Whether of all the humors blood alone nourishes.
Further one asks whether of all the humors blood alone nourishes.
1. It seems so, because there are four powers in every animated being: the attractive, retentive, digestive, and expulsive. But the attractive only attracts the sweet, as the Philosopher implies in the second book of On the Soul, because only the sweet nourishes. Therefore, the digestive power will digest only the sweet.
But only what is digested nourishes. Therefore, since only the blood is sweet, only the blood nourishes.
2. The same view seems defended by the authority of the physicians, who posit that only the blood nourishes.
3. Moreover, the other humors have their own proper receptacle, like yellow bile [cholera] which has the gall bladder, and melancholy the spleen, which would not be the case if they contributed something to the animal's nutriment.
To the contrary. Nature does nothing in vain. But all the humors can be generated from any part of the nutriment, according to its diverse disposition. Therefore, for the same reason, if one nourishes, so too will another.
To this, when one asks whether the blood alone nourishes, one must reply that blood can be considered in three ways. In one way, such that blood alone nourishes and no other humor does so. But this understanding is false, because nutrition is the complete assimilation of the nutriment to the one nourished. It is therefore necessary that whatever is lost be restored by the nutriment. But what is lost can be choleric or phlegmatic or melancholic or sanguineous, because the member that has to be nourished may be of this type. Therefore, one must say that all the humors can enter into the nutriment for a complete restoration and perfect assimilation to occur.
That blood alone nourishes can be understood in another way, because it nourishes in a solitary manner without another humor present, and this understanding is true, because blood can nourish though not conjoined with or mixed with other humors, and this is not the case for the other humors.
It can be understood in a third way, that blood alone nourishes effectively and the other humors nourish only materially, because they nourish only with the blood mediating their activity, just as one may say that only medicine heals because it acts effectively, even though food and drink heal materially because they generate the material for health.
Therefore, one must reply to the question that the other humors nourish along with the blood, because there is no mixture so choleric or so melancholy in and of itself that it cannot be nourishment for certain birds like the ostrich. Nevertheless, that they do not nourish all animals stems from the fact that the complexion of an animated being is not always proportioned to the mixture.
1. On to the arguments. To the first, one must reply that in the nutriment and in the attractive power, that by reason of which something is attracted is sweet. But nothing prevents that which is attracted from being bitter, sharp, or pontic, or having other tastes.
2. To the second, one must reply that the physicians say that only the blood nourishes because it alone nourishes effectively and in a solitary manner, but nevertheless it is not the only one that nourishes; rather, the other humors nourish as well.
3. To the third argument one must reply that choler exists in an animal's body in two ways: one is necessary, and the other is helpful. The choler that is necessary flows with the blood and makes a path for the blood, which is thick, by penetrating and opening the narrow veins of the body, because choler is very penetrating. The other, helpful choler is received into the gall bladder for three reasons: (1) so that it may arouse the expulsive power by stimulating the intestines; (2) so that it may wash away the oily phlegm and scrape it from the wrinkles of the stomach; (3) to strengthen the digestive power with its heat. Therefore, the choler that is necessary can contribute to the nutriment of any part, and the one that is helpful acts as a nutriment for the gall bladder [bursiculae fellis] and for those three forms of assistance that have been mentioned. And the same must be said for the melancholy, with respect to the spleen and the attractive power, which it strengthens.