles. 
In the mammalia the circulation is double, and the aerial 
respiration simple, that is, it is performed in the lungs only ; 
their quantity of respiration is, consequently, superior to that 
of reptiles, on account of the form of their respiratory organ, 
and to that of fishes from the nature of their surrounding ele- 
ment. 
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