MANY experiments have been undertaken in the effort to measure the inertia or sluggishness of the various senses. Inasmuch as the nervous structure is a physical system set in operation by the incidence upon it of external agencies, in each of its parts it requires a certain time in order to be set going; and, once set in operation, acquires a certain momentum which necessitates that a certain time elapse before it is again in a state of equilibrium. At least the sense organs all show such inertia, so that, in a given case, only a limited number of distinct sensations can be produced in a given time by successive stimulation. A measure that has often been used for expressing such facts is the maximum number of separate excitations to which the sense organ responds in a unit of time, as one second. This measure, to be sure, varies considerably with numerous conditions and circumstances, such as the nature of the stimulus, the part of the sense organ affected, its previous condition, the intensity of the stimulation, etc. This measure, which may be said roughly to indicate the duration of a sensation (including its positive after image), is very short for touch, somewhat longer for sound, and still longer for vision. Because of the nature of the stimuli in taste and smell and the difficulty of accurately controlling their application and removal, satisfactory measures of the inertia of these sense organs have never been secured. _Reaction Time to Taste Stimuli_ One fact, however, seems to be fairly clear from experiment, although it would by no means be suspected from casual observation, namely, that the various taste qualities are not equally prompt in the time required for them to appear after the application of the stimulus. Salt and sweet come rather quickly as compared with sour and bitter, the order of speed being salt, sweet, sour, bitter. How much this may depend merely on such differences as may exist in the structure and location of the various taste buds it is impossible to say. Kiesow points out that taste sensations are tardy and gradual in their appearance. If the person being stimulated be required to indicate by a signal the instant at which the taste quality appears, it is possible to measure, in very small units of time, the interval between the superficial application of the solution and the appearance of the sensation. This is called the “reaction time” to the taste stimulus. When the stimuli were applied to the tip of the tongue Kiesow found the following figures to represent average reaction times to his different solutions: Seconds ─────────────────── Salt .307 Sweet .446 Sour .536 Bitter 1.082