Heat, again, tends to
make the body erect; and thus it is that man is the most erect of
animals, and the vivipara more erect than other quadrupeds. For no
viviparous animal, be it apodous or be it possessed of feet, is so
given to creep into holes as are the ovipara.
The lung, then, exists for respiration; and this is its universal
office; but in one order of animals it is bloodless and has the
structure described above, to suit the special requirements There
is, however, no one term to denote all animals that have a lung; no
designation, that is, like the term Bird, applicable to the whole of a
certain class. Yet the possession of a lung is a part of their
essence, just as much as the presence of certain characters
constitutes the essence of a bird.
7
Of the viscera some appear to be single, as the heart and lung;
others to be double, as the kidneys; while of a third kind it is
doubtful in which class they should be reckoned. For the liver and the
spleen would seem to lie half-way between the single and the double
organs. For they may be regarded either as constituting each a
single organ, or as a pair of organs resembling each other in
character.
In reality, however, all the organs are double. The reason for
this is that the body itself is double, consisting of two halves,
which are however combined together under one supreme centre.
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