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.]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.