So far we have seen that the brain has the two functions of receiving impressions from the sense-organs and of sending out orders to the muscles. There is a further mechanism that must now be described. When messages are received in the sensory areas, it is necessary that there be some means within the brain of transmitting them over to the motor area so that they may be acted upon. Such an arrangement is provided by another group of nerve-cells in the brain, having as their function the transmission of the nervous current from one area to another. They are called association neurones and transmit the nervous current from sensory areas to motor areas or from one sensory area to another. For example, suppose you see a brick falling from above and you dodge quickly back. The neural action accompanying this occurrence consists of an impression upon the nerve-cells in the eye, the conduction of the nervous current back to the visual area of the brain, the transmission of the current over association neurones to the motor area, then its transmission over the motor neurones, down the spinal cord, to the muscles that enable you to dodge the missile. The association neurones have the further function of connecting one sensory area in the brain with another. For example, when you see, smell, taste and touch an orange, the corresponding areas in the brain act in conjunction and are associated by means of the association neurones connecting them. The association neurones play a large part in the securing and organizing of knowledge. They are very important in study, for all learning consists in building up associations.