Page:Mind (Old Series) Volume 11.djvu/232

 THE TIME TAKEN UP BY CEREBRAL OPERATIONS. 231 been a favourite subject for physiological research, 1 but the re- sults as yet reached are unsatisfactory. Exner, in Vol. ii. of Hermann's Handbuch der Physiologie 2 gives, as result of the "perfectly irreproachable measurements" of Helmholtz and Baxt, the rate of transmission in a motor nerve as 62m. the second ; whereas, in the same volume 3 and likewise as the re- sult of experiments by Helmholtz and Baxt, Hermann gives the rate as 33 -9005m. the second. The fact seems to be that the rate depends on the temperature and other conditions, chiefly brought about by the method of experiment. Determinations made on the sensory nerve give results still more discordant and unsatis- factory. We can for the present do nothing better than assume the average rate of transmission in both motor and sensory nerve to be 33m. the second. It is probable that the rate is slower in the spinal cord, and that the nervous impulse is delayed in enter- ing and leaving the cord, as also in passing through a ganglion. 4 As a temporary hypothesis we can suppose that when the re- action, lasting 150<r, is made, 50<r is used in transmitting the ner- vous impulse from the retina to the brain, and from the brain through the spinal cord to the muscle of the hand. The latent period when the muscle of the frog is stimulated by means of an induction-shock, is between 5 and 10<r ; 5 and is perhaps the same when the muscle of the hand is innervated by means of a will- impulse. There is also undoubtedly a latent period in the sense- organ while the stimulus is being converted into a nervous impulse. In the so-called mechanical senses this period is very short, but when the retina is stimulated by light a chemical pro- cess (as we suppose) takes place, and the time may be quite long. 6 We know that a light must work on the retina for a considerable time in order that the maximum intensity of the sensation may be called forth; from this time, however, we can draw no exact inferences as to the length of the process here under considera- tion. I have shown 7 that a coloured light of medium intensity must work on the retina '6 to 2'75<r (varying with the observer and colour) in order that a sensation may be excited ; the time be- comes however much longer when a white light follows the 1 See for references Hermann, Handb. d. PhysioL II., ii., 14 ff. 2 ii., 272. 3 i., 22. 4 Exner, Pfluger's Archiv, viii., Archiv. f. Anat. u. Phys., 1877 ; Francois- Franck et Pitres, Gazette Hebd., 1878; Wundt, Mechanik der Nerven, ii., 45. 8 Tigerstedt, Archiv f. Anat. u. Physiol., 1885, and references there given. 6 V. Wittich (Zeitschr. f. Rat. Med., xxxi.) and Exner (Pflilger's Archiv, vii.) found the reaction-time to be shorter when the optic nerve was stimulated by an electric current than when the retina was stimulated by light. This difference may, however, be due to other factors of the reaction- time as well as to the latent period in the sense-organ. 7 Philosophische Studien, iii., 1 ; Brain, Pt. 31.