THE LIMBS. 305 



of the hand — a pre-hallux. Supernumerary digits, when they 

 appear, are commonly situated on the radial side of the thumb or 

 ulnar side of the little finger, but they may represent merely a 

 fission of the normal pollex or little finger. The pisiform has been 

 regarded as the vestige of a post-minimal digit ; the sesamoid on 

 the trapezium, in which a slip of the extensor ossis metacarpi 

 pollicis ends, as a remnant of a pre-hallux. It is possible also to 

 regard the pisiform as a sesamoid developed in the tendon of the 

 flexor carpi ulnaris — for that muscle is originally a flexor of the 

 metacarpus and ends on the 5 th metacarpal — the pisi-metacarpal 

 ligament representing the terminal part of the tendon. The pisi- 

 form, however, is developed with the rest of the carpal bones and 

 before the tendon of the flexor carpi ulnaris. In mammals generally, 

 but not in man, the pisiform articulates with the ulna as well as the 

 cuneiform, and its synovial facet opens into the wrist joint. It 

 may be represented in the foot by the heel epiphysis of the os 

 calcis. The gastrocnemius, which represents the flexor carpi 

 ulnaris in the leg, is also primitively a flexor of the metatarsus ; 

 the long plantar ligament, from which it is separated by the 

 growth of the heel, represents the continuation of its tendon. 



The Eversion of the Foot and Development of the Arch. — 

 The human foot has been highly modified for upright progression. 

 The chief modifications are : 



(1) Gradual eversion of the foot, so that the sole can be 

 applied to the ground. Even at birth — and for some time after 

 — and always up to and before the 7 th month of foetal life, the 

 soles of the feet are inverted, so that when the foetal limbs are in 

 their natural position they are directed towards the belly of the 

 child. In club foot the natural process of eversion does not take 

 place. The ape's foot is kept normally in the inverted position, 

 an adaptation for prehension. The following factors assist in 

 producing eversion : 



(a) The neck of the astragalus (Fig. 248), which in the foetal 

 foot is long and directed downwards and inwards at 

 an angle to the axis of its body, becomes relatively 

 shorter and directed more in line with the axis of the 

 articular surface of its body (Fig. 248). Further, the 

 outer border of the tibial articular surface of the astra- 

 galus is prominent in the foetus ; the inner border is 

 u 



