The change in a nerve-fibre can also produce changes in other organs. If the nerve-fibre reaches the cells in the spinal cord it may set up changes in these which result in a trans- mission of nerve-currents or shocks along other fibres, as in the phenomena of reflex action. Again, if the fibre passes to the brain, it excites changes in nerve-cells connected in some way with consciousness, and we thus come to know of something which affected the fibre at its commencement. Thus when light falls on the eye it affects the nervous structure called the retina; the retina is connected with the brain by nerve-fibres which are affected by the changes occurring in the retina, and nerve-currents travel along these nerve-fibres to the brain. In the brain they set up changes in nerve-cells which result in the consciousness of light, that is to say, we have a sensation which we call light. In all these instances, the nature of the change in the nerve-fibre and the mode of its transmission are the same. The results are different because the fibres end in different kinds of terminal structures.
Thus an electric current travelling along a wire may do very different things according to
