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The Phalanges of the Foot (Phalanges Digitorum Pedis).
The phalanges of the foot correspond, in number and general arrangement, with those of the hand; there are two in the great toe, and three in each of the other toes. They differ from them, however, in their size, the bodies being much reduced in length, and, especially in the first row, laterally compressed.
First Row.—The body of each is compressed from side to side, convex above, concave below. The base is concave; and the head presents a trochlear surface for articulation with the second phalanx.
Second Row.—The phalanges of the second row are remarkably small and short, but rather broader than those of the first row.
The ungual phalanges, in form, resemble those of the fingers; but they are smaller and are flattened from above downward; each presents a broad base for articulation with the corresponding bone of the second row, and an expanded distal extremity for the support of the nail and end of the toe.
Articulations.—In the second, third, fourth, and fifth toes the phalanges of the first row articulate behind with the metatarsal bones, and in front with the second phalanges, which in their turn articulate with the first and third: the ungual phalanges articulate with the second.
Ossification of the Bones of the Foot (Fig. 289).—The tarsal bones are each ossified from a tingle center, excepting the calcaneus, which has an epiphysis for its posterior extremity. The centers make their appearance in the following order: calcaneus at the sixth month of fetal life; talus, about the seventh month; cuboid, at the ninth month; third cuneiform, during the first year; first cuneiform, in the third year; second cuneiform and navicular, in the fourth year. The epiphysis for the posterior extremity of the calcaneus appears at the tenth year, and unites with the rest of the bone soon after puberty. The posterior process of the talus is sometimes ossified from a separate center, and may remain distinct from the main mass of the bone, when it is named the oa trigonum.
The metatarsal bones are each ossified from two centers: one for the body, and one for the head, of the second, third, fourth, and fifth metatarsals; one for the body, and one for the base, of the first metatarsal.1 Ossification commences in the center of the body about the ninth week, and extends toward either extremity. The center for the base of the first metatarsal appears about the third year; the centers for the heads of the other bones between the fifth and eighth years; they join the bodies between the eighteenth and twentieth years.
The phalanges are each ossified from two centers: one for the body, and one for the base. The center for the body appears about the tenth week, that for the base between the fourth and tenth years; it joins the body about the eighteenth year.
The hand and foot are constructed on somewhat similar principles, each consisting of a proximal part, the carpus or the tarsus, a middle portion, the metacarpus, or the metatarsus, and a terminal portion, the phalanges. The proximal part consists of a series of more or less cubical bones which allow a slight amount of gliding on one another and are chiefly concerned in distributing forces transmitted to or from the bones of the arm or leg. The middle part is made up of slightly movable long bones which assist the carpus or tarsus in distributing forces and also give greater breadth for the reception of such forces. The separation of the individual bones from one another allows of the attachments of the Interossei and protects the dorsi-palmar and dorsