SCIENTIFIC SUMMARY. 
287 
exhibition of some indication of its reception, we find that the lime 
occupied can be measured. Thus Hirsch, by means of a suitable apparatus, 
found that a touch upon the face was recognized and responded to by a 
predetermined signal operated by hand in one-seventh of a second. There 
is no doubt some loss in the purely mechanical operation of making the 
signal ; but when the different senses are tested in this manner, and a mean 
taken of all the experiments, we find not only that the act of thinking is not 
so rapid as was imagined, but that the speed varies with different senses. 
Thus the sense of touch was found to respond in one-seventh of a second, 
that of hearing required one-sixth of a second to respond, and when the eye 
was tested, one-fifth of a second was occupied in recognizing the signal. 
The distances travelled by the nervous impulses in each of these cases, was 
as nearly as possible the same, and it follows therefore that the recognition 
of them required more time in some cases than in others. Simple as it may 
seem, a number of operations must be performed by the brain in receiving 
and recording the reception of the impression. There is the transmission of 
the sensation to the brain, its recognition, and then the determining to 
make the signal, the transmission of the determination to the muscles, and 
the movement of those muscles. Hirsch showed, as explained above, that 
less time was required to recognize a touch than a sound, and that it took 
more time to see than to hear, but the question still remained as to what 
part of the time occupied was consumed in the act of recognition. Donders, 
by means of some very ingeniously constructed apparatus, solved the ques- 
tion. He found that the double act of recognizing a sound and giving the 
response, occupied seventy-five thousandths of a second, of which forty 
thousandths were occupied in simple recognition, leaving thirty-five 
thousandths for the act of volition. One twenty-fifth of a second was 
occupied in judging which was first of two irritants acting upon the same 
sense ; but a slightly longer time was necessary to determine the priority of 
signals sent by different senses, as those of hearing and seeing. These 
results were obtained from a man of middle age, the young were slightly 
quicker; but the average of many experiments showed that the time 
required for a simple thought was never less than the fortieth of a second. 
From these experiments we learn that the mind cannot perform more than 
twenty-four hundred simple acts in a minute, and that the stories we have 
heard from persons rescued from drowning are simply exaggerations. 
— 
ZOOLOGY. 
Paired Fins of Sharks and Rays. — Mr. F. M. Balfour lately read before 
the Zoological Society a paper on the development of the skeleton of the 
paired fins of Elasmobranchs, considered in relation to its bearings on the 
nature of the limbs of the Vertebrata. The object of the investigations re- 
corded in this paper was explained by the author to be twofold, — viz., on the 
one hand to test how far the study of the development of the skeleton of the 
