In animal physiology again, there is no essential difference between the effect of the direct and indirect stimulation. In a nerve-and-muscle preparation, for example, indirect stimulation at the nerve induces the same contraction as the direct stimulation of the muscle. The only difference lies in the latent period, which is found to be longer under indirect stimulation by the time interval necessary for the excitation to travel along the conducting nerve. It is probable that stimulus gives rise to dual impulses in the animal tissue as in the plant. But the detection of the positive impulse in the animal nerve is rendered exceedingly difficult on account of the high velocity of conduction of excitation. I have explained that the separate effects of the two impulses can only be detected if there is a sufficient lag of the excitatory negative behind the positive, so that the relatively sluggish responding organ may exhibit the two impulses one after the other. In a highly conducting tissue the lag is very slight, and the negative will therefore mask the positive by its predominant effect. In spite of the difficulty involved in the problem, I have recently been successful in demonstrating the dual impulses in the animal nerve. In any case it is important to remember the following characteristic effects of indirect stimulation. TABLE XXII.--SHOWING THE EFFECT OF INDIRECT STIMULATION. +-----------------------------------------------------------------+ | Intensity of | Character of intervening | Responsive effect. | | Stimulus. | tissue. | | +-----------------+--------------------------+--------------------+ |Moderate |Highly Conducting | Contraction. | | " |Non-conducting | Expansion. | | " |Semi-conducting | Expansion followed | | | | by contraction. | |Feeble | " " | Expansion. | +-----------------------------------------------------------------+ These effects of indirect stimulation have been fully demonstrated in the case of pulvinated organs (p. 136) and growing organs (p. 215). Having demonstrated the fundamental reactions of direct and indirect stimulation, we shall next study the tropic effects induced in growing organs by the effect of unilateral application of indirect stimulus.