If the sole of the dissected foot is observed it is seen that the plantar arch lies approximately over a triangle of which the base is formed by the _transverse adductor_ muscle of the great toe, across the heads of the metatarsal bones, and the two sides by the _oblique adductor_ of the great toe and the _short flexor_ of the little toe. It extends, of course, somewhat further back under the heel-bone, but this is its highest part. In the changing foot the _tibialis posticus_, which was originally a flexor of the metatarsal bones, obtains a secondary attachment to the _scaphoid_ bone, and the _tibialis anticus_ becomes inserted anew into the internal _wedge-shaped_ and metatarsal bones. “Both of these muscles, thus modified, help to maintain the arch of the foot. So does the tarsal part of the tendon of the _tibialis posticus_.” (Keith). The three _peronei_ muscles, especially the new _peroneus tertius_, attached to the little toe, are called in by increased walking to redress the balance of forces in the foot and produce that eversion, with some supination, which is essential to the arch. No arch was possible till these muscles came into some preponderance of action over the _flexors_, so beloved of gorilla C. The _short flexor of the digits_ becomes modified so that its attachment to the tendons of the _long flexors_ in the sole has its _origin_ completely transferred to the heel-bone in man (Keith). “It can thus act more powerfully in maintaining the arch,” and finally the _flexor accessorius_, a muscle which cannot fail to surprise the dissector when he first penetrates into the deep layer of muscles of the sole, and which is a detached piece of the _long flexor of the great-toe_, becomes especially well-developed and helps to maintain the arch. The order of events then is: first, increased and altered muscular function; second, growth of bones and adjustment; third, binding together of these by new or modified ligaments. If it were possible to separate in this way the age-long formation of such a living tool as the human foot, this is the order in which alone, I submit, the sequence of events can be placed. It is a convenient, because simple and plain example of initiative in evolution, and I cannot say how much I owe to Professor Keith’s teaching on the subject.[71] [71] It is not sufficiently noticed by some writers how important is Professor Keith’s teaching as to the maintenance of the arch by muscular action rather than ligamentous union. And it is a very practical matter from my own point of view in connection with the prevention of flat-foot in the young. If indeed the poor deformed feet of the sufferers can only be corrected by attention to the lowly-organised ligaments, and the muscles will not avail, I can but add “God help them!” CHAPTER XXII. MUSCLES.