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Whether veins are necessary in an animal body.
We have already discussed the disposition of the members," etc. With respect to this third book we will first ask whether veins are necessary in an animal body.
1. It seems not. The natural power [vis naturalise is in charge of providing nourishment for plants and in animals. But there are no veins in plants delegated for nourishment. But a vein does not exist for another purpose, and therefore veins are not necessary.
2. Moreover, bones are nourished just like other members, but there are no veins in bones. It is therefore not necessary to posit the existence of veins in other parts for the sake of the distribution [delegation of nourishment.
3. Moreover, the conversion of the nutriment is almost completed in the third digestion, but veins are not required during the third digestion, and are therefore not required in the other digestions either.
To the contrary: every bodily operation occurs by means of an intermediate body. But the distribution of the nutriment is a bodily operation, as is clearly evident in its own right [ de se]. Therefore, it occurs by means of a bodily medium. But such a medium is nothing other than a vein; therefore, etc.
One must reply that the word "vein" is an equivocal noun which has two meanings. These are "pulsating," which is properly called an artery, and "non-pulsating," which is properly called a vein in the human. Now, a pulsating vein (that is, an artery) is necessary for the sake of sustaining the heat or to bear the spirit to individual members. Since these members are like a medium for life, their arteries arise on the left side of the heart, where the heart's heat and the spirit are especially abundant. But a non-pulsating vein is necessary for the distribution of the nutriment, that is, the humors, to individual members and for the sake of sustaining the heat and the spirits and to restore what has been lost, because when aliment [alimentum] is received into a particular part, it will be unable to restore whatever has been lost in these individual members unless it has access to individual members.
Moreover, everything that has a part in which it is generated, has also a part in which it is kept; otherwise, it would be generated in vain. But the blood is generated in a given part in the body such as in the liver and the heart. It therefore requires a part in which it is kept. But this part is the vein, and this is why, etc.
Nevertheless, one must observe that sometimes the blood is in a remote power, and sometimes it is in a power that is close to conversion, and sometimes it is in a middle power. When it is in the stomach it is in a remote power. From there it has a pathway through which it comes to the stomach (like the esophagus) and a pathway through which it is distributed from the stomach (like the delicate or mesenteric pathways through which it is sent to the liver). It is in a power close [to conversion] when it is dispatched to a given member, and then it requires no connection beyond the member. But it exists in a middle power in the liver where the second digestion occurs, and this is why the liver requires veins through which the blood may be dispatched to other members and through which it can receive the moistures not converted into blood as a sediment [hypostasis]. And this is why the veins are necessary.
1. On to the arguments. To the first, one must reply that the nutritive power in the animal exists under the natural and animal regimen. But in a plant it is not ruled by the animal regimen, and this is why the power requires more in the animal than in the plant. And in addition, the plant has a marrow in the middle of every part in place of a vein. But these "veins" extend from the root to the extremities. But in animals the nutriment is taken into the middle, as it were, and the veins extend out from here just as if from the center to the circumference.
Moreover, plants always have their aliment at hand, and they therefore do not absorb more than they can digest. But animals do not have their aliment always at hand, and as a result they frequently eat in a single day more than they can digest over several days, and this is why certain parts are necessary in an animal where this undigested or unconverted nutriment may be stored. Veins are parts of this sort.
2. To the second argument one must reply that every animal part, although it may appear solid, like bone, is nevertheless porous and capable of receiving moisture. Thus blood is conveyed to the extremities through the veins, and in the extremities the blood seeps into the parts connected to them. Therefore, although bones do not receive nutriment through external veins, they nevertheless do receive it from internal veins through internal pores.
3. To the third argument one must reply that the digestive power is weak in the third digestion, and this is why it should not receive much at one and the same time, because if it did receive a lot, and more than it could digest, then some disability would occur in it. This is why there are veins in which the blood is stored and gradually and successively sent to the members. And this is why there are no veins in the first or third digestion, but only in the second.
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