Theses medicae de divina et admiranda hominis in utero formatione

XXXIX.
Therefore, this seems to be a truer account of the conformation: Once the generative faculty is excited, the conception is by no means idle, but each of its parts applies itself to its work. Thus, all parts of the fetus equally obtain the outline and rudiment of their conformation, even if they do not all attain their perfection equally. But first of all, three bubbles appear in the middle of the seed, which correspond to the three principal members. Thus, the delineated parts are subsequently and gradually fashioned, until they attain perfection and absolute form.

XL.
The principal members are formed; the liver indeed from venous blood with veins diffused around it: the heart from arterial blood, growing together with membranes and fibers already formed from the seed: but the brain from the purer matter of the seed, as it was to be the instrument of the noblest functions.

XLI.
There is a great contention between Philosophers and Physicians regarding the order of the conformation of the principal members, some of whom hold that the liver, others that the heart, is formed first. In which controversy it is very difficult to establish anything certain, since there is the utmost necessity of both viscera for life and nutrition. For just as the heart does not live without blood, which the liver sends, so neither does the liver live without the vital heat, which the heart infuses. We, however, for the sake of discussion, establish that the necessity of the heart is greater than that of the liver. And since nature in generating looks to what is more necessary, we acknowledge with Aristotle that that is also made first.

XLII.
Meanwhile, while the parts of the fetus are delineated, Nature generates blood—which a woman, by a singular privilege of nature, requires more than is needed for her own nourishment—and pours it out to the uterus through certain vessels, so that the fetus may be nourished and increased by it.

XLIII.
The vessels of the uterus are double: some are called descending, others ascending. The descending ones are two veins and two arteries, called seminal: the right of the veins arises from the trunk of the vena cava: the left, however, from the left emulgent: the arteries from the trunk of the aorta below the kidneys.

XLIIII.
These vessels are later divided into two branches, of which the smaller is inserted into each female testicle for the concoction of the seed: the other, however, and the largest, is distributed through the uterus as its seat.