Where the signal is of touch, the place to which it is applied makes a difference in the resultant reaction-time. G. S. Hall and V. Kries found (Archiv f. Anat. u. Physiol., 1879) that when the finger-tip was the place the reaction was shorter than when the middle of the upper arm was used, in spite of the greater length of nerve-trunk to be traversed in the latter case. This discovery invalidates the measurements of the rapidity of transmission of the current in human nerves, for they are all based on the method of comparing reaction-times from places near the root and near the extremity of a limb. The same observers found that signals seen by the periphery of the retina gave longer times than the same signals seen by direct vision. The _season_ makes a difference, the time being some hundredths of a second shorter on cold winter days (Vintschgau _apud_ Exner, Hermann's Hdbh., p. 270). _Intoxicants_ alter the time. _Coffee_ and _tea_ appear to shorten it. Small doses of _wine_ and _alcohol_ first shorten and then lengthen it; but the shortening stage tends to disappear if a large dose be given immediately. This, at least, is the report of two German observers. Dr. J. W. Warren, whose observations are more thorough than any previous ones, could find no very decided effects from ordinary doses (Journal of Physiology, viii, 311). _Morphia_ lengthens the time. _Amyl-nitrite_ lengthens it, but after the inhalation it may fall to less than the normal. Ether and chloroform lengthen it (for authorities, etc., see Buccola, p. 189). Certain _diseased states_ naturally lengthen the time. The _hypnotic trance_ has no constant effect, sometimes shortening and sometimes lengthening it (Hall, Mind, viii, 170; James, Proc. Am. Soc. for Psych. Research, 246). The time taken to _inhibit_ a movement (e.g. to cease contraction of jaw-muscles) seems to be about the same as to produce one (Gad, Archiv f. (Anat. u.) Physiol., 1887, 468; Orchansky, _ibid._1889, 1885). An immense amount of work has been done on reaction-time, of which I have cited but a small part. It is a sort of work which appeals particularly to patient and exact minds, and they have not failed to profit by the opportunity. CEREBRAL BLOOD-SUPPLY. The next point to occupy our attention is the _changes of circulation which accompany cerebral activity_. [Illustration: FIG. 23.--Sphymographic pulse-tracing. _A_, during intellectual repose; _B_, during intellectual activity. (Mosso.)]