to an animal forces an immediate response so long as the muscular system is not exhausted; but where, as in series 1 and 2 of the electrical stimulus, the stimulus is not harmful, the reason for a sudden reaction is lacking unless fear enters as an additional cause.  Just as long as an animal is fresh and unfamiliar with the stimulus there is a quick reaction to any stimulus above the threshold, and as soon as a few experiences have destroyed this freshness and taught the subject that there is no immediate danger the response becomes deliberate.  In other words, there is a gradual transition from the flash-like instinctive reaction, which is of vast importance in the life of such an animal as the frog, to the volitional and summation responses.  The threshold electrical stimulus does not force reactions; it is a request for action rather than a demand, and the subject, although startled at first, soon becomes accustomed to the experience and responds, if at all, in a very leisurely fashion.  The reaction time to tactual stimuli, soon to be considered, was determined by giving a subject only three or four stimulations a day; if more were given the responses failed except on repetition or pressure; for this reason the data on fatigue, or lengthening of reaction time toward the end of a series, are wanting in touch.  A few tests for the purpose of discovering whether the time would lengthen in a series were made with results very similar to those of the threshold electrical stimulus; the chief difference lies in the fact that the responses to touch fail altogether much sooner than do those to the electrical stimulus.  This, however, is explicable on the ground that the latter is a stimulus to which the animal would not be likely to become accustomed so soon as to the tactual.

First Half.          Second Half.       Second % Greater. 
Series 1  244.8[sigma]        356.8[sigma]         46 per cent
Series 2  216.6                246.6               14       "
Series 3  101.0                105.9                5       "

If pure fatigue, that is, the exhaustion of the nervous or muscular system, appears anywhere in this work, it is doubtless in series 3, for there we have a stimulus which is so strong as to force response on penalty of death; the reaction is necessarily the shortest possible, and, as a matter of fact, the motor reaction (jump forward) here occupies little more time than the leg-jerk of a decapitated frog.  This probably indicates that the reaction is a reflex, and that the slight increase in its length over that of the spinal reflex is due to occasional cerebellar origin; but of this there can be no certainly from the evidence herewith presented.  At any rate, there is no possibility of a voluntary reaction to the strong current, and any changes in the general character of the reaction time in a series will have to be attributed to fatigue of the nervous or muscular systems.  The second halves of the sets of series 3 are 5 per cent. longer