The spirit is affected through one of the body parts according to the distinctive nature of that part, and this has an effect upon the second body part that is the natural partner of the first. When activated in one part, the overall form sends a current to the second part, which is also activated there in its place in the same way through the overall form that connects the two of them, while the intervening parts are not affected. (Thus “the eye muscles depend upon the understanding in the heart”, Avodah Zarah 28b.) For no other part will be affected in this way except one that is the natural partner of the first and structurally coupled with it.
This is comparable to the way water is affected according to the place where it is contained. The influence of the stars also proves this point, for the stars send their influences through the entire atmosphere yet their effects are felt only in the place prepared for them: there alone and nowhere else does the particular influence flash forth to act.