Page:Popular Science Monthly Volume 2.djvu/377

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headquarters of the will. Here is one second lost. What takes place then? How much time does reflection need? That depends on circumstances; but it is certain that the will does require a measurable time to make a decision. Then it acts; the order is sent out to the tail to thrash the boat to bits. Another second elapses before the message reaches its destination: total, two seconds gone, during which the boat and sailors get off clear by vigorous rowing.

It will be asked, How have philosophers succeeded in measuring the rapidity of the onward movement of nervous stimulus? Several methods of calculating it have been devised. A doctor of the middle ages, cited by Haller, gave some thought to it long ago. He conceived—a singular notion—that the speed of the nervous fluid might be deduced from that of the blood in the aorta; these two rates, he fancied, must be in the inverse ratio of the sizes of the aorta and the nerve-tubes. That calculation assigned, as the speed of the nervous fluid, 600,000,000,000 of yards a minute—600 times the rapidity of the motion of light.

Haller himself undertook the task in a different way. Reading the "Æneid" aloud, he counted the number of letters he could pronounce in a minute with a very rapid utterance. He found 1,500 the extreme limit, or one fifteen-hundredth part of a minute for each letter. Now the letter r requires, Haller says, ten successive contractions of the muscle that gives the tongue vibration, and from that, he adds, we may conclude that in one minute this muscle can contract and relax 15,000 times, which represents 30,000 simple motions. The distance from the brain to the muscle in question is a little over three inches. If the nervous fluid travels it 30,000 times, that makes more than 9,000 feet, and 9,000 feet a minute represents a speed of 154 feet in a second. This reasoning is a mere sequence of mistakes, and the approximation to the right view that Haller gained is the more astonishing because his method was not in the least likely to ascertain it. The "Æneid" justifies, in this instance, its ancient pretensions as a book of oracles.

Not until 1850 were these researches resumed by a new method that led to the solution of the problem. It is due to Helmholtz, the most famous of the German physiologists, who unites, to rare talent as an observer, the profound learning of a consummate mathematician. His first method is founded on the use of the chronoscope of Pouillet. A galvanic current of very brief duration acts at a distance on a magnetized needle, and swings it away from the normal position; the range of the deviation is measured, and the length of the current deduced thence by calculation. A means is thus gained for measuring intervals of time not exceeding a few thousandths of a second. Helmholtz applied this method in the following way: One of the muscles of a frog's leg is fixed at one extremity in a nip, and attached at the other extremity to a little lever forming part of a galvanic circuit. A