This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
If one's finger unexpectedly touches a very hot object, pain is felt and the hand is suddenly snatched away; that is to say, sensation is aroused and certain muscles are caused to contract. If, however, the nerves passing from the arm to the spinal cord have been divided, or if they have been rendered incapable of activity by disease, no such results follow. Pain is not then felt on touching the hot body nor does any movement of the limb occur; even more, under such circumstances the strongest effort of the Will of the individual is unable to cause any movement of his band. If, again, the nerves of the limb have connection with the spinal cord, but parts of the cord are injured higher up, between the brain and the point of junction of the nerves of the arm with the cord, then contact with the hot object may cause the hand to be snatched away, but no pain or other sensation due to the contact will be felt, nor can the will act upon the muscles of the arm, either to make them contract or to prevent their contraction. From the comparison of what happens in such cases (which have been observed again and again upon wounded or diseased persons), with what occurs in the natural condition of things, several important conclusions may be reached:
What usually results when a hot object is unexpectedly touched? Under what circumstances do these results not occur? Can the Will cause movement of the muscles of an arm whose nerves have been cut? When the arm-nerves are intact but the spinal cord is injured near the brain, what happens on touching a hot body?
1. The feeling of pain does not reside in the burned part itself; for it is found that applying a hot object to the skin or pinching it arouses no sensation if the nerves between the skin and the nerve-centres be diseased or divided.
2. The hot object when the nerves are intact originates some change which, propagated along the nerves, excites a condition of the nerve-centres accompanied by a feeling, in this particular case a painful one. This is clear from the fact that loss of sensation immediately follows division of the nerves of the limb, but does not immediately follow the injury of any of its other parts. The change propagated along the nerve-trunks and causing them to excite the nerve-centres is called a nervous impulse.
3. When a nerve in the skin is excited it does not directly call forth muscular contractions; for if so, touching the hot object would cause the limb to be moved even when the nerve had been divided high up in the arm, while, as a matter of observation and experiment, we find that no such result follows if the nerve-fibres have been cut in any part of their course from the excited, or, in physiological phrase, the stimulated, part to the spinal marrow. It is therefore through the nerve-centres that the nervous impulse trans-mitted from the excited part of the skin is " reflected" or sent back to act upon the muscles.
4. The preceding fact makes it probable that nerve-fibres pass from the centre to muscles as well as from the skin to the centre. This is confirmed when we find that if the nerves of the limb be divided the Will is unable to act upon its muscles, showing that these are excited to contract through their nerves. That the nerve-fibres concerned in arousing sensation and muscular contractions are distinct, is shown also by cases of disease in which the sensibility of the limb is lost while the power of voluntarily moving it remains; and by other cases in which the opposite is seen, objects touching the hand being felt, while it cannot be moved by the Will. We conclude therefore that certain nerve-fibres when stimulated transmit something (a nervous impulse) to the centres, and that these, when excited by the nervous impulse conveyed to them, may radiate impulses through other nerve-fibres to distant parts, the centre serving as a connecting link between the fibres which carry impulses from without in, and those which convey them from within out.
How do we know that our feeling of pain does not reside in a burned or pinched part of the skin?
What does a touched hot object originate when the nerves are healthy? What is a " nervous impulse"?
Does a skin-nerve when excited produce directly a muscular movement? Give reason for your answer. What happens to the nervous impulse transmitted from the excited part of the skin?
Is it probable that other nerve-fibres than those arising from the skin are connected with the nerve-centres ?
5. Further we conclude that the spinal cord can act as an intermediary between the fibres carrying in nervous impulses and those carrying them out, but that sensations cannot be aroused by impulses reaching the spinal cord only, nor has the Will its seat there ; volition and consciousness are dependent upon states of the brain. This follows from the unconscious movements of the limb which follow stimulation of its skin after such injury to the spinal cord as prevents the transmission of nervous impulses farther on; from the absence, in such cases, of sensation in the part whose nerves have been injured; and from the loss of the power of voluntarily causing its muscles to contract.
Point out a fact tending to prove that the muscles are normally excited to contraction through their nerves. State facts showing that the nerves of sensation and those governing the muscles are distinct. What purpose does the nerve-centre serve?
What further conclusions may we draw from the facts already considered in this chapter? Give reasons for your answer.
6. Finally, we conclude that the spinal cord in addition to being a centre for unconscious movements serves also to transmit nervous impulses to and from the brain; this is confirmed by the histological observation that in addition to the nerve-cells, which are the characteristic constituents of nerve-centres, it contains the simply conductive nerve-fibres, many of which pass on to the brain. In other words the spinal cord, besides containing fibres which enter it from, and pass from it to, the skin and muscles, contains many fibres which unite it to other centres.