Hippocrates says that the origin of men's sperm derives from the
brain, and from the lungs and testicles of our parents, where the
final decocture is made, and all the other limbs transmit their
substance to this sperm by means of expiration, because there are no
channels through which they might come to the sperm.

[Footnote: The works of Hippocrates were printed first after
Leonardo's death.]