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Whether blood is necessary for an animal.

One asks further about the blood. And first, whether blood is necessary for an animal.

1. It seems not, because "we exist from and we are nourished by the same things." But blood is posited as necessary only for the sake of the nutriment and its preservation. Therefore it is not necessary [to the animal].

2. Moreover, in the seventeenth book of this work the Philosopher argues in this way: whatever is not present in all animals is not necessary to an animal. But the testicles are not present in all animals, and therefore, etc. The same argument can be made about the blood, since it is not present in every animal, and therefore, etc.

3. Moreover, one and the same thing cannot be the cause of contraries. But blood is the cause of an animal's corruption, as is said in the text. Therefore it is not necessary to it.

To the contrary: but that thing is necessary to an animal which, when absent, means the animal cannot survive. But blood is something like this, as is said in the text, and therefore, etc.

One must reply to this that both order and degree exist in animals. Now the human is the noblest animal. But some animals are closer to plants, like the immobile animals, and some are closer to the human, like the horse and cow and ones similar to these. Some occupy the middle ground, like fish and serpents. Therefore, those that are nearer to plants lack blood, and there are four genuses of these, as is said in the beginning of book four. But those that are nearer to the human have a great deal of blood, while those that occupy the middle ground have blood but have less of it. Therefore, blood is not necessary to all animals, but only to those that have blood, because their life depends on warmth and moisture, and blood is naturally warm and moist. And this is why it acts especially to preserve life, since preservation stems from a like thing just as corruption results from a contrary. For blood is said to be the seat of life according to the physicians and the Peripatetics and especially according to Plato.

1. On to the arguments. To the first, one must reply that "we exist from and we are nourished on the same things" in a mediated but not in an immediate sense. For we exist from mixed things, since we are nourished by semen and mixed things. This is why everything that nourishes is mixed. Thus certain birds eat iron and other birds eat poison, like flies, just as the quail eats henbane [iusquiamum] and the she-goat and the stag eat serpents. Nevertheless not every mixed thing nourishes, owing to a defect in the heat and a disproportion in those things by which nourishment occurs. Nevertheless, the proximate principles of generation and nutrition are not the same, since semen is a principle of generation and blood is a principle of nutrition.

2. To the second argument one must reply that blood is not necessary to an animal insofar as it is an animal, because it is not present in every animal; nevertheless, it is necessary for some animals, as was said.

3. To the third argument one must reply that the same thing is not a cause of contraries uniformly and in the same way or according to one and the same reason, but it is entirely possible in another way. Thus it is when proportionally disposed blood is the cause of preservation. If, however, the blood is superfluous or diminished, it can be a source of corruption.

Whether blood is a principle of the animal's preservation or is a natural source of corruption.

One asks whether blood is a principle of the animal's preservation or a natural source of corruption.

1. And it seems that it may be the occasion or principle of corruption. Corruption is of two types. One is combustion, and this terminates in ashes; the other is putrefaction, and this is a natural and proper corruption, as is said at the beginning of the fourth book of the On Meteorology. But putrefaction arises from heat and moisture, and therefore, since the blood of all the parts is especially warm and moist, it will especially be the principle of putrefaction, so it seems.

2. Moreover, of all the animals, those having blood are more prone to corruption, according to the physicians. But this would not be so were the blood not the principle of corruption, and therefore, etc.

To the contrary. The philosophers chose to study in areas near to the sea, which are warm and moist. But this would not be so unless heat and moisture were the principle of preservation. Therefore, since the blood is particularly hot and moist, it is especially the principle of preservation, so it seems.

One must reply that blood can be considered in two ways. One way is that which falls under nature's regimen, and in this way blood is especially the principle of preservation, because life depends on heat and moisture, which are found in blood, and this is why a sanguineous complexion, according to which the individual is under nature's regimen and under the requisite temperament, is the best and suited to long life.

Blood can be understood in another way, to the extent that it is beyond nature's control on account of some defect. But this defect can also be of two types: either specifically, such as blood corrupted in an abscess, or generally, as in a fever, which arises from the blood. And this latter defect causes more corruption than does the first. But this defect can still be understood with respect to the substance and to the quantity and quality [of blood]. For if blood is considered according to its substance, then it is hardly corruptible at all and hardly passes over into the matter or into the nature of poison. This is not the case for the other humors because a potent poison arises from a quartan fever, which results from a hard, earthy, dry melancholy.

But if one considers blood in terms of its quantity, then it is especially corruptible, because it is very abundant in the body. And in like manner if one considers it with respect to its quality, because it contains within itself the father of putrefaction, which is heat, and its mother, which is moisture. But in terms of its own substance, since it is very well digested and very pure, it does not putrefy, etc.

Thus one must reply to the question that blood that falls under nature's regimen is not a principle of corruption but of preservation. If, however, it falls outside nature's control, then it can be a principle of corruption, not, however, by reason of its substance, but by reason of its quantity and quality, as was seen.

1. On to the arguments. To the first, one must reply that heat is of two types: natural and accidental. Putrefaction arises from accidental heat, which is external and foreign. Because external heat opens the body's pores and causes the internal, natural heat to escape and as a result it releases the moisture that is the subject of the natural heat. Thus one must reply to the form of the argument that heat and moisture are natural things in the blood, but putrefaction does not arise from the natural heat and moisture but from a heat and moisture accidentally acquired in the blood itself.

2. To the second argument one must reply that corruption is of two types: namely, natural and accidental. The more sanguineous bodies are, the more slowly are they naturally corrupted because the heat and airy moisture that are in the blood are the cause of long life. Nevertheless, such things are more quickly corrupted accidentally because the complexion of bodies of this sort is more temperate and therefore more easily moves away from the mean and from a temperate condition, and this is why the complexion of such bodies can easily become distempered, etc.

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