The heart

in all animals has cavities inside it. In the case of
the smaller animals even the largest of the chambers is scarcely
discernible; the second larger is scarcely discernible in animals of
medium size; but in the largest animals all three chambers are
distinctly seen. In the heart then (with its pointed end directed
frontwards, as has been observed) the largest of the three chambers is
on the right-hand side and highest up; the least one is on the
left-hand side; and the medium-sized one lies in betwixt the other
two; and the largest one of the three chambers is a great deal
larger than either of the two others. All three, however, are
connected with passages leading in the direction of the lung, but
all these communications are indistinctly discernible by reason of
their minuteness, except one.