Page:Popular Science Monthly Volume 76.djvu/434

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430
THE POPULAR SCIENCE MONTHLY

justly described as stabbing, aching, burning, it owes its individual character to the form of its onset and to its duration, and these in turn depend upon either the vascular condition of the part affected—the pumping of blood through the vessels of a tissue free to expand, or packed in a bony case—or they are due to the effect upon the inflamed or injured part of muscular contractions. If the injured part be inaccessible, pain has no "local sign." If it be on or near the surface of the body the pain felt in it has or seems to have a topographical meaning; but it is very doubtful whether the mind can localize the source of pain in the absence of evidence simultaneously afforded by the nervous apparatus of the sense of touch. Many instances are on record of disease or injury to the central nervous system resulting in complete loss of sensitiveness to pain, whilst sensitiveness to touch and pressure remained undiminished. But there are no recorded cases, so far as we are aware, of complete paralysis of the mechanisms of touch and of the recognition of heat, of cold and of pressure, with the retention of normal sensitiveness to pain. Such a condition, if it were established, would make it possible for an investigator to ascertain whether skin-pain, by itself and unsupported by collateral evidence, has a topographical meaning, or "local sign"; and whether the expression pain-spot may be legitimately used, as meaning a sounding spot in the midst of a dumb area, and not merely a focus of sensitiveness at which the weakest stimulus which can evoke pain is effective.

Dr. Henry Head caused the large cutaneous nerve of the thumb-side of the forearm and hand to be cut in his own arm, in order that he might study carefully the revival of sensations which follows on nerve repair. He found that, long before he regained the ability to distinguish degrees of warmth, to feel as separate the two points of a pair of compasses, or to recognize a touch with cotton-wool, he regained his power of recognizing stimulation by agents that do harm—hot things, cold things, pricking with a pin—but his power of localizing the spot injured was extremely vague. Trotter and Davies have made similar experiments in their own persons on a still more extensive scale and have confirmed and amplified Head's results.

Investigations with the aid of new histological methods has shown that the epithelial tissues are supplied with nerve-filaments in inconceivable abundance. It is probable that the conclusion is justified that every cell of the skin, of the mucous membranes, of the lining epithelium of the air-chambers in the lungs, of the pleural and peritoneal cavities, of the various glands, is connected with a nervous thread. It is certainly true also of every muscle-fiber in the walls of the alimentary tract, of ducts and of blood-vessels. By these filaments the cells of the body-surfaces both external and internal, the central nervous system and all motile organs, are bound together.

Superimposed on this basal system are the various specialized sys-