Thus it is that stimulation which in the simpler organisms was diffused throughout the protoplasm, has in the complex organisms become the specialized property of a particular tissue. 51. Two general facts of supreme importance must now be stated: One is the law of stimulation--_every excitation pursues the path of least resistance_. The second is the condition of stimulation--_unlike mechanical impulsion, it acts only at insensible distances_. 52. This means that although a nerve may be excited by any stimulus external to it which changes its molecular condition, no propagation of that change (i. e. no stimulation through the nerve) is possible except through continuity of substance. Mere physical contact suffices to excite the nerve; but if there be an interruption of continuity in the nerve itself, no stimulus-wave passes across that line. Cut a nerve, and bring the divided surfaces once more into close contact, there will still be such a solution of continuity as to arrest the stimulus-wave, mere physical contact not sufficing for the propagation. Whereas across the cut ends of a divided nerve, even visibly separated, the electric current easily passes. This necessity for the vital continuity of tissue in the propagation of stimulation must always be borne in mind. The presence of a membrane, however delicate, or of any tissue having a different molecular constitution, suffices to arrest or divert the wave. I conceive, therefore, that it is absolutely indispensable that a nerve should terminate in and _blend_ with a muscle or a centre, otherwise no stimulation of muscle or centre will take place through the nerve. [Illustration: Fig. 13.] 53. The difference between excitation from contact and stimulation from continuity may be thus illustrated. In Fig. 13 we see the legs of a frog attached to the spine by the lumbar nerves (_l_), and lying on the muscles (_m_) of one leg is the nerve (_c_) of another frog’s leg. Applying the electrodes to (_l_), the muscles (_m_) are violently contracted; not only so, but their contraction excites the other nerve (_c_), and the leg attached to this nerve is thereby thrown into contraction. This “secondary contraction,” as Dubois Reymond calls it, might be supposed to be due to a diffusion of the electrical current; but that it is due to a change in the muscles (_m_) is proved by delicate experiments showing that the movements in the detached leg are of precisely the same kind as those in the legs directly stimulated. If there is only a muscular shock in the one case, there is only a muscular shock in the other; if there is tetanus in the one, there is tetanus in the other; if the muscles of the first leg are fatigued and respond slowly and feebly, the response of the second is slow and feeble. Moreover, the secondary contraction may be produced by chemical or mechanical stimulus, as well as by the electrical.