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tarsus determines the side-to-side roundness of the instep, whilst its plantar surface forms arches in both a transverse and a longitudinal direction, in which the softer tissues of the sole are lodged, and so protected from injury.
Ossification. Unlike the carpus, the tarsus is at birth partially ossified. At this period there is a well-marked osseous nucleus within the body and neck of the talus, and the calcaneus is extensively ossified. In the latter the deposition of earthy matter appears as early as the sixth month of foetal life, whilst in the talus the ossific centre makes its appearance in the later weeks of gestation. Shortly before or after birth the
cuboid begins to ossify, succeeded early in the first year by the third cuneiform, followed in order by the second cuneiform, first cuneiform, and navicular. The ossific centre of the latter appears at the third year or somewhat later. An epiphysis, which forms a cap over the extremity of the tuberosity of the calcaneus, appears from the seventh to the ninth year, and fusion is completed between the ages of sixteen and twenty.
To emphasize the different conditions which obtain in the wrist and ankle, at, and for some time after birth, drawings of radiographs of both are given.
The metatarsal bones, five in number, in their general configuration resemble the metacarpal bones. They are, however, slightly longer, their bases are proportionately larger, their bodies are more slender and are compressed from side to side, and their heads are proportionately smaller. They are named numerically the first, second, third, fourth, and fifth metatarsal bones, in order from the tibial to the fibular side. The first can be readily recognised on account of its stoutness; it is also the shortest of the series. The second is the longest of the five, and the fifth can easily be distinguished by the projecting tubercle at its base.
The first metatarsal, or metatarsal bone of the great toe, the shortest of the series, is remarkable for its stoutness. Its proximal end or base, where the bone
GROOVES FOR SESAMOID BONES
is provided with a reniform facet for articulation with the first cuneiform, is wider from the dorsal to the plantar aspect than from side to side. The concavity of the kidney-shaped articular area is directed to the fibular side. rule the lateral aspects of the base are non-articular, though occasionally on its lateral side there is a "pressure" facet for the base of the second metatarsal bone. The plantar basal angle projects proximally and laterally, and forms a prominent tubercle which is pitted for the insertion of the tendon of the peronæus longus muscle, whilst its tibial margin is lipped by a surface for the attachment of part of the tendon of the tibialis anterior. The body, short, thick, and prismatic on section, tapers rapidly towards the head, the distal and plantar surfaces of which are articular. The former is convex in both directions, and supports the proximal or first phalanx. It is confluent with the plantar articular surface, which is divided by a median ridge into two shallow grooves, of which the tibial is the wider. In these grooves are lodged the two sesamoid bones of the metatarsophalangeal joint. On either side of the head, the bone is pitted for the strong collateral ligaments of the joint. The second metatarsal, the longest of the series, has a base of wedge-shaped
PRESSURE FACET FOR
TUBEROSITY Tibialis anterior
FIG. 273. THE FIRST METATARSAL
FIG. 274.-VIEW OF THE BASES AND SHAFTS OF THE SECOND, THIRD, AND FOURTH METATARSAL BONES OF THE RIGHT FOOT.
form, the proximal aspect of which articulates with the second cuneiform. its tibial aspect, near its dorsal edge there is a small circular facet for the first
cuneiform; distal to this and near the plantar surface there is sometimes a tubercle with a "pressure" facet on it, where the bone comes in contact with the base of the first metatarsal. On the fibular side of the base there is one, more usually two small facets, each divided into two parts, a proximal for articulation with the third cuneiform, and a distal for the base of the third metatarsal. The bodies of this and the succeeding three bones are slender and compressed from side to side. The heads are small and narrow, and display a pronounced side-to-side and vertical convexity.
The third metatarsal bone also possesses a base of wedge-shaped form, the proximal surface of which articulates with the third cuneiform. On its tibial side it is provided with one, more usually two, small facets, for articulation with the base of the second metatarsal. Laterally the base has a larger facet for articulation with the base of the fourth metatarsal, more or less conical in outline, and having its plantar edge sharply defined by a narrow groove which underlies it.
The fourth metatarsal possesses a base more cubical in shape. Its proximal aspect articulates with the cuboid, whilst medially an elongated oval facet, divided by a slight vertical ridge, provides surfaces for articulation with the third metatarsal distally and the lateral side of the first cuneiform proximally. On the lateral side there is a demi-oval facet, bearing a slightly saddleshaped surface, for articulation with the tibial side of the base of the fifth metatarsal.
