THE BONES OF THE LOWER LIMB 265 



separate ossicle ; (3) the internal cuneiform may be divided into two parts, 

 dorsal and plantar ; or (4) there may be an additional ossicle in the space at 

 the antero-intemal part of the os calcis, or between the internal cuneiform and 

 the second metatarsal. 



Ossification. — The tarsal bones ossify in cartilage, each from one centre, 

 (except the os calcis, which has one primary, and one secondary, centre), and 

 at the following periods approximately : 



Os calcis, 6th month (intra-uterine). External cuneiform, ist year. 



Astragalus, 7th month (intra-uterine). Internal cuneiform, 3rd year. 



Caboid, 9th month (intra-uterine). Middle cuneiform, 4th year. 



Navicular, 4 th year. 



For the secondary centre of the os calcis, see page 258. The 

 external tubercle on the posterior border of the astragalus has some- 

 times a secondary centre, and then it remains separate as the os 

 trigonum. 



The Metatarsus. 



The metatarsus is composed of five long bones, which are named 

 nuiherically from within outwards, that of the great toe being the 

 first. Each bone is di\'isible into a shaft and two extremities, 

 proximal and distal. The shaft, which is triangular, is massive in 

 the first, slender and much compressed laterally in the second, 

 third, and fourth, and compressed from above downwards in the 

 fifth. Each shaft, except that of the first, is longitudinally convex 

 on its dorsal aspect, and they are all longitudinally concave on their 

 plantar aspects. The shaft presents three borders and three 

 surfaces. In the outer four bones the borders are two lateral, and 

 a plantar. The lateral borders, external and internal, extend from 

 the sides of the proximal end or base, close to the dorsal aspect, 

 to the dorsal tubercle on either side of the distal end or head, and 

 their outline is sharp. The plantar border, rotmd behind, but 

 sharp in front, extends from the centre of the plantar aspect of 

 the base forwards in the middle line to near the head, where it 

 bifurcates, the divisions passing to the comua on the plantar 

 aspect of the head. The dorsal surface lies between the external 

 and internal borders, and is narrow. Each lateral surface is situ- 

 ated between the lateral and plantar borders. The lateral surfaces, 

 which are extensive and sloped, boiuid the interosseous spaces, and 

 give attachment to the interosseous muscles. The shaft of the first 

 metatarsal has its borders disposed as supero- external, infero- 

 extemal, and internal. The dorsal surface is convex, and is directed 

 upwards and inwards. The plantar surface is concave, and sup- 

 ports the tendon of the flexor longus, and the flexor brevis hallucis. 

 The external surface, which is practically vertical, is narrow in front, 

 but wide behind. 



The heads of the four outer metatarsal bones are much compressed 

 laterally. The cartilage is prolonged more on the plantar than on 

 the dorsal aspect, and in the former situation it ends in a concave 



