Structure of the tarsal bones.—Like all the short bones, those of the tarsus are composed of a mass of spongy tissue surrounded by a thin and papyraceous layer of compact. llence these bones are remarkable for their extreme lightness.
Del:elopement. — In the third month the cartilaginous framework of these bones is already apparent. The largest two begin to ossify before birth; the os calcls commences at from the fifth to the seventh month, by a single point of ossification situate about the middle of the bone rather nearer to its anterior part, and the ossification is not completed till eight or ten years after birth, when another point appears in the posterior part of the bone, and by the extension of it to the first point, which is finished about the fifteenth year, the process is completed. The ossification of the astragalus commences about the sixth month. The cuboid and navi cular begin to ossify immediately after birth by one point each, and the three cuneiform bones are ossified, the internal about the end of the first year, the middle and external about the fourth year.
II. Metatarsus (der Mittelfuss).—This seg ment of the foot is composed of five bones placed parallel to each other in front of the tarsus, with which their posterior extremities are articulated. These bones are distinguished numerically, counting from within outwards ; a distinct interosseous space intervenes between each pair of bones, which in the recent state is filled by muscle. From the arched form of the tarsus, the metatarsus naturally takes a similar arrangement by reason of its articula tion with it, and consequently we observe that it is convex on its dorsal surface and concave on its plantar, The metatarsal hones possess certain general characters in common; they belong to the class of long bones, and consequently each has its shaft and two extremities. The shaft in all is prismatic, slightly curved, convex on the dor sal, concave on the plantar surface; two of the surfaces of the shaft are lateral, and correspond to interosseous spaces ; the third is superior, and corresponds to the dorsum of the foot.
The posterior or tarsal extremity of each metatarsal bone is wedge-shaped, the base of the wedge being on the dorsal aspect. Three
articular facets may be noticed on each, ex cepting the first and fifth. The posterior of these is triangular and plane, articulated with the tarsal bones ; the remaining two are lateral, and adapted to corresponding ones on the metatarsal bones on each side.
The anterior or digital extremity of each metatarsal bone presents an articular head or condyle, flattened upon the sides, oblong from above downwards, and much more extended inferiorly or in the direction of flexion than superiorly or in that of extension. This is articulated with the posterior extremity of the metatarsal phalanx. On each side of the con dyle there is a depression, and behind that an eminence to which the lateral ligament of the metatarso-plialangeal joint is attached.
In addition to the characters above men tioned, there are certain special characters belonging to particular metatarsal bones which enable us to distinguish them from each other.
The first, or metatarsal of the great toe, is distinguished, 1. by its considerable size and its being the shortest of the five bones ; 2. its tarsal extremity is semilunar and concave, and has no lateral articular facet ; 3. its digital extremity has on its plantar portion two con cavities separated by a ridge, with which the sesamoid bones articulate. The second cha racteristic is one NN hich peculiarly distinguishes this bone.
The second is the longest; it extends farther backwards than any of the others, and is lodged in a mortise-shaped cavity formed by the three cuneiform bones.
The fifth has the following characters :-1. it is shorter than the second, third, and fourth ; 2. it has no lateral articular facet on the outer side of its tarsal extremity ; 3. on this same side it is prolonged backwards and outwards into a long pyramidal process, which gives insertion to the tendon of the peroneus brevis. This process being quite subcutaneous, it is a useful guide to surgeons in the partial amputation of the foot at the tarso-metatarsal articulation.