Page:Popular Science Monthly Volume 76.djvu/435

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tems of nerves which originate in organs in which their ends are so modified and so enveloped as to render them sensitive in the highest degree to one particular order of stimulus, whether of smell, sight, taste, hearing, touch, heat, cold, pressure or traction, and inaccessible to stimuli of every other class. These nerves, with the chains of neurones which link them to the muscles, via the spinal cord and brain, stand out as a pattern on the basal system, like the pattern formed of thicker fibers and coarser knots on a sheet of lace.

In a book recently published, "The Body at Work," I have endeavored to present a picture of the nervous system and its activities, which, although not original in any of its details, is new in their grouping and in its comprehensiveness. It is based upon the teaching that the two great functions of the nervous system, notwithstanding that they grade one into the other, must, for purposes of analysis and description, be considered apart. By the basal system of protopathic nerves all the cells of the body, with the exception of those of the connective tissues, bones, tendons and so forth, are bound together into a continuous inseparable whole. No change can occur in the nutritive condition of any part of the skin or of an internal epithelium without the induction of a nutritive change in the central nervous system and thence, onward, in the plain muscle-fibers of arteries and other structures of the segment of the body in which the inducing change occurs. As contrasted with the influence which spreads through this basal system the "impulses" which travel up the nerves of special sense are peculiar in kind or, at any rate, in intensity. In order that they may overcome the resistance of a chain of neurones they have a certain potential, and progress in pulsations or waves.

Pain is explained as due to the setting up in a particular segment of the axial nervous system of a focus "pain-conditioned" sympathetically with the injured tissues. Consciousness of pain depends upon the direction of attention to impulses which ascend through the pain-conditioned segment from end-organs of nerves of special sense. If the seat of injury be the skin it is through the specialized nerves of the injured spot that modified impulses reach the cortex of the brain. If the seat of injury be an internal organ no effect is produced in consciousness until the pain agitation of the spinal cord has become sufficiently intense, and sufficiently wide-spread, to modify impulses which ascend to the cortex from skin areas of the segment in which the viscus is situate. The pain in angina pectoris is felt on the left side of the breast bone at its lower end. This shows that the nerves of the aorta have their centers in the same region of the spinal cord as the cutaneous nerves of this area on the surface of the chest.

To give an illustration of the difference of mechanism of pain and of sensation. In a railway station lavatory I recently observed a man who absent-mindedly placed his fingers on a free-standing iron stove