General discussion about nerves

The usefulness of nerves is twofold: one is inherent in them essentially, the other is incidental. The usefulness essentially consists in the fact that the brain transmits sensations and movements to other organs through the nerves, and the secondary usefulness inherent in the nerve includes, for example, strengthening meat and giving strength to the body.

This also includes awareness of damage that occurs to organs that lack sensitivity, such as, for example, the liver, spleen, lungs; the fact is that these organs, although they lack sensitivity, are covered with a nervous membrane and are shrouded in a nervous film. When they swell or are stretched by the winds, the severity of the swelling or the stretching caused by the winds reaches the shell and its beginning, so that the severity pulls it back, the winds stretch it, and the person feels it.

The place where nerves begin, according to the generally known idea, is the brain, and the place where their branching ends is the outer side of the skin. The fact is that the skin is pierced by thin threads and nerves from nearby organs diverge in it.

The brain is the origin of the nerves in two ways. For some nerves it serves as the origin by itself, and for other nerves it originates through the spinal cord, which flows from it.

From the nerves emanating from the brain itself, only the organs of the head, face and internal parts of the body receive sensation and movement. As for other organs, they receive sensation and movement from the nerves of the spinal cord. Galen pointed out the great care shown to the nerves descending from the brain to the entrails. After all, the Creator took precautions to protect these nerves that do not take place in relation to other nerves. The fact is that since the nerves going down to the insides go far from their origin, it was necessary to give them greater strength.

Therefore, the creator covered them with a body, which in its substance is something between a nerve and cartilage and adapts to what happens to the body of the nerve during twisting. This is observed in three places, one of which is at the larynx, another where the nerve goes to the base of the ribs, and the third when the nerve passes past the chest area.

As for the other nerves emanating from the brain, those of them whose function is to transmit sensations pass from their origin directly to the desired organ, because movement in a straight line leads to the goal by the closest route. Here the action emanating from the initial source is stronger, since for the sensory nerves, unlike the motor nerves, compaction is undesirable, which would force them to move away from the brain substance in a tortuous way, so that they only gradually lose their resemblance to it in terms of softness. On the contrary, the softer the sensory nerves, the better they conduct the ability of sensation.

As for the motor nerves, they are directed towards the goal, passing through a tortuous path in order to move further away from their origin and gradually become denser. The necessary hardening and softening of both types of nerves is facilitated by the substance of the place from which they grow. The fact is that most of the nerves that transmit sensations come from the anterior lobe of the brain, and the anterior lobe is softer in composition. Most of the nerves that transmit movement come from the posterior lobe of the brain, and the posterior lobe of the brain is denser in composition.