Flat-foot is a sinking of the instep, with flattening and broadening of the sole of the foot.

Two strong arches, a longitudinal and a transverse, are formed by the arrangement of the numerous small bones of the foot. The longitudinal arch rests on two points—the two tubercles on the under surface of the os calcis at the back and on the heads of the three middle metatarsal bones in front ; the inner side of the arch is formed by the astragalus, scaphoid, cuneiform, and the three inner metatarsal bones ; the outer side is formed by the os calcis, cuboid, and the outer metatarsal bones. Rather more than the middle third of the inner border of the normal foot is raised from the ground ; the outer border is more or less in contact with the ground. From the inner border the bones arch over to the outer border and form the transverse arch. The greatest convexity is across the cuneiform and cuboid bones. The weakest part of the arch is the joint between the astragalus and the scaphoid. It is here that it is liable to yield when the muscles of the calf are fatigued and relaxed, and too much weight is in consequence thrown on the ligaments. In standing, there is always some yielding of both arches, owing to slight elasticity in the interosseous ligaments, and it is in long-continued standing, coupled perhaps with muscular weakness, that the arches yield abnormally, the result being flat-foot.

The tarsal and metatarsal bones are bound together by interosseous, plantar, and dorsal ligaments. The most important ligaments in preserving the arch of the foot are the long and short plantar, calcaneo-cuboid, and the inferior calcaneo-scaphoid.

The long calcaneo-cuboid ligament is attached to the under surface of the os calcis in front of the tuberosities, and to the ridge on the under surface of the cuboid and bases of the three middle metatarsal bones. It is the longest tarsal ligament, and in crossing the groove on the under surface of the cuboid forms a canal for the tendon of the peroneus longus to pass through to its insertion. The short calcaneo-cuboid, only 1 inch in length and very broad, is attached to the anterior tubercle of the under surface of the os calcis, and to the cuboid behind the ridge. The inferior calcaneo-scaphoid is attached to the anterior margin of the sustentaculum tali of the os calcis and to the under surface of the scaphoid. Beside uniting these two bones, this ligament supports the head of the astragalus. When it yields the head of the astragalus is pressed downwards, inwards, and forwards, and the foot becomes flattened. The tendon of the tibialis posticus, being attached to nearly all the bones of the foot, gives this ligament support underneath. Tendons of muscles attached to the under surface of the bones of the foot, and which assist in maintaining the arch, are flexor longus pollicis, tibialis posticus, peroneus longus, flexor brevis pollicis, flexor longus digitorum. The plantar fascia, which is of great strength, is also a factor in maintaining the arch.

In massage for flat-foot all the movements for the leg, foot, and toes are used, to which are added deep kneading with thumbs on the sole and a quick flapping movement

done crosswise with palm of hand, also on the sole. Each joint of the toes is exercised separately with care and precision. In circumduction of the metatarso-phalangeal joints traction is made. A vibration may be given to each toe by taking the end between the thumb and index-finger, and conveying a trembling movement to it. The tarsal bones are well manipulated. The ankle-joint is flexed, extended, adducted, abducted, circumducted inwards, and inverted.

The leg is vigorously stroked. Each group of muscles is kneaded separately with fingers and thumbs and heel of hand, then deeply and thoroughly kneaded en masse between the two hands, to get better at the deep muscles of the calf. The thumbs are made the fixed points in front, while the fingers work at the back, thus reversing the usual way of working. Fulling is done firmly, making it a deep movement. Brisk tapotement ; finish with rolling effleurage. The foot is taken in both hands, thumbs on top and fingers underneath, and vibration given to the whole limb, traction being made at the same time.

Exercises—Active.—1. Patient, standing erect, with hands on hips and toes turned in ; rises to tiptoe, then slowly brings heels back to the ground. This is repeated as often as the strength of the patient permits. It strengthens the flexors of toes and extensors of foot.

2.   Standing in same position. The inner borders are turned upwards till the weight falls on the outer borders ; slowly brought down after a few seconds and repeated. This throws up the arch of the foot and stretches the peronei muscles.

3.   Standing on heels and toes alternately. This strengthens the tibialis posticus and anticus.

Position of rest to be assumed after exercises : Sitting on a couch, with knees abducted and legs crossed, outer border of feet resting on couch.

Bathing the feet in warm water, followed by douches of cold water, before massage is helpful. Flat feet usually perspire freely, and require to be frequently dried while being massed. It is noticeable that they cease to perspire after a few weeks of massage.

The patient should rest for half an hour after treatment, which should be given twice a day, and when possible three times. Before rising in the morning and on going to bed at night are desirable occasions, and it is a further advantage to have it in the middle of the day ; in any case, rest in the position above described should be taken at that time.

In severe cases, when all the muscles of the lower limbs are weak, absolute rest for a time is enjoined, and the thighs are massed as well as the legs. Exercises should always stop short of fatigue, and be increased gradually—at first passive, then resistive, and later the active movements done by the patient standing.

Knock-knee is sometimes found 111 connection with flat-foot, in which case treatment for it is added.