Theses medicae de divina et admiranda hominis in utero formatione

XLV.
The ascending vessels of the uterus are also two veins and two arteries, arising from the principal branches of the vena cava and the great artery not far from the groins: which, variously dispersed through the exterior sides of the uterus, are finally inserted into its very base.

XLVI.
Through these vessels, blood, both venous and arterial, is poured forth like dew into the interior spongy tunic of the uterus, which is pervious like a sponge or pumice stone with many holes, with the arteries and veins mixed and confounded there together: upon which gradually concreting, a certain soft and round flesh is made, which some call the placenta from its circular figure, and others call the "uterine liver" from the duty it seems to perform.

XLVII.
That blood by which the fetus is nourished is otherwise called menstrual, because women who are not pregnant are accustomed to be purged by it every month.

XLVIII.
For since a woman is moister and colder than a man, she therefore abounds more in blood, concocting food as much as is expedient: nevertheless, because of the weakness of her heat, she is not able to digest what is redundant through exhalation. Therefore, so that this superfluous blood may not cause any harm in the body, it is purged by nature every month through the female parts.

XLIX.
This blood is by no means poisoned, as many mistakenly think, interpreting Hippocrates as speaking about diseased women: but it is at fault only in its abundance and quantity. In which respect it is called vicious and sickly by Aristotle.

L.
It erupts in virgins around the fourteenth year: because the heat then becomes more acute and pours it out: secondly, because it is no longer consumed in growth.

LI.
The cause of the eruption is not the movement of the moon, since it does not flow in everyone at the same time: but it is partly due to the abundance, and yet not that alone, since even those who collect little blood have their own periods: it is partly also due to the temperament and the custom of nature.

LII.
It is purged most of all through the neck of the uterus, into which certain small branches from the ascending vessels are implanted.