364 THE POPULAR SCIENCE MONTHLY. 



tubes undergo great changes at their entrance into the spinal mar- 

 row, where, according to Van Deen, they cease to be sensitive to 

 the action of electricity, of chemical substances, mechanical inju- 

 ries, etc. 



It follows, from all these experiments, that the nerve-current makes 

 its way with a speed that is relatively inconsiderable. The hand in 

 thowing a stone parts the air with the quickness of nearly 68 feet a 

 second, which is qu'te 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 27f 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 inter- 

 vention 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 sub- 

 ject 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 risrht hand when the shock came from the left side. 

 The interval between the shock and the signal was found to be T 2 ^- 

 of a second when the person knew beforehand, which side the shock 

 would come from, and T s ^g- when he was not forewarned ; thus T fo 

 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 experi- 

 ments 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 Jae- 

 ger made the experiment a little differently. One pronounced some 

 syllable, which the other repeated as soon as he heard it, while a pho- 

 nontograph 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 acci- 

 dental 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 



