Whether the first digestion, which occurs in the stomach, changes the nutriment into a different species.
One asks further whether the first digestion, which occurs in the stomach, changes or alters the nutriment into a different species.
1. And it seems not. For just as the nutriment is converted into the animal's substance, so too is it converted into the plant's substance. But in the first digestion the species of the plant's nutriment are not changed, and therefore neither are they changed in the first digestion of the animal.
2. In addition, digestion changes the species only by the power of heat. But the stomach is cold by nature, because it is nerve-filled. Therefore, it cannot change the species.
To the contrary. The species of the nutriment is changed in the second digestion, which occurs in the liver, because [there] it is converted into blood. Therefore, for the same reason, this occurs in the first digestion.
One must respond that the first digestion, which occurs in the stomach, when it proceeds by a natural path, changes the species of the nutriment. And I say "by a natural path" because if the natural power were outside its natural disposition, for example, if the retentive power were too weak and the expulsive power too strong, then it could happen that the food would be excreted under the same species in which it was consumed. But when it proceeds by a natural path, it happens otherwise, because in the beginning the nutriment becomes dissimilar, and in the end, similar. But this can happen only if it is changed into a contrary disposition. Thus the nutriment in the stomach is segregated into several parts by the power of the natural heat; of these parts, the chyle is said to be purer, and the waste product is said to be more impure. This chyle, however, is changed into blood during the second digestion, and in the third digestion this blood is changed into the members. Thus its species is changed.
1. To the arguments. To the first argument one should respond that the first digestion in a plant changes the species of the nutriment, as is the case in an animal. But that digestion that is in the ground does not occur through the mediation of any part of the plant. Thus, if this digestion is compared to the digestion that occurs in an animal, the argument does not succeed.
2. To the second argument one must respond that the stomach is cold and dry in its upper part, but it is warm in its lower part. Nevertheless, it does not have its own heat adequate for digestion; rather, it participates in the heat of the heart, the lungs, the spleen, and the liver. This is why the stomach is placed over them, just as a pot is placed over a fire. Thus, just as the pot is in itself cold, nevertheless by means of the heat of the fire surrounding it, it cooks the food. And so too, the stomach is in itself cold, but nevertheless it digests by the power of these other members, etc.
Whether the four humors are generated from the same nutriment.
One inquires further whether the four humors are generated from the same nutriment.
1. And it seems not. Now, an agent is of two types: higher and lower. A higher agent acts according to the exigency of its power, but a lower agent acts according to the disposition of matter. Since, then, the agent in digestion is one and the material is uniform, it will generate a single thing.
2. Besides, sometimes the food is proportional to one humor, because sometimes it is cholericlike pepper and others like thisand sometimes it is phlegmatic. Therefore, since in those that have an affinity the transformation will be easier, the food is converted into only a single humor.
The Philosopher says the opposite.
One must say that the four humors can be generated from the same nutriment because, whatever the nutriment is, however uniform it is, it is a mixture because, according to the Philosopher in the first book of On Generation [and Corruption] and in On Sense and the Sensed, simples do not nourish. Thus the power of the four mixables is in every nutriment. And this is why the four elements can be separated from every nutriment, and in like manner the four humors, which correspond to the four elements, can be separated from every nutriment. For just as there are four substances in milk, and, with the addition of a coagulant three of the substances can be separated from it, one of whichlike cheesecorresponds to earth, and another like buttercorresponds to air, and a thirdlike wheycorresponds to water or, according to some, to fire. So too, diverse humors can be separated from nutriment through a natural power, because the earthy parts are more converted into melancholy, the watery into phlegm, the airy into blood, and the fiery into choler.
1. To the arguments. To the first argument one should respond that although the first matter of nutriment is one, nevertheless it itself has a varied disposition and has power under various powers. Therefore, because it is the material of a mixture, and is acting on a single thing that is nevertheless disposed in various ways, it can produce diverse effects. Natural heat acts one way when using the power of the soul on earthy parts, and another way on airy parts. This is because natural heat first assembles homogenous parts, as when lead and silver are joined together at the same time, and fire separates one from the other. This is how a natural heat acts when digesting nutriment, etc.
2. To the second argument one should respond that although some kind of nutriment may correspond more to one humor than to another, nevertheless it exists in potency toward any of them, and this is why although any humor can be generated from it nevertheless that humor will be generated most from the one to which it corresponds, etc.