The third and fourth resemble each other very closely ; the third, however, is a little longer than the fourth, and the posterior ar ticular facet on the fourth is more quadrangular than triangular.
The structure of the metatarsal bones is that of the long bones in general.
Developement.—Each metatarsal bone has two points of ossification ; one for the body, the other for the anterior extremity, except in the ease of the first, in which the second ossific point is for the tarsal extremity. Be tween the third and fourth months the osseous point of the body commences, and in the full-developed fmtus the body is completely ossified. In the course of the second year the point for the extremity appears ; the epi physis of the first metatarsal bone is united first, about the eighteenth year, and this union precedes that of the others by about twelve months.
Toes (Digit; pedis ; Fr. les orteil s ; Germ. die Zehen).—The toes are numbered from the inner or great toe; they gradually diminish in length from the first to the fifth : the four outer ones consist each of three portions or phalanges; the great toe has only two. The phalanges are best named from their relations, viz. metatarsal, middle, and ungual.
The metatarsal phalanges are considerably the longest. The shaft in each is prismatic, like that of the metatarsal bones, convex on the dor sal, concave on the plantar surface. On the pos terior extremity is a concave facet, articulated with the anterior head or condyle of the cor responding metatarsal bone. The anterior ex tremity is less swollen than the posterior : it is marked by an articular surface, which ex tends much more on the inferior surface than the superior ; this is concave transversely, but convex from above downwards, and is arti culated with the posterior extremity of the middle phalanx. All the metatarsal phalanges possess these general characters : that of the great toe is very considerably thicker than the others, and is slightly longer; the remaining ones differ but little in size : they progressively diminish towards the fifth.
The middle phalanges are very short, but possess pretty nearly the same general characters as the metatarsal. The posterior extremities are articulated with the last-named phalanges by means of an articular surface, concave from before backwards and convex transversely. The articular surface on the anterior extremity is convex. The great toe is deficient in the middle phalanx ; they diminish in size from within and outwards. They have been com
pared to the pieces of the coccyx, but may be easily distinguished by the articular surfaces.
The ungual phalanges (so called from being next the nail, unguis) are five in number, and decrease in size from the first to the fifth ; that belonging to the first very much exceeding the rest in size. The posterior extremity of each is expanded, and has an articular facet for articulation with the middle phalanx. The central part or shaft is flattened, slightly convex on its dorsal surface : its anterior ex tremity is still more flattened and slightly expanded, presenting a thin convex margin. It is rough on its inferior surface where the dense and adipose cellular tissue constituting the pulp of the toe is connected with it, and on its superior surface it is smooth, where the nail is applied upon it.
The structure and mode of developement of the phalanges are pretty much the same as those of the metatarsal bones : their complete ossification, however, takes place at a much later period.
For the modifications in the number, forms, and arrangement of the bones of the foot in the animal series, see OSSEOUS SYSTEM (Comp. Anat.) and the articles on the various classes.
JoINTS OF TII E rooT.—These may be classed as the joints of the tarsus, metatarsus, and toes.
faints of the tarsus.—The bones constituting the first row of the tarsus are connected to gether by means of two articulations, one posterior, the other anterior. The first (pos terior astragalo -ealeanien articulation) is formed by a convex oval surface on the os calcis, which is received into a deep concavity on the astragalns. A synovial sac lines these surfaces ; the posterior part of this sac is covered by the fatty substance which is placed between the back of the ankle-joint and the tendo Achillis, and on the removal of the fat the sac is observed to be strengthened, especially in its centre, by a few ligamentous fibres. On the inner side this sac is strengthened by the tendon of the flexor pollicis proprius and its sheath behind, and by the internal lateral ligament of the ankle-joint in front; both of which very much protect the articulation and strengthen the union of the bones. Anteriorly there are no proper fibres applied upon the synovial membrane ; but the interosseous liga ment to be described presently, amply supplies the want of them. On the outside a few ligamentous fibres are applied to the synovial membrane.