We have just seen that the underlying arrangement of the nervous system is one which makes communication possible between any sense organ in the body and any muscle; also that in the working of the nervous system there are certain paths from given sense organs to particular muscles which we may call the selected paths, and over which nervous disturbances easily pass. These paths work out in such a way that the muscles that are thrown into activity as the result of the arousing of any sense organ are those most likely to serve the needs of the animal. There are a number of respects in which a nervous system, having no more than we have thus far described, would fall short of meeting the requirements of any animal except perhaps the very simplest. The thing that is missing that is of most importance, perhaps, is the ability to perform actions in response to information received at some past time; in other words, the ability to profit by experience. As the nervous system has been described up to this point, only immediate effects upon the sense organs are transmitted to the muscles to throw them into activity. As a matter of fact, we know that we ourselves, as well as many of the higher animals, are able to profit by past experience. That is to say, we can make our muscles move in accordance with information received at some time in the past. There must be in the nervous system, then, some arrangement for doing this, and our present task is to describe it. In the chapter on the distance sense organs considerable emphasis was placed on the fact that the advantage of having distance sense organs is to allow time for a choice among the various responses that the animal might make to the information received by the sense organs. It is this opportunity for choice that lies at the foundation of the higher nervous activities that we are now beginning to describe. As soon as the element of choice enters, the nervous disturbance cannot pass from the sense organ to the muscle in the least possible time. In order for there to be a choice there must be a delay while the selection of the muscle to be aroused to activity is being made. We have, then, a new feature in connection with the operation of distance sense organs in that the nervous disturbance can come to a stop at some point in the course of its progress from sense organs to muscles. Every animal that has distance sense organs shows this feature of possible delay in the movement of the nervous disturbance over the nervous pathway, and in every one the particular part of the nervous system in which this occurs is known as the brain. In fact no other nerve cells except some of the nerve cells of the brain have this ability of stopping the nervous disturbance and holding it for a time before sending it on.