The fifth metatarsal can be readily recognised by the peculiar shape of its base, from the lateral side of which there projects proximally and laterally a prominent tuberosity (tuberositas ossis metacarpi quinti). To the posterior extremity of this is attached the tendon of the peronæus brevis muscle. Into its dorsal surface the tendon of the peronæus tertius is inserted, whilst its plantar surface provides an origin for the flexor digiti quinti brevis muscle. The medial surface of the base is provided with a demi-oval, slightly concave facet, for the lateral side of the base of the fourth metatarsal, whilst proximally it articulates with the cuboid by means of a semicircular facet.
Vascular Foramina.-The canals for the nutrient vessels open, FIG. 275.-FIFTH RIGHT
as a rule, on the plantar aspects of the middle of the bodies. Those of the lateral four metatarsals are directed towards the bases of the bones, whilst that for the metatarsal of the great toe passes towards its head.
Ossification. In correspondence with the mode of ossification which obtains in the metacarpus, the primary centres for the metatarsus appear as early as the third month of foetal life. In the case of the second, third, fourth, and fifth, these centres furnish the bases and bodies of the bones, the heads being developed from secondary centres which appear from two to four years after birth, fusion with the body being usually completed about the eighteenth year. In striking contrast to this is the mode of ossification of the first metatarsal. From its primary centre the head and body is developed; the secondary centre appears at its base about the second or third year, and fuses with the body about eighteen. In this respect, therefore, the metatarsal bone of the great toe resembles the phalanges in its mode of development. Mayet, however (Bull. Soc. Anat. Paris, 1895), describes the occurrence of two ossific centres in the proximal epiphysis. These fuse early, and he considers that the one represents the metatarsal element, whilst the other may be regarded as phalangeal in its origin.
Phalanges Digitorum Pedis.
The phalanges of the toes differ from those of the fingers in the striking reduction of their size, and in the case of the bones of the first row, in the compression of their bodies from side to side. Each toe is provided normally with three phalanges, except the great toe, which has only two. In their general
configuration and in the arrangement of their articular facets they resemble the phalanges of the fingers, though owing to the reduction in their size, the bodies, particularly those of the second row, are often so compressed longitudinThe
FIG. 276.-THE PHALANGES
ally as to reduce the bone to a mere nodule. proximal end of each of the bones of the first row is proportionately large, and is provided with a simple hollow in which the head of the metatarsal bone rests; the distal ends are furnished with condyloid surfaces. The proximal extremities of the second row are each provided with two small concavities, separated by a slight ridge, for articulation with the condyles of the first row. The joint between the second and third row displays the same arrangement-the third, or ungual phalanx, being easily distinguished by the spatula-shaped surface at its extremity, on which the bed of the nail is supported.
The phalanges of the great toe, two in number, differ from the others in their size and length. Into the base of the first phalanx are inserted the short muscles of the great toe, whilst the second phalanx receives on its plantar aspect the insertion of the flexor hallucis longus muscle, the tendon of the extensor hallucis longus being inserted into the dorsal aspect.
Ossification. Each phalanx is developed from two centres one primary for the body and distal extremity, the other for the epiphysis on the proximal end. The primary centres for the ungual phalanges are the first to appear, commencing to ossify from the eleventh to the twelfth week of foetal life. The centre for the ungual phalanx of the great toe makes its appearance before that of its corresponding metatarsal bone. The primary centres for the phalanges of the first row appear from the fourteenth to the sixteenth week. The primary centres for the middle phalanges of the second and
A. About the end of the third month.
B. A little later. The centres for the terminal phalanges of the medial four toes are
C. About the fourth month. The centres for all the terminal phalanges as well as
D. About the fifth month. In this the centre for the second phalanx of the second toe has
1 This specimen displays the occurrence of anomalous centres within the tarsus the significance of
third toes begin to ossify about the sixth month, those for the fourth and fifth toes not till later the body of the middle phalanx of the fourth toe being frequently cartilaginous at birth, the normal condition in the case of the fifth toe (Lambertz). The proximal epiphyses do not begin to ossify until about the fourth year, and are usually fused with the diaphyses about the age of sixteen or eighteen. Union between the bodies and epiphyses of the first row precedes that of the second and third rows.
As in the hand, small independent nodules of bone, called sesamoid bones, are met with in the ligaments and tendons of the foot. The most constant of these are found in connexion with the metatarso-phalangeal articulation of the great toe, where they lie in grooves on the plantar surface of the head of the metatarsal bone in connexion with the tendons of the short muscles of the great toe. Small osseous nodules occupying a corresponding position are occasionally met with in the other toes, and instances have been recorded of like ossicles occurring on the plantar aspect of the interphalangeal joint of the great toe.
An osseous nodule is not infrequently met with in the tendon of the peronæus longus as it turns round the lateral border of the foot to lie in the groove on the under surface of the cuboid.