This section is from the book "Health", by W. H. Coefield.
These spinal nerves, then, are distributed to different parts of the body. Those from the upper part of the spinal cord are distributed chiefly to the muscles and skin of the neck, and the parts near to the neck. Those a little farther down are distributed to the arms-to the skin, and to the muscles of the arms. Those in the dorsal and lumbar regions are distributed to the walls of the chest and abdomen, both to the muscles and the skin; while the lowest of them form the largest nerves in the body, called the sciatic nerves (from which the disease sciatica gets its name), which go to the lower limbs.
I have told you that these spinal nerves are distributed to the muscles of the parts, and to the skin of the parts. Now, of what use are they ? What is done by means of them ? If you take a nerve belonging to this system, a branch of one of the spinal nerves, in any part of the body of an animal, and irritate it by means of a needle, or in any other way, two things happen. In the first place, the animal feels pain and shows signs of it; and, in the second place, the muscles to which that -nerve goes contract. And where is the pain felt ? The pain is not felt where the nerve is irritated at all; the pain is felt in that part of the skin to which the nerve goes, so that if we have irritated the trunk of the nerve-say in the fore-leg of the animal-the pain is felt just as if you irritated the extremities of the nerve in the skin. Now suppose you take the trunk of a spinal nerve and cut it in two, and then irritate the end of it which is still in communication with the part to which that nerve goes, but which is no longer in communication with the spinal cord, then only one thing happens: the muscles to which that nerve goes contract, but the animal feels no pain whatever, and shows no sign of it; suppose, on the other hand, you irritate the end which is no longer in communication with the part to which the nerve goes, but which is still in connection with the spinal cord, then the animal feels acute pain and shows signs of it, but no muscles contract; that shows you that the stimulus which causes the muscles to contract travels along these nerves from the spinal cord towards the muscles that are contracting, and by contraction moving the different parts of the body, and that the stimulus which causes pain travels, on the other hand, along these nerves towards the spinal cord; and, more than that, if you cut through a piece of nerve and irritate the end of it which is still in connection with the spinal cord, the animal refers the pain to the extremities of the nerve, just as if they were being irritated.
That shows us that these nerves have two properties, -that they are capable of conveying stimuli from the spinal cord where they rise to the muscles to which they go to cause them to contract, and they are capable of conveying stimuli from the skin, to which some of the branches go, to the spinal cord; they, therefore, convey stimuli both ways, and for that reason are called mixed nerves.
Now let us go on to consider the two roots. Suppose that in an animal you cut through the front or anterior roots of some of the spinal nerves and leave the others alone, what happens ? It is found that the animal is no longer capable of contracting the muscles to which these nerves go by means of the will, but that he feels pain in the parts to which these nerves go, just as he did before; that shows at once that the stimulus which is started by means of the animal's will travels through these anterior or front roots of the nerves; it travels through them and along the nerves to the muscles to which the nerves go; these anterior roots are for that reason called the motor roots of the nerves; it is through these anterior or front roots that the stimuli travel which produce contraction of the muscles and so movement, and that can be proved to still greater demonstration by taking these cut pieces and irritating with a needle that part of the root which is still in connection with the nerve; that irritation produces no pain whatever, but the muscles contract.
Suppose, instead of cutting the anterior roots of the nerves, you cut the posterior roots of a few of these nerves, then it is found that the animal no longer feels at all in those parts of the body to which the nerves go, the posterior roots of which have been cut through, but he is perfectly capable of moving the muscles of those parts. Suppose, for instance, you cut through two or three of the posterior roots of the nerves that go to the fore-leg of the animal or to the arm of a man, then the animal is perfectly capable of moving the muscles of that limb, but he is no longer capable of feeling in that limb, or in the parts of the limb to which they go. It may be put into the fire, into strong acid, anything may be done with it, and the animal knows nothing whatever about it, so that the stimuli of sensation travel along the mixed nerves and to the spinal cord by means of the posterior roots of those nerves, and for that reason the posterior roots of the spinal nerves are called sensory roots; and you see now still better why these spinal nerves go by the name of the mixed nerves-they are made up of these two roots, and these two roots have, distinct properties,-one of them conveying stimuli from the spinal cord towards the parts of the body to which the nerves go, and the other from those parts of the body towards the spinal cord,-the one the anterior or motor root, the other the posterior or sensory root.
Let us speak a little about the properties of the spinal cord itself.
If you cut through the spinal cord of an animal somewhere in the back, right across, then the animal has no longer any control whatever over the muscles of the parts of the body which are supplied by nerves coming from the part of the cord below the cut, and he is also deprived of sensation in those parts of the skin supplied by the same nerves.