106 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



THE SESAMOID BONES OF THE FOOT. 

 Two sesamoid bones, remarkable on account of their size, are constantly found at the 

 metatarsophalangeal joint of the great toe (Fig. 153). Inconstant sesamoids are also found in 

 the tendon of the peroneus longus muscle, at the interphalangeal joint of the great toe, and less 

 frequently in the tendon of the tibialis posterior muscle. 



THE SKELETON OF THE FOOT AS A WHOLE. 



The skeleton of the foot (Figs. 153 and 155) differs from that of the hand not only in the 

 number and form of the component elements of the tarsus, but also in certain peculiarities, chiefly 

 due to the functional adaptation of the foot as a support for the erect body. While the axis of 

 the hand is situated in the direct continuation of that of the arm and forearm, the axis of the foot 

 is placed at almost a right angle to that of the lower extremity, and while in the hand the phalanges 

 take up about one-half of the length of the skeleton, in the foot the tarsus alone occupies the proxi- 

 mal half and the metatarsus and phalanges together form the anterior half. The phalanges 

 make up only a fifth of the entire length of the foot. 



The foot shows a much more pronounced curvature than do the relatively flat and closely 

 approximated bones of the hand, and this curvature is practically a constant one. The convexity 

 is directed toward the dorsal, the concavity toward the plantar surface, and the deepest point 

 of the concavity is situated at the apex of the middle cuneiform bone, the dorsal surface of the 

 same bone likewise forming the highest point of the middle portion of the arch. 



The arch of the foot is supported posteriorly by the tuberosity of the calcaneus and anteriorly 

 by the heads of the metatarsal bones. The tarsal arch is formed exclusively by the tarsus and 

 metatarsus and is open internally, since the inner border of the foot is much higher than the outer 

 one, which is in contact with the ground throughout its entire length. The sinus of the tarsus 

 (see page 102) is a striking formation which gradually becomes narrower as it passes inward 

 and backward from the outer side of the dorsal surface. The tarsus is much narrower posteriorly 

 than anteriorly. 



The phalanges of the second to the fifth toe do not lie in one plane even during extension, 

 but are strongly curved with the convexity upward and seem to be bent upon the heads of the 

 metatarsal bones like claws, so that only their tips touch the ground. 



The second toe is the longest and marks the longitudinal axis of the foot. 



Usually the only tarsal bones possessing centers at birth are the calcaneus (sixth month) and the talus (astragalus) 

 (seventh month); the center for the cuboid appears at about the time of birth. The external cuneiform is the first of 

 the three cuneiform bones to ossify (first year), the internal is the next (third year), and the middle one is the last, its 

 ossification and that of the navicular In me occurring respectively in the fourth and the fourth to the fifth year. The cal- 

 caneus alone has a disc-like epiphyseal center upon its posterior surface, corresponding to the tuberosity; this appears 

 in the tenth year and fuses with the rest of the bone at from the fifteenth to the sixteenth year. 



The metatarsal bones ossify much earlier than do the tarsal bones and ossification proceeds in a manner quite 

 similar to that of the metacarpal bones. The diaphyseal nuclei appear in the eighth to the ninth fetal week, and the 

 epiphyseal centers also arc like those of the metacarpal bones, appearing in the third to the fourth year and not uniting 

 with the diaphyseal center until after puberty. 



The ossification of the phalanges of the foot also corresponds exactly to that of the phalanges of the hand. The 

 diaphyseal centRrs appear in the third fetal month, the epiphyseal centers in the third to the fourth year, and the union 

 of the epiphyses with the diaphyses, as in the metatarsal bones, occurs after puberty. 



