In fractures by _muscular action_ the bone is broken by "traction" or "tearing." The sudden and violent contraction of a muscle may tear off an epiphysis, such as the head of the fibula, the anterior superior iliac spine, or the coronoid process of the ulna; or a bony process may be separated, as, for example, the tuberosity of the calcaneus, the coracoid process of the scapula, or the larger tubercle (great tuberosity) of the humerus. Long bones also may be broken by muscular action. The clavicle has snapped across during the act of swinging a stick, the humerus in throwing a stone, and the femur when a kick has missed its object. Fractures of ribs have occurred during fits of coughing and in the violent efforts of parturition. Before concluding that a given fracture is the result of muscular action, it is necessary to exclude the presence of any of the diseased conditions that lead to pathological fracture. Although the force acting upon the bone is the primary factor in the production of fractures, there are certain subsidiary factors to be considered. Thus the age of the patient is of importance. During infancy and early childhood, fractures are less common than at any other period of life, and are usually transverse, incomplete, and of the nature of bends. During adult life, especially between the ages of thirty and forty, the frequency of fractures reaches its maximum. In aged persons, although the bones become more brittle by the marrow spaces in their interior becoming larger and filled with fat, fractures are less frequent, doubtless because the old are less exposed to such violence as is likely to produce fracture. Males, from the nature of their occupations and recreations, sustain fractures more frequently than do females; in old age, however, fractures are more common in women than in men, partly because their bones are more liable to be the seat of fatty atrophy from senility and disease, and partly because of their clothing--a long skirt--they are more exposed to unexpected or sudden falls. [Illustration: FIG. 1.--Multiple Fracture of both Bones of Leg.] #Clinical Varieties of Fractures.#--The most important subdivision of fractures is that into simple and compound. In a _simple_ or subcutaneous fracture there is no communication, directly or indirectly, between the broken ends of the bone and the surface of the skin. In a _compound_ or open fracture, on the other hand, such a communication exists, and, by furnishing a means of entrance for bacteria, may add materially to the gravity of the injury. A simple fracture may be complicated by the existence of a wound of the soft parts, which, however, does not communicate with the broken bone. Fractures, whether simple or compound, fall into other clinical groups, according to (1) the degree of damage done to the bone, (2) the direction of the break, and (3) the relative position of the fragments.