Page:Popular Science Monthly Volume 2.djvu/380

From Wikisource
Jump to navigation Jump to search
This page has been validated.



tubes undergo great changes at their entrance into the spinal marrow, where, according to Van Deen, they cease to be sensitive to the action of electricity, of chemical substances, mechanical injuries, etc.

It follows, from all these experiments, that the nerve-current makes its way with a speed that is relatively inconsiderable. The hand in throwing a stone parts the air with the quickness of nearly 68 feet a second, which is quite comparable with that of the nervous fluid; and the race-horse, the hare, and the leveret, move quite as rapidly. The arterial wave, which passes through 27¾ feet in a second, moves only three times more slowly.

When the sensation transmitted to the spinal marrow occasions a reflex action, that is, an involuntary movement determined by the intervention of the ganglionic cells, the reflex motion always proceeds more slowly than that produced by the direct action of the exciting current on the muscles; the retarding varies from a thirtieth to a tenth of a second. It may be inferred from this that reflex action in the spinal marrow takes twelve times longer than the transmission of a stimulus through nerves of sensation or motion.

The time employed in the brain's operations is also some tenths of a second. Dr. Jaeger measured it in the following way: The subject of the experiment was made to touch the electric key with the left hand as soon as he received an electric shock on the right side, and with the right hand when the shock came from the left side. The interval between the shock and the signal was found to be 20100 of a second when the person knew beforehand, which side the shock would come from, and 27100 when he was not forewarned; thus 7100 of a second were used in reflection. Hirsch found that at least two-tenths of a second elapse before an observer marks by signal the preception of a luminous spark or a sudden sound. In other experiments it was arranged that the observer should touch the key with the left hand for a white spark, and with the right for a red one, and he lost, in that case, from three to four-tenths of a second. Therefore reflection took from one to two-tenths of a second. Donders and Jaeger made the experiment a little differently. One pronounced some syllable, which the other repeated as soon as he heard it, while a phonontograph registered the vibrations of the word. When the syllable to be repeated was agreed on beforehand, the delay observed was two-tenths of a second; when not, it was three-tenths.

As we see, then, thought does not spring instantaneously; it is a phenomenon subject to natural laws of time and space. In different observers the time lost is not alike; one perceives, reflects, and acts, more briskly than another; it is a matter of temperament and of accidental circumstance. This explains the differences always remarked between astronomers busied in observing the same phenomenon. Two persons never saw at the same instant the passage of a star across a