As you grew older and learned to speak, your vocal organs imitated the sound of the word "salt" as you heard it expressed by others and thus you learned to speak that word. At that stage, your brain was capable of calling up three mental pictures--an auditory picture, or a picture of the sound of the word; a graphic or visual picture, or a picture of the appearance of salt and a third, which we have called a motor-memory picture, which represents the muscular movements necessary to speak the word. A little later on, after you had gone to school and learned to write, you added to these pictures a fourth, the movements of the hand necessary to write the word "salt." At the sight of the mother, a child may, for instance, be heard to say the word "Mom" while at the sight of the pet dog whose name is "Dot," be heard to say "Dot" in his childish way. Here we have the first example in this child of the association of ideas. The child has heard, repeatedly, the word "Mama" used in conjunction with the appearance of the smiling face of his mother. Thus has the child acquired the habit of associating the word "Mama" with that face--and the sight of the countenance after a time recalls the sound of the associated word. Thus a visual image of the mother transmitted to the child through the medium of the eye, links up a train of thought that finally results in the child's attempt to say "Mama." To take another example of the association of ideas or the co-ordination of mental images necessary to the production of speech, let us suppose, for instance, that the child has been in the habit of petting the dog and hearing him called by name "Dot" at the same time. Now, if the dog be placed out of the child's sight and yet in a position where the hand of the child can reach and pet him in a familiar way, this sense of touch, like the sense of sight, will set up a train of thought that results in the child making his childish attempt to speak the name of the dog "Dot." In other words the excitation of any sensory organs sets up a series of sensory impulses which are transmitted along the sensory nerve fibres to the brain, where they are referred to the cerebellum or filing case, locating a set of associated impulses which travel outward from the motor area of the brain and result in the actions, or series of actions, which are necessary to produce a word. It will make the action of the brain clearer if the reader will remember the sensory nerve fibres as those carrying messages only TO the brain, while the motor nerve fibres carry messages only FROM the brain.