Now, as the nature of blood and the nature of the veins have
all the appearance of being primitive, we must discuss their
properties first of all, and all the more as some previous writers
have treated them very unsatisfactorily. And the cause of the
ignorance thus manifested is the extreme difficulty experienced in the
way of observation. For in the dead bodies of animals the nature of
the chief veins is undiscoverable, owing to the fact that they
collapse at once when the blood leaves them; for the blood pours out
of them in a stream, like liquid out of a vessel, since there is no
blood separately situated by itself, except a little in the heart, but
it is all lodged in the veins. In living animals it is impossible to
inspect these parts, for of their very nature they are situated inside
the body and out of sight. For this reason anatomists who have carried
on their investigations on dead bodies in the dissecting room have
failed to discover the chief roots of the veins, while those who
have narrowly inspected bodies of living men reduced to extreme
attenuation have arrived at conclusions regarding the origin of the
veins from the manifestations visible externally.
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