Illetatarso-phalangcal articulations.—The convex articular surface of the anterior extre mity of each metatarsal bone is adapted to the concave surface on the posterior extremity of each posterior or metatarsal phalanx. A sepa rate synovial membrane lines the articular sur faces of each joint ; and two lateral ligaments, one on either side, maintain the surfaces in apposition. On the dorsal aspect each joint is strengthened and protected by the extensor tendons; and on the plantar a strong, thick, almost cartilaginous substance is extended from the metatarsal bone to the phalanx. This substance protects the joint inferiorly ; it is grooved on its inferior surface, and contributes to form the sheath for the flexor tendou, which runs along the plantar surface of each toe.
The tnetatarso-phalangeal articulation of the great toe presents some points of difference from the others ; its surfaces are snore exten sive, and on the plantar aspect the head of the metatarsal bone has a pulley-like form, from the existence of a ridge in its centre, on either side of which there is a superficial depression : each depression receives a sesamoid bone, which, being formed in the substance of the inferior ligament, thus contributes greatly to strengthen the joint in this situation.
Articulations of the toes.—These are gin glymoid joints, all closely resembling each other both in the forms of the articular surfaces, and also in the bonds of union by which the contiguity of these surfaces is maintained. The articular surfaces are pulley-like; an in ternal and an external lateral ligament belong to each joint ; and the plantar aspect of each is protected by a ligamentous structure similar to that already described in the metatarso-phalan geal joints.
illations of the metatarsal joints.—At the tarsal extremities the metatarsal bones enjoy but a very limited mobility in consequence of the strong and compact manner in which they are articulated with the tarsus ; their motions consist in a very limited and scarcely percepti ble gliding upwards and downwards. At their phalangeal extremities, however, the metatar sal bones are capable of a greater, although still a very limited, degree of motion.
.11fations of the metatarso-phalanral joints. —These are flexion and extension, with a slight degree of lateral inclination or abduction and adduction, and also, of course, circumduction or the rapid succession of the preceding four. The lateral motions are very limited, being most manifest in the joint of the great toe. Flexion is limited by the extensor tendon and the superior fibres of the lateral ligaments ; extension by the inferior fibres of the lateral ligaments, by the inferior ligament, and by the flexor tendon.
illotiuns of the phalangeal joints.—Flexion
and extension only are enjoyed by these joints, the extent of which is principally controlled by the lateral ligaments and by the due anta gonism of the flexor and extensor muscles.
Viewing the human foot as a whole, we cannot frail to notice how admirably it is adapted as an instrument of support, and for the purposes of progression. For the former end the solid and yet elastic mechanism of the tarsus is mainly useful ; this part is placed immediately under the tibia, which transmits the weight of the body to the astragalus, the highest bone of the tarsus; from this bone, again, the weight is transmitted to the os calcis in the backward direction, and to the anterior row of tarsal bones in front, where the transverse extent of the tarsus is consider ably increased, in order to enlarge the basis of support. It is worthy of remark that the solidity of the anterior part of the tarsus is less on its inner than on its outer side, the effect of which is to increase the elasticity of the former part without materially diminishing its strength. The object of this arrangement appears to be ex plained by the observation that the weight of the body is transmitted by the astragalus principally to the inner side of the tarsus. It is toward the inner side also that the concavity of the under surface of the tarsus is most evident, by which not only can the sole of the foot adapt itself to the irregularities of surface to which it is applied, but it is enabled to yield under the superincumbent weight, and so to counteract the effects of sudden concussion in walking, leaping, &c.
In the foot anatomists have described two arches as connected with its mechanical arrange ments. The first is best seen in a profile view of the foot ; it is termed the antero-posterior arch ; upon this arch we rest when the toes are applied to the ground, the posterior extremity of it being the heel, the anterior the balls of the toes, and the astragalus resembling the key stone of the arch. The second is the transverse arch, which may be most satisfactorily demon strated by a transverse section made along the line of the cuneiform bones. The effect of the constant and violent exercises of the foot to which public dancers are accustomed is to in crease the mobility of the different parts of the foot, to an extent which unfits it, in a great measure, for its office as an instrument of sup-. port in standing or walking, as may be ob served, says Sir C. Bell, in any of the retired dancers and old figurantes. By standing so much on the toes, he adds, the human foot is converted to something more resembling that of a quadruped, where the heel never reaches the ground, and where the paw is nothing more than the phalanges of the toes.