Whether the arteries take their origin from the heart or the brain.
One asks whether the arteries take their origin from the heart or the brain.
1. It seems it is from the brain. An artery is a vehicle for spirits. Therefore arteries naturally arise from a place where spirit abounds. But spirit naturally arises abundantly from the area of the brain, as is said in the book On the Motion of the Heart; therefore, etc.
2. And the argument is confirmed in this way. The lesser world imitates the greater world, but transparent and gleaming bodies (such as air, fire, and supercelestial bodies) are especially abundant in the superior part of the world. Therefore, it will be this way in the human, who is a lesser world. Now the human is called a microcosm, and microcosm is said from micros, which means "lesser," and cosmos, which means "world"; it is a sort of a lesser world, just as the "macrocosm" is the greater world.
3. Besides, the arteries arise from that part in which they [the spirits] especially abound, but this is from the area of the brain. For its one web, which is called the "marvelous net" [retemirabile], is made from arteries.
To the contrary. That which comes from a source attests to its source. But the arteries have a tough substance and composition, whereas the brain is soft. They therefore cannot arise from it, since like arises from its like.
One must reply that both virtually and corporeally the arteries have their origin in the heart. This is clear first from the fact that all the powers that exist in an animal are rooted in the vital power, which is in the heart. Second it is evident that the artery is the vehicle for the natural heat, the spirits, and the vital power. An artery draws its origin corporeally from that area in which these are generated. But these are all generated in the heart, and this is why, etc. Thus, just as the vein takes its radical and virtual origin from the heart (because this is the first part in an animal) but takes its corporeal origin from the liver (in which is generated the blood for which veins provide a vehicle), so too does an artery take both its corporeal and virtual origin from the heart: the virtual because the heart is the first member, and the corporeal because the generation of those things for which the artery is the vehicle lies in the heart.
1. On to the arguments. To the first, one must reply that spirit is of two types, namely, the vital and the animal, which is itself divided into sensible and motive. Animal spirit abounds in the brain, and the author indicates this in the book On the Motion of the Heart. But the vital spirit abounds in the heart.
2. To the second argument one must reply that the heart in an animal is analogous to superior or supracelestial bodies, as the Philosopher indicates in book two of On Heaven and Earth. For although the solar body is not the center of the world in the way that the heart is located in the animal, it is nevertheless the center of nature.
3. To the third argument one must reply that some of the arteries are just like roots and these are located near the left ventricle of the heart. Other arteries are like branches, and the web in the head is covered with these. In this way a solution to the whole is apparent.
Whether the nerves are necessary.
Next one asks about the nerves. And first, whether the nerves are necessary.
1. It seems not. Because a power that exists indiscriminately in every part does not require a determinate part as its bearer. But sensation exists in every part of an animal. Therefore, it does not require a part bearing it but only one for sustaining it. But nerves are only posited for the sake of bearing sensation and are therefore posited superfluously.
2. Besides, every part necessary to an animal is ordered to sense and exists for the sake of sense. But we do not experience [ sentio] by means of hairs and nerves, according to the Philosopher in the third book of On the Soul, and therefore, etc.
To the contrary. "Nature does nothing in vain." But nerves exist in every animal; therefore, they do not exist in vain, but are necessary.
To this, one must reply that there are four powers in a human: the nutritive or natural, which is in the liver; the vital, which is in the heart; the animal, which is in the brain; and the generative, which is in the parts designated to generation (specifically the testicles in men and women). In just the way an artery corresponds to the vital power (for the power flows through the artery to the individual members), and a vein corresponds to the natural power, so too does a nerve correspond to the animal power. And because the animal power consists in sense and motionfor by these we distinguish an animal from a non-animal, as the Philosopher says in the first book of On the Soul this is the reason why some nerves are sensitive and some are motive nerves. Therefore, just as sense and motion are necessary to an animal, so too are the nerves.
Moreover, bone is not directly connected to bone in an animal because, if it were, then no flexion could occur between the bones. This is why they are positioned appropriately next to one another and are joined by nerves.
1. On to the arguments. To the first, one must reply that although sense exists in every part in terms of participation, it nevertheless exists in one part in terms of its roots [radicaliter], for it flows from here to the other parts by means of specific members. For example, the power of sight flows to the eye through the optic nerve. Thus sense exists in one part only for bestowing [a power], as in it does in the heart, and in another part only for receiving [a power], and in still another part it exists both for giving and receiving.
2. To the second argument one must reply that some nerves carry sense and motion, and these are very sensitive [sensibiles].
Thus an injury to the arm muscles is especially onerous owing to the branching of the nerves. Other nerves only join bones together, but these are not properly called nerves, as the Philosopher notes. Nerves like this are hardly sensitive at all, so that the bones can be moved without pain in the joints, for otherwise intolerable pain would be caused by their continuous motion. This is why they do not have sensation.