The commonest, as well as the most effective, means of making motions is by cells specially developed for that purpose. These are called muscle cells, and every highly organized animal depends on them for most if not all of the motions which take place in its body. In muscle cells the functional metabolism takes the form of forcible changes in shape of the cells by which bodily motions are brought about. A muscle cell might be described as a mechanical device for transforming the chemical energy of burning fuel into the energy of motion. We have something comparable in the automobile cylinder, where the energy obtained from the explosive burning of the air-gas mixture drives the piston and so propels the car. Of course the two devices are not even remotely alike in the actual way in which they operate; their resemblance is purely the general one of converting one type of energy (chemical) into another type (motion). There are three kinds of muscle cells in our bodies. The simplest are those that are found in the wall of the stomach and intestines and other internal organs that are capable of movements; the next kind is found only in the heart; the third, and most complex, makes up the bulk of our muscular tissue; it is the muscle that moves the bones. The first kind, because it shows no particular markings when examined through the microscope, is usually called smooth muscle; the second kind is known as heart muscle; the third kind, because it moves the skeleton, is named skeletal muscle. We shall devote most of our attention to this third kind of muscle, because it is a much more efficient machine than the others, and also because it has to do with our general bodily movements instead of with motions of internal organs. A single skeletal muscle cell is an exceedingly slender fiber, much smaller than the finest thread; it may also be very short, not more than a twenty-fifth of an inch long, or it may be as much as an inch long. A muscle is made up of many of these fibers grouped side by side in bundles, and also, if the muscle is long, placed end to end. The fibers are held in place and fastened together by connective tissue. Lean meat consists of thousands of these muscle cells with their connective tissue fastenings. In coarse meat there is relatively more connective tissue and less actual muscle tissue than in the finer grades. In every muscle the connective tissue is loose enough to allow body fluid to penetrate among the muscle cells. Blood vessels are also distributed through the mass of the muscles between and among the cells; thus their nutrition is provided for.