accept this general statement as one which is universally true. And, if so, they will agree with Hobbes that all our knowledge of the external world is a knowledge of motion. Now, if it would have been thus a joy to Hobbes to have heard to-day how thoroughly he has been justified in his views touching the external world, with no less joy would he have heard that he has been equally justified in his views touching the internal world. For it has now been proved, beyond the possibility of dispute, that it is only in virtue of those invisible movements which he inferred that the nervous system is enabled to perform its varied functions. To many among the different kinds of movement going on in the external world, the animal body is adapted to respond by its own movements as best suits its own welfare; and the mechanism whereby this is effected is the neuro-muscular system. Those kinds of movement going on in the external world which are competent to evoke responsive movements in the animal body are called by physiologists stimuli. When a stimulus falls upon the appropriate sensory surface, a wave of molecular movement is sent up the attached sensory nerve to a nerve-centre, which thereupon issues another wave of molecular movement down a motor nerve to the group of muscles over whose action it presides; and when the muscles receive this wave of nervous influence they contract. This kind of response to stimuli is purely mechanical, or non-mental, and is ordinarily termed reflex action. The whole of the spinal cord and lower part of the brain are made up of nerve-centres of reflex action; and, in the result, we have a wonderfully perfect machine in the animal body considered as a whole. For while the various sensory surfaces are severally adapted to respond to different kinds of external movement--the eye to light, the ear to sound, and so on--any of these surfaces may be brought into suitable relation with any of the muscles of the body by means of the cerebro-spinal nerve-centres and their intercommunications.