Although not all muscle cells are exactly equal in power, on the whole the _force_ that muscle can show is the force of one cell multiplied by the number of cells that can join in the pull. A strong muscle must have many cells side by side; in other words it must be thick. Also, the _distance_ through which muscles can make movements depends on their length, so a muscle that has to pull for a considerable way must be long, and since single muscle cells are short there will have to be a good many cells end to end to make the whole muscle long enough for its task. The actual make-up and arrangement of muscles in the body depends in part, therefore, on the thickness and length needed for the particular work to be done, and in part on the architecture of the part of the body where the muscle is located. For example, the strongest muscle in the body is that by which one rises on the toes. This muscle operates by pulling upward at the back of the heel. If it were located right at the ankle, where it would have to be if attached directly to the place where its force is exerted, the resulting clumsiness can easily be imagined. By shifting it up to the middle of the lower leg room is found for the large mass of muscle needed for the work. The connection with the heel is made by means of a long and very strong tendon, known as the tendon of Achilles, because that was the part Achilles’ mother failed to immerse when she was dipping the infant in the river Styx to make him invulnerable. Other equally good examples are the muscles for operating the fingers. If placed in the hands, the latter would be too bulky and clumsy for any kind of efficient use. By placing them up in the forearms out of the way, and connecting them with the fingers by long tendons, delicacy is secured for the hands. The muscles of the arms and legs are arranged in groups about the joints, and these groups always include opposing sets. Thus if the joint is a simple hinge, as at the elbow, where the only motion possible is back and forth, there will be one muscle to bend the joint and another opposing muscle to straighten it out again. The first is known as a _flexor_; the second as an _extensor_. In the arm the biceps, on the upper surface, is the flexor and the triceps, on the under side, the extensor. Joints that permit of motion in several directions have correspondingly more opposing sets of muscles acting upon them. The same scheme applies to the trunk, but since in the trunk instead of a few very movable joints we have the whole row of slightly movable vertebræ, the grouping of muscles is more complicated. Not all the skeletal muscles work about joints. The tongue, the muscles of the lips and about the eyes, those along the front of the abdomen, and some others are attached to bones only at one end or not at all, and do their work by pulling upon one another. [Illustration: THE BICEPS MUSCLE AND THE ARM BONES (From Martin’s “Human Body”)]