As we have said over and over, the purpose of the nervous system is to arouse the muscles to activity, and to guide them in that activity. We must ask next, then, how the nerves are distributed to the muscles. If we dissect the body of any animal or bird, we can find nerves passing to various parts. For example, a large nerve goes down each leg; this nerve subdivides here and there. Since we know that the nerve consists of a great many axons bundled together, we will realize that this subdivision is not a real branching, but simply a passing of some of the axons away from the main trunk along the smaller stem. Some of these smaller stems can be traced to endings in the skin; these contain the axons connecting with sense organs. Others lead directly into muscles. Some of these axons may also connect with sense organs, since, as we have already seen, every muscle has embedded in it the organs of muscle sense, but in addition any nerve that leads to a muscle contains a great many axons which pass directly to the muscle fibers. These are the axons by which the muscles are aroused to activity. It is a general rule of the nervous system that no nerve cell extends without a break from any sense organ to any muscle fiber. The axon which communicates with the sense organ belongs to one nerve cell; the axon which connects with the muscle fiber belongs to a different nerve cell. The first is called a _sensory_ nerve cell, the second a _motor_ nerve cell. Some idea of the appearance of these cells can be gotten from the figures on page 126.