Whether all bones are nourished.

One asks further whether all bones are nourished.

1. It seems not. For some bones are not hollow, like the bones of feet and hands. Therefore, no marrow, which is the nourishment for the bones, is found in ones like these. Therefore, etc.

2. In addition, whatever can be nourished can also grow and, as a result, extend. But bones, since they are dry, hard, solid, and compact, cannot be extended. Therefore, etc.

On the contrary. Everything participating in life is nourished. But every bone participates in life, and therefore, etc.

To this, one must respond that every bone is nourished. And an indication of this is that when the body grows the bones are proportionally increased in size, either in length or in thickness. But this can only be if the bones are nourished. And besides this, just as the heat in the parts surrounding a single bone acts on it, so too is this the case in the part surrounding any given bone. Just as that one needs nourishment, so too does each one.

1. To the arguments. To the first argument one must reply that although certain bones are solid (except for their hollow area), they nevertheless remain porous and therefore can be nourished.

2. To the second argument one must respond that bones can exist in two conditions. They are sometimes perfectly hard, and other times imperfectly hard. In the first condition they neither grow nor extend, but in the second condition they can grow, just as other members do.

Whether the marrow is hot.

One inquires further into the complexion of the marrow, and whether it is hot and moist.

1. It seems not. For what is derived [rescinditur] from something else bears witness to it, according to Galen in his Tegni: but marrow is derived from the bones' nourishment, which is cold and dry. Therefore, marrow will be cold and dry.

2. In addition, every effect bears witness to its principle. But the first marrow is the marrow of the brain and the second is the spinal cord [nucha] crossing through the vertebrae. But these marrows are cold and moist, and therefore so will be all the others.

The Philosopher says the opposite.

One must reply that marrow is of two types: one is ordered to nutrition, and this one is generated from spermatic blood or from the nutriment, and this marrow is situated in bones. The other marrow is ordered to sensation and towards influencing motion, and this kind of marrow is the marrow of the brain and of the vertebrae. But the first is hot and moist, because provident and learned nature always has ordained that one contrary be next to another in the body, so that they might temper one another. And nature situated the first marrow in bones, which are cold and dry. It is necessary, then, that this marrow be hot and moist.

Moreover, the first marrow shares the nature of blood, and an indication of this is that it changes its heat in accordance with a change in the blood, which has one heat in youths and a different heat in old people. But blood is hot and moist. For this reason, the marrow is hot and moist. But the second marrow is cold and moist. And the reason for this is that sensation consists in being acted on and being altered. Therefore, that which serves the senses must be especially capable of being acted on and hardly active at all. But the cold is less active than the hot, and the moist is more passive than the dry, and this is why the marrow of the brain and vertebrae is cold and moist. But nevertheless the spinal cord, which is the marrow for the vertebrae, is hot in comparison to the brain, but cold in comparison to the other marrow, because it is nearer to the brain and for this reason receives coldness from it. But it is nearer the heart than the brain, and this is why it is hotter than the brain itself.

1. To the first argument one must reply that something may be derived from something else in two ways. It may be according to the path of division, and something thus derived bears witness to the one from which it is derived. Or it may be according to the path of separation. By this path, the four elements can be derived from the same mixed body, and one derived in this way does not always resemble the one from which it is derived. It is in this way that marrow is derived from the bones' nourishment.

2. To the second argument one must reply that not every marrow takes it origin from the brain or from the spinal cord, but the bones' marrow takes its origin from the spermatic blood, and this is why it has a yellowish color, whereas the spinal cord has a white color.

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