4.  Assistant stops chronoscope and takes reading.

[Illustration:  FIG. 7.  Reaction Key. l, lever swung on pivot; p, p, posts for contacts with platinum plates on base; b, upright bar for string; s, spring for clamping string; w, wheel to carry string; c, c, chronoscope circuit; 1 and 2, points which are brought into contact by animal’s reaction.]

The steps of this process and the parts of the apparatus concerned in each may be clearly conceived by reference to the diagram given in Fig. 6.  The various forms of stimulating apparatus used and the modification of the method will be described in the sections dealing with results.  The same reaction key was used throughout (see Fig. 7).  Its essential features are a lever l, pivoted in the middle and bearing a post at either end, p, p.  From the middle of this lever there projected upward a small metal bar, b, through the upper part of which a string to the animal ran freely except when it was clamped by the spring, s.  This string, which was attached to the subject’s leg by means of a light elastic band, after passing through the bar ran over a wheel, w, and hung tense by reason of a five-gram weight attached to the end.  Until everything was in readiness for an experiment the string was left free to move through the bar so that movement of the animal was not hindered, but the instant before the ready-signal was given it was clamped by pressure on s.  The diagram shows the apparatus arranged for a reaction.  The current is broken, since 1 and 2 are not in contact, but a slight movement of the animal turns the lever enough to bring 1 against 2, thus making the circuit and stopping the chronoscope.  When the motor reaction of the subject was violent the string pulled out of the clamp so that the animal was free from resistance, except such as the string and weight offered.  The five-gram weight served to give a constant tension and thus avoided the danger of error from this source.  Between experiments the weight was placed on the table in order that there might be no strain upon the subject.

That the subject might be brought into a favorable position for an experiment without being touched by the operator a special reaction box was devised.

The animals used in these studies were specimens of Rana clamitans which were kept in a tank in the laboratory throughout the year.

VI.  ELECTRIC REACTION TIME.

The reaction time to electrical stimuli was determined first because it seemed probable that this form of the pain reaction would be most useful for comparison with the auditory, visual, olfactory and tactual reactions.  In this paper only the electrical and the tactual reaction times will be considered.  The former will be divided into two groups:  (1) Those resulting from a stimulus given by touching electrodes to the leg of the frog, and (2) those gotten by having the frog resting upon wires through which a current could be passed at any time.