All that has been said about the nervous system thus far has applied to its duty of controlling the activities of skeletal muscles; namely, the muscles by which the body carries on its principal motions. These, as already stated in Chapter VII, are not the only kind of muscles in the body. There are also the smooth muscles and heart muscle. There is also another kind of functional metabolism besides the metabolism of muscular motion over which the nerves have control. This is the metabolism of secretion, or the manufacture of special chemical substances by gland cells. In later sections of the book we shall hear more about what the glands do, but here we have to say something about the way they are controlled through the nervous system. We speak of them along with the smooth muscles, because it happens that the same nervous arrangements are found in both cases; the smooth muscles and the glands have a precisely similar kind of nervous control. In appearance the nerves which lead from the central nervous system to the smooth muscles and the glands do not differ much from other nerves. They are composed of nerve cells, just as are all the other nerve pathways. It happens to be true that the sheaths which inclose the axons of these particular nerves usually have little or no white material in them, so that nerve trunks which are made up of these fibers look gray instead of white, as do the other nerves. Just why there should be this difference, we do not know. Of more importance than this difference in appearance is the fact that the connection from the central nervous system to the smooth muscle cell and the gland cell in no case is made up of a single nerve cell, but always of a chain of two. In the case of nerve paths to skeletal muscles the cell body of the motor nerve cell lies either in the spinal cord or the brain and its axon extends all the way from there directly to the muscle fibers. In the case of smooth muscles and glands a connecting nerve cell extends out from the central nervous system some distance and makes its nerve junction with the motor nerve cell part way between the central nervous system and the muscle or gland. These connecting cells are much branched, as are all connecting cells; also the motor cells which connect with smooth muscles or glands are very much branched; the result is that a single nerve cell starting from the spinal cord finally connects with a large number of smooth muscle cells or gland cells. We do not have, then, in these organs the fine subdivision of control that there is in the skeletal muscles, for a single skeletal motor cell in the spinal cord connects with one or, at the most, a very few muscle fibers. If we consider the uses to which smooth muscles are put, we shall see that finely subdivided control is not important, as it is in the case of the skeletal muscles, where the movements have to be graded very accurately. Smooth muscles are present in the stomach and