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Why a human's chest is broader, with respect to its size, than that of other animals.
Further one asks why a human's chest is broader, with respect to its size, than that of other animals.
For it seems that the chest should come together more narrowly and subtly, since it is of a more subtle complexion.
One must respond that a broad human chest can occur for three reasons. One is the abundance of the heat in the heart, distributing and expanding the parts near it. Another reason is that the human walks upright, and other animals walk with their stomach facing the earth. And for this reason in other animals a wide chest might hinder movement, but in the human it contributes to movement because the chest is located more equidistantly above the feet. This is valid as long as the chest is wide and not stretched into an arc, because then it would be extended in front of or behind the feet. As it is, it is placed directly over them. The third reason is to guard the heart and the respiratory members. A broad chest performs this better than a narrow one.
Whether the motive power proceeds from the posterior or anterior part of the brain.
One inquires further about the motive powers. And it is asked whether the motive power proceeds from the posterior or anterior part of the brain.
1. It seems that it proceeds from the anterior part because motion follows sense, according to Avicenna. But sense proceeds from the anterior part, and therefore motion will as well.
2. Likewise, motion flows from the brain, and therefore from that part to which the brain inclines. But the brain is located on the anterior part, as was said above, and therefore, etc.
3. Likewise, powers are known through their objects. But the same thing is the object of sense and motion. Therefore, the sensitive and motive powers are in the same part.
To the contrary: one and the same thing is not the principle of contraries. But the anterior part is the principle of the sensitive power, which is passive, and therefore it is not the principle of the motive power, which is active.
One must say that the motive power proceeds immediately from the posterior part of the brain. For just as moisture is necessary for the reception of sensation, so too is dryness necessary for motion. But the first part of the brain is moister, and the posterior is drier. Therefore the motive nerves proceed more properly from the posterior part than from the anterior part, and the opposite is so for the sensitive nerves.
Likewise, sense operates through apprehending. But we apprehend things better and more easily which are in front of us. But movement is from behind, because motion is from a starting point to an end point. This is why a movable immediately leaves behind the starting point at which it formerly was. And for this reason, rationally, the motive power is in the posterior part, whereas the sensitive power is in the anterior part. An indication of this is that if there is some injury to the spinal chord [nucha] descending from the brain through the vertebrae, the animal immediately ceases to move.
1. To the first argument one must respond that although motion follows sense it is not necessarily the case that it proceeds from the same part. Now motion follows sense because what we sense first, later we acquire by motion.
2. To the second argument one should respond that although the motive power proceeds from that part to which the brain inclines, nevertheless the position of the brain is not indivisible. And for this reason it can proceed from one part or from the other, although it does so more suitably from the posterior, because this is drier.
3. To the third argument one should respond that although the object of the motive power and the sensitive power may be the same with respect to the thing, nevertheless it differs in ratio, just as the sensible and the appetible can be the same under a different ratio. And sense and appetite are therefore different powers.
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