In all sanguineous animals the case stands as here set forth in
regard to the points of departure and the courses of the chief
veins. But the description does not hold equally good for the entire
vein-system in all these animals. For, in point of fact, the organs
are not identically situated in them all; and, what is more, some
animals are furnished with organs of which other animals are
destitute. At the same time, while the description so far holds
good, the proof of its accuracy is not equally easy in all cases,
but is easiest in the case of animals of considerable magnitude and
supplied abundantly with blood. For in little animals and those
scantily supplied with blood, either from natural and inherent
causes or from a prevalence of fat in the body, thorough accuracy in
investigation is not equally attainable; for in the latter of these
creatures the passages get clogged, like water-channels choked with
slush; and the others have a few minute fibres to serve instead of
veins. But in all cases the big vein is plainly discernible, even in
creatures of insignificant size.