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Whether the motion of birds, which is flight, is natural.
Further one asks whether the motion of birds, which is flight, is natural.
1. It seems not, because every mixed thing is moved according to the nature of the element that is dominant in it. But water and earth dominate in every mixed thing and their natural motion is downward. Therefore, the ascent of a mixed thing is contrary to nature.
2. Besides, if the flight of birds were natural, it would be so either by reason of the body or by reason of the soul. Now, not by the body, because once the soul has withdrawn from the bird, its body, placed in the air, will descend. And it is proved in the same way that it is not by reason of the feathers. Neither is it by reason of the soul because the operation of a nobler form is itself nobler; but the human's soul is nobler than the soul of any other animal. Thus flight will be much more natural to the human than to the bird, but this is false, and therefore the premise is false.
To speak to the opposite, natural motion is one whose principle is from within per se and not per accidens, but flight in birds is from their intrinsic principle, because it is by their power; therefore, etc.
To this question one should reply that the motion of flight can be related to a bird in two ways. It can be by reason of the body, just as the Philosopher responds in the fourth book of the Physics, and thus the flight of a bird is an unnatural motion because the motion of the mixed body follows the nature of the element that is dominant in it. But in every mixed body the heavy elements are dominant. Or it can be related to the bird by reason of the soul, and in this way flight is a natural motion. Nevertheless, it is not natural absolutely, like the motion of the heavy and the light, but it is the motion of an animal even with apprehension, because, simply put, a natural motion is toward the one end in which the movable [mobile] naturally rests, like the heavy going down and the light rising up. And on account of this the Philosopher said in the second book of On Interpretation [Peri Hermeneias] that a natural power does not relate to opposites. But animal motion can be toward several ends owing to the difference of its apprehension.
1. On to the arguments. To the first, one should reply that flight is not a natural motion for a bird with respect to its body, and reason proves this, but rather it is natural with respect to and by reason of its soul.
2. And you prove that it is not by reason of the soul because it is of a nobler form, etc. One must say that a nobler operation is appropriate to a nobler form. Thus the proper operation of the human soulwhich is to understand is nobler than the operation of another animal as far as this is concerned. Nevertheless nothing prevents some brute beast from surpassing the human in certain operations. For example, the eagle and the lynx surpass the human in sight, the ape in taste, the dog and the vulture in smell, the wild boar in hearing, the spider in touch, and so on for others. Whence the verse:
The wild boar surpasses us in hearing, the spider by touch, The vulture in smell, the lynx in sight, and the ape in taste.
Whether any animal ought to lack a spleen, a gall bladder, which is its reservoir [cista], and lungs.
Further one inquires whether any animal ought to lack a spleen, a gall bladder, that is, its reservoir [cista], and lungs.
1. It seems not, because the heart is a more principal member than the brain. But no animal having a brain lacks protection for the brain, which is afforded it by the cranium, its webs, and its tunics. Therefore, by analogy, no animal that has a heart will lack a protector for the heart. But the lungs are the protector of the heart, because the heat of the heart is tempered through the air or through the cold of the air that has been breathed in; and therefore, etc.
2. Moreover, the gall bladder [cistis fellis] is the receptacle for bile [cholera], in order to aid in digestion and elimination, and it is something purified from the blood. Therefore, similarly, every animal that has blood requires the gall bladder [or "bile," fel] and something else for the sake of digestion and the elimination of feces. Therefore, etc.
3. The same can be argued with respect to the spleen since it is the receptacle of black bile [melancholia] and there is no animal without black bile. Therefore, etc.
But the Philosopher says the opposite.
One must reply that the lungs are a necessary assistant and fan for the heart. For by expanding they draw in air for tempering the heat of the heart, and by contracting they expel air that was drawn in and warmed. In some there is therefore an abundance of heat in the heart, and for these the lungs are even more necessary. But in those lacking blood, heat does not abound in the heart owing to the coldness of their complexion. Bloodless creatures are like this and neither does heat abound in fish, because they continuously live and are nourished in a cold element, namely, water, and such animals therefore lack lungs.
Similarly bile is something purged from the blood, as was argued. For the gall bladder is a receptacle for bitter, red bile [ cholera rubea]. But in some animals the whole nourishment [nutrimentum] gives way and is converted over to the nourishment [ fomentum] of the animal and to the restoration of what has been lost. This is the case in birds on account of the abundance of their heat and their dryness. And this is why there is no gall bladder in them distinct from other parts, but rather [the bile] is scattered among various parts along with the nour-ishment. And if some bile [cholera] remains in the liver, it immediately sends it to the stomach in order to strengthen their digestion. And because they use little food in comparison with quadrupeds, their elimination therefore obeys the urge for expulsion without the need of bile [cholera] as a stimulant. And there is still another reason for this: not all birds drink, neither do they emit urine, as is apparent in birds with curved talons, and this is why they do need some moisture to enable the food to penetrate the narrow vessels of the members better. For this reason both cholera and melancholia are mixed into their blood in the vessels, so that the nourishment might have easier transit to the individual parts of the members, because cholera can penetrate by virtue of its heat and melancholia does so by virtue of its acidity, because it is acidic. This is why the Philosopher says that every animal has a bladder, but according to more and less. For some have a proper receptacle for the bile, like the human, the cow, the dog, and such animals, and some do not have their bile in a dedicated receptacle but rather dispersed among various parts. So, in the stag, the bile is dispersed through the intestines, and this is why the intestines are bitter tasting and dogs will not eat them unless they are starving. And this is why hunters hunt them at a very fast pace, because otherwise it would be impossible to capture them using dogs. For the cholera is very much aroused in their intestines by all the motion, and cholera flees there from other parts of the body and corrodes and ulcerates them. As a result of the excessive pain they will not run further and thus are detained by the dogs, because they cramp from the pain in the intestines just as a human does suffering from colic [cholerica passio]. You can assign the same reasons concerning the spleen and the melancholia in such animals.
1. On to the arguments. To the first, one must say that just as the brain needs an exterior body as a protector, so too does the heart need one, but nevertheless it needs lungs only if there is an excess of heat in it. This is why the relationship of the cranium to the brain is not the same as that of the lungs to the heart.
2. To the second argument one should say that although every animal has what is purged from the cholera and melancholia of the blood, nevertheless not all have a proper receptacle for them. Instead, in some animals these are dispersed through various parts. Now because the spleen is the proper receptacle for melancholia, and the gall bladder for cholera, and these are distributed among various parts in some animals, thus not every one has a spleen or a gall bladder.
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