Albertus Magnus's Little Book,

...mentioned original: "tiuntur"; this completes the sentence from the previous page regarding how different women are affected by their cycles, and there are many signs concerning this part—namely, that the menses flow—about which more will be said later. Here the author raises four questions in order, as appears in the text. Note that the flow of the menses is blood-red in color. The cause of this is that the menses is a superfluity superfluity: an excess substance or byproduct produced during a biological process that the body must expel of the second digestion, namely that of the liver original: "hepatis"; since the liver is a red organ and extremely hot, the fluid takes on its likeness in both color and form as much as it possibly can. But on the contrary, it would seem that the menses ought to be white, because it is extremely cold, and coldness is the "mother of whiteness." I respond, following Albertus, that according to the nature of the menses itself, it would indeed be white; however, because the organ producing it is red (the liver), it takes on that same appearance. Then a doubt arises: why is a man's semen original: "sperma viri" not red, since it is nevertheless hot and well-digested? It is said that male semen is actually red by its own nature, because it is the blood of the third digestion, but it is turned white in the testicles due to the coldness of the testicles. For this reason, it does not act as seed original: "spermatizat" unless it is first warmed by the movement of the testicles. Note, the color red is generated because, in an earthy liquid, the moisture is consumed through the "burning" of that moisture, as is clearly seen in a red brick original: "latere rubeo". For a brick is not red before it is fired, but it is made red through the heat consuming its moisture, so that the dry, earthy part predominates. This is how it is with a woman's menses: because the moisture is not entirely consumed into dry earthiness by the heat of the liver, it becomes red in a certain way. But in women who are very stout and of an "earthy" temperament, there is so much moisture left unconsumed,