+Acquired Flat Foot.+ The common form of acquired flat foot is the static variety, which is an expression of a disproportion between the body weight and the sustaining power of the muscles and ligaments. +Common Causes.+ 1. The use of improper shoes is by all means the most frequent cause of flat foot, and frequently makes all of the following causes more pronounced. 2. Weakness and insufficiency of the muscles, resulting from poor general condition; advancing age; convalescence from acute illness; from childbirth; and from injuries of the leg, especially fractures. 3. Prolonged standing, especially on hard wood and stone floors. 4. Rapid body growth. 5. Rapid increase in body weight. 6. Excessive weight bearing. 7. Shortened condition of the gastrocnemius muscle. Other causes are rickets; inflammation of the ankle joint, as in tuberculosis; or, as a result of a badly treated fracture of the ankle-joint; or, as a result of paralysis of the muscles of the inner side of the leg. +Pathology of Acquired Flat Foot.+ The pathologic condition is due to change in the relations of the bones rather than to any change in the bones themselves. The abnormal position is an exaggeration of the normal yielding of the foot under weight bearing. The front of the astragalus rotates inward, and with it the bones of the leg turn at the hip-joint. The deformity is essentially a displacement of the astragalus on the bones of the tarsus. The scaphoid, cuneiform, and the base of the first metatarsal move downward and inward with the head of the astragalus; the outer border of the foot is made more concave and the inner border becomes convex in extreme cases. In the severest cases, the head of the astragalus, and scaphoid may be displaced below the plane of the other bones. The ligaments are respectively shortened and stretched in the severest cases and there is a loss of motion in certain of the tarsal articulations, due to faulty apposition of joint surfaces, and to constant strain. +Symptoms.+ The feet burn and tire easily and feel stiff and lame. They may swell, and the size of the shoe worn must be then increased. Later, a painful period generally begins in which walking is avoided and a dragging pain in the arch and behind the inner malleolus is noticed. This is increased by walking and standing and tender points may be found under the scaphoid and on the upper surface of the heel. The foot feels strained and irritated and is a constant source of discomfort. The inner malleolus is generally more prominent and the foot is displaced outward in relation to the leg. The height of the arch is somewhat diminished; it may be much lowered, or it may be flat on the ground. When the foot is really flattened, it presents two types, one the _flexible flat foot_, in which the arch can be restored by gentle manipulation; the other, the _rigid foot_, which is held by structural changes in the position of deformity.