254 SPECIAL ANATOMY OF THE SKELETON 



Development of the Foot (Fig. 206). 



The tarsal bones are each developed from a single centre, excepting the os calcis, which has an 

 epiphysis for its posterior extremity. The centres make their appearance in the following order: 

 os calcis, at the sixth month of fetal life; astragalus, about the seventh month; cuboid, at the 

 ninth month; external cuneiform, during the first year; internal cuneiform, in the third year; 

 middle cuneiform and scaphoid, in the fourth year. The epiphysis for the posterior tuber'osity 

 of the os calcis appears at the tenth year, and unites with the rest of the bone soon after pubertv. 

 When this part remains as a separate bone, it is called the os triyonum, 



The metatarsal bones are each developed from two centres one for the shaft and one for the 

 digital extremity in the four outer metatarsal ; one for the shaft and one for the base in the metatar- 

 sal bone of the great toe. 1 Ossification commences in the centre of the shaft about the ninth week, 

 and extends toward either extremity. The centre in the proximal end of the first metatarsal 

 bone appears about the third year, the centre in the distal end of the other bones between the fifth 

 and eighth years; they unite with the shaft between the eighteenth and twentieth years. 



The phalanges are developed from two centres for each bone one for the shaft and one for 

 the proximal extremity. The centre for the shaft appears about the tenth w r eek, that for the 

 epiphysis between the fourth and tenth years; they join the shaft about the eighteenth year. 



Construction of the Foot as a Whole (Figs. 207, 208). 



The foot is constructed on the same principles as the hand, but modified to 

 form a firm basis of support for the rest of the body when in the erect position. 

 It is more solidly constructed, and its component jmrts are less movable on each 

 other than in the hand. This is especially the case with the great toe, which has 

 to assist in supporting the body, and is therefore constructed with greater solidity; 

 it lies parallel with the other toes, and has a very limited degree of mobility, 

 whereas the thumb, which is occupied in numerous and varied movements, is 

 constructed in such a manner as to permit of great mobility. Its metacarpal bone 

 is directed away from the others, so as to form an acute angle with the second, 

 and it enjoys a considerable range of motion at its articulation with the carpus. 

 The foot is placed at right angles to the leg a position which is almost peculiar 

 to man, and has relation to the erect position which he maintains. In order to 

 allow of its supporting the weight of the whole body in this position with the 

 least expenditure of material, it is constructed in the form of an arch. This 

 antero-posterior or longitudinal arch is made up of two unequal limbs. The 

 hinder one, which is made up of the os calcis and the posterior part of the astragalus, 

 is about half the length of the anterior limb, and measures about three inches. 

 The anterior limb consists of the rest of the tarsal and the metatarsal bones, and 

 measures about seven inches. It may be said to consist of two parts, an inner 

 segment made up of the head of the astragalus, the scaphoid, the three cuneiform, 

 and the three inner metatarsal bones; and an outer segment composed of the os 

 calcis, the cuboid, and the two outer metatarsal bones. The summit of the arch 

 is at the superior articular surface of the astragalus; and its two extremities 

 that is to say, the two piers on which the arch rests in standing are the tubercles 

 on the under surface of the os calcis posteriorly, and the heads of the metatarsal 

 bones anteriorly. The weakest part of the arch is the joint between the astrag- 

 alus and scaphoid; and here it is more liable to yield in those who are over- 

 weighted, and in those in whom the ligaments which complete and preserve the 

 arch are relaxed. This weak point in the arch is braced on its concave surface 

 by the inferior calcaneoscaphoid ligament, which is more elastic than most other 

 ligaments, and thus allows the arch to yield from jars or shocks applied to the 

 anterior portion of the foot and quickly restores it to its pristine condition. This 



1 As was noted in the first metacarpal bone, so in the first metatarsal, there is often to be observed a ten- 

 dency to the formation of a second epiphysis in the distal extremity. 



