The third property of nerve-cells which is important in study is _modifiability_. That is, impressions made upon the nerve-cells are retained. Most living tissue is modifiable to some extent. The features of the face are modifiable, and if one habitually assumes a peevish expression, it becomes, after a time, permanently fixed. The nervous system, however, possesses the power of modifiability to a marked degree, even a single impression sufficing to make striking modification. This is very important in study, being the basis for the retentive powers of the mind. Having examined the action of the nervous system in its simplicity, we have now to examine the ways in which the parts of the nervous system are combined. We shall be helped if we keep to the conception of it as an aggregation of systems or groups of pathways. Some of these we shall attempt to trace out. Beginning with those at the outermost parts of the body, we find them located in the sense-organs, not only within the traditional five, but also within the muscles, tendons, joints, and internal organs of the body such as the heart, and digestive organs. In all these places we find ends of neurones which converge at the spinal cord and travel to the brain. They are called sensory neurones and their function is to carry messages inward to the brain. Thus, the brain represents, in great part, a central receiving station for impressions from the outside world. The nerve-cells carrying messages from the various parts of the body terminate in particular areas. Thus an area in the back part of the brain receives messages from the eyes; another area near the top of the brain receives messages from the skin. These areas are quite clearly marked out and may be studied in detail by means of the accompanying diagram. There is another large group of nerve-cells which, when traced out, are found to have one terminal in the brain and the other in the muscles throughout the body. The area in the brain, where these neurones emerge, is near the top of the brain in the area marked _Motor_ on the diagram. From here the fibers travel down through the spinal cord and out to the muscles. The nerve-cells in this group are called motor neurones and their function is to carry messages from the brain out to the muscles, for a muscle ordinarily does not act without a nervous current to set it off.