Page:Popular Science Monthly Volume 76.djvu/436

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436
THE POPULAR SCIENCE MONTHLY

to ascertain whether it was hot. It was a frosty morning in November. Obviously, the stranger had a strong prejudice that the station-master would not have thought it necessary to order a fire to be lighted so early in the winter. As nearly as I could estimate, three seconds elapsed between the touching of the stove and the ejaculation which announced with unnecessary emphasis that the man had obtained the information he desired. Had he, expecting to find the iron hot, directed his attention to the modification of his skin sensation he would have withdrawn his fingers in one seventh of a second.

One of the characteristics of incipient pain is exaltation of reflex actions. I can not by any effort of will prevent my muscles from withdrawing my hand from hot iron (although the resolute withdrawal of attention from pain-modified sensations and the forcing of a conviction that it does not exist, has in certain cases a remarkable effect in suppressing pain). Equally characteristic of established pain is the inhibition of action. A whitlow abolishes all temptation to shake the finger.

Physiologists can not investigate the phenomena of pain, although they make elaborate studies of its threshold value and of the distribution of "pain spots." It would take us too far were we to consider the evidence of a degree of specialization in the protopathic nerves of the skin which is held by some to justify the use of the expression "pain-nerves," and of the allied question of neuronic conduction of incipient or threshold pain along pain-tracts in the spinal cord.

The chief interest of the hypothesis of structural continuity through the protopathic nervous system, with its corollary of sympathetic nutritional change, lies in the explanation which it affords of the influence upon reflex action of the establishment of a pain-condition in the axial nervous system in circumstances in which, consciousness not being affected, there is no "pain."

Pain-condition which inhibits reflexes due to impulses which start in the damaged organ or skin area, greatly increases in many instances the conductivity of the portion of the nervous system which it affects for impulses which do not come from the damaged part. Such a reinforced reflex is the attack of sneezing to which many persons, most monkeys, and some breeds of dogs are subjected when the eye is stimulated by a bright light. When the gaze is directed towards a bright cloud, excessive stimulation of the retina sets up a pain-condition in the mid-brain. In the progress of evolution this portion of the cerebrospinal axis has undergone great changes. Its sensory nerves with their protopathic constituents have been drawn backwards into the great bundle of the fifth nerve, which joins the hind-brain, whilst the nerve from the retina has established a secondary connection with the mid-brain. The mid-brain receives in consequence the protopathic nerves of the eye. But the nose being the real tip of the body and anterior to the eye the sensory fibers of the skin which lines it al-