THE LIMBS. 307 



ground. The arch is produced as the child learns to walk. The 

 chief factor in its production is the growth of the tarsal bones — 

 especially of the scaphoid and internal cuneiform — and 1st meta- 

 tarsal. Hence in rickets, where the normal tarsal growth is 

 disturbed, the occurrence of flat foot. Amongst the structures 

 which help to maintain the arch are : 



(a) The growth of the os calcis into the heel separates the 



tendon of the plantaris from its prolongation in the sole 

 — the middle part of the plantar fascia, which assists 

 in maintaining the arch. In lower primates the two parts 

 are continuous, the tendon of the plantaris plying across 

 the os calcis in a cartilage-lined groove. 



(b) The internal lateral ligament of the ankle (anterior part) 



and the inferior calcaneo-scaphoid ligaments undergo 

 great development in man. 



(c) The flexor brevis digitornm which in lower primates arises 



principally from the long flexor tendons in the sole, has 

 its origin completely transferred to the os calcis in man. 

 It can thus act more powerfully in maintaining the 

 arch. The flexor accessorius, a detached part of the 

 flexor longus hallucis, is specially well developed and 

 helps to maintain the arch of the foot. 



(d) The tibialis posticus, originally a flexor of the metatarsus, 



corresponding to the flexor carpi radialis in the hand, 

 obtains a secondary attachment to the scaphoid. The 

 tibialis anticus, which answers to the extensor ossis 

 metacarpi pollicis, becomes permanently inserted to the 

 internal cuneiform and metatarsal. 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. 



(e) The long plantar ligament, originally a part of the tendon 



of insertion of the gastrocnemius — also assists to main- 

 tain the arch. 

 (4) The development of the great toe and the peculiar 

 arrangement of its muscles must also be regarded as adaptations 

 in the foot to upright posture and progression. 



Comparison of the Muscles of the Upper and Lower 

 Extremities. — As a mental exercise it may be worth the 



