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Whether the arteries are necessary to an animal.
Again one asks whether the arteries are necessary to an animal.
1. It seems not. For the arteries exist for the sake of conveying life and the natural heat. But natural heat and life exist in every part of the body. Therefore, since these exist in the animal without the arteries, the arteries will not be necessary.
2. Moreover, according to the Philosopher in On the Difference between the Soul and the Spirit, the soul is united to the body by a mediating spirit. But the soul is united to every part of the body; therefore, there is spirit in every part. But it is not in the arteries, and therefore the arteries are not necessary for the sake of the spirits. But they are posited for the sake of the spirits; therefore, etc.
3. Besides, life and heat exist in a plant as well as in an animal. But arteries are not necessary for the plant, and are therefore not necessary for the animal either.
To the contrary. An animal cannot exist without life and heat. But the vital power and the natural heat are borne from the heart to the individual members by the arteries. Therefore, the arteries are necessary.
One must reply that the arteries are necessary because two things are required for the animal's preservation: namely, the restoration of what has been lost and the maintenance or preservation of what has been acquired. But the veins, which are quiet and non-pulsating, are posited for the sake of the restoration of what has been lost, so that the bloodthat is, nutrimentmay be conveyed through them to the individual members. But the arteries, which are pulsating veins, are posited for the preservation of what has been acquired, so that through them the vital power and the natural heat and spirits, without which no operation occurs, descend from the heart to the individual members. Thus, just as the <lungs'> arteries are necessary to attract air to cool and temper the heart, so too the arteries proceeding from the heart are necessary for [delivering] the power of its influence to the other parts.Thus the Philosopher says, in book fourteen, that every corporeal operation (sensation and life are corporeal operations) occurs with the mediation of the heart and the heart's influence on the other parts. This therefore occurs through a corporeal medium, and that medium is the artery that is filled with spirit and natural heat. And because of the continuous pulsing of the spirit in the heart and in the arteries, every artery has two tunics. But a vein has only one tunic, except for that one that is directed from the heart to the lungs. For it contains not only blood but also spirit, and this is why it has two, just as an artery does.
Another reason is that it conveys nutriment. And it must be firm on account of this, lest it burst on the way from the weight of the nutriment that travels through it and lest, as a result, the entire animal fail, etc.
1. On to the arguments. To the first, one must reply that although the natural heat and vital power are in every part, these nevertheless are connected only through the influence they receive from the heart. For heat is flowing from the heart and the vital power flows from the heart through the arteries, and this is why, etc.
2. To the second argument one must reply that the soul is not said to be united to the body by spirit, as if the spirit were the medium, but rather that it operates with the mediation of the spirit. But spirit alone is conveyed through the arteries. For even if other parts operate by means of spirit, these spirits nevertheless flow through the arteries, and this is why, etc.
3. To the third argument one must reply that animal life has a more perfect regimen than plant life, and this is why many things are necessary for the animal that are not necessary for the plant.
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