United by the joints, the bones of the skeleton, taken as a whole, approach the form of the body. But in order to put these bones in motion, and to bring these joints into play, we must call to our aid an external force. By itself, if we may be permitted a very familiar comparison, the skeleton is a puppet of which the different parts are put in motion by threads. The threads by which the skeleton is moved are the muscles. The name muscles is given to the masses of red tissue which constitute the flesh. We have already described the elements of the muscular tissue; how the primitive microscopic bundles, united into secondary ones, become muscular or fleshy fibres easily distinguished by the naked eye. These fibres are parallel or divergent according to the muscle, and assume different forms. Sometimes it is that of a ribbon (sartorius, sternohyoid, &c); sometimes a broad web-like tissue of a texture more or less firm, like the transverse muscles of the abdomen; in one region the muscle, swollen in the centre, and drawn out like a thread at the ends, resembles a spindle in form (biceps, straight muscle of the thigh); in another it is fan-shaped (temporal, obturator), or like a ring (orbicular muscle of the lips and eyes); or the fibres converge like the radii of a circle (the diaphragm); or are disposed in parallel lines like the feathers of a pen (extensor muscles of fingers). Lastly, certain organs, the heart for example, are nothing but muscle, or rather an assemblage of muscles intimately united.
The muscles determine the form and volume of the body, and especially of the limbs. The outline depends upon their projection, and changes incessantly as they are in action or in repose. They are disposed in layers, deep or superficial, and united in groups or separated by sheaths and membranous partitions. Their colour varies from deep red to pale rose, according to the region of the body they occupy, age, sex, the constitution and richness of the blood. The stronger the muscle the redder it is, and it becomes still brighter under the influence of exercise.
The muscles of the human body number about 350, and they are distinguished by names suggested by their form, their locality, their functions, or their attachments. Some are fixed to the skin, as several of the muscles of the face; others to muscles in their vicinity, as in the face and tongue; others still to the cartilages, but the largest number to the bones by means of the tendons or the aponeuroses, of which we proceed to speak.