Whether the nerves take their origin from the brain or the heart.
One asks next whether the nerves take their origin from the brain or the heart.
1. It seems, from the heart. For "flesh is the medium of touch," according to the Philosopher in the second book of On the Soul, and an organ is something internal, "near the heart," as the Philosopher says in On Sense and the Sensed. But a nerve is the organ of touch according to Avicenna. Therefore, the nerves take their origin from the heart.
2. Besides, the nerves are very sensitive; therefore, they take their origin from a very sensitive part, and the heart, but not the brain, is such a part. Therefore, etc.
To the contrary. The nerves arise from that part from which sense and motion flow, and they carry sense and motion as well. But this is immediately from the brain; therefore, etc.
Moreover, the nerves arise from that part at whose injury the operation of sense and motion cease. But this occurs following injury to the brain; therefore, etc.
One must reply, as was done earlier regarding the veins, that origin is spoken of in two ways: one is virtual and radical, and this is how the nerves and all the official parts of an animal arise from the heart; another is corporeal and immediate, and this is how the nerves arise from the web of the brain itself and from its nucha.
The first two arguments proceed according to the first way, and the arguments to the contrary proceed according to the second. And thus the solution is clear.
Whether the nerves are of a melancholic or phlegmatic complexion.
One asks next about the complexion of the nerveswhether they are of a melancholic or phlegmatic complexion.
1. It seems that they are of a phlegmatic complexion. For it is said in the text that a watery moisture surrounds the nerves. But phlegm is harmful owing to its viscosity, blood is harmful owing to its abundance, bile [choler] owing to its sharpness, and black bile [melancholia] owing to its harshness. Since viscosity and wateriness are attributed to phlegm, the nerves will have just this sort of complexion.
2. Moreover, a thing attests to its origin. But the nerves arise immediately from the brain, which is cold and moist. The nerves will therefore be of the same sort.
To the contrary. Whatever is made of a hard substance has a melancholy complexion. But the nerves are of this sort, and therefore, etc.
One must reply that the nerves can be considered in two ways: either in terms of the matter from which they arise or in terms of the type of complexion they have. If in terms of the matter, then the nerves are cold and moist, because if they were cold and dry they would be dark and hard in the manner of earth or rock. Nerve, however, is transparent and clear, which attests to the nature and matter of water and, as a consequence, to the nature and matter of phlegm. If, however, they are considered in terms of the type of their complexion, they are cold and dry, and this is why they suffer injury from excessive moisture, as is clear in the case of paralysis and spasm. According to the physicians, this is also very clear among animals, because goose flesh, in terms of the matter, is cold and dry, and it therefore generates a sickness on the basis of the cold and dry matterquartan fever, for example. But in terms of its composition it is moist and warm, and is the opposite of chicken flesh.
With this a response to the arguments is clear. The first ones prove that the nerves are cold and moist, and this is true in terms of the matter. But reason proves the opposite, that they are cold and dry, and this is true in terms of the manner of their composition or complexion.
-  Nucha: Either the marrowy matter of the brain itself or, more likely, the spinal chord. Cf. DA 22.214.171.1247 (SZ 1: 141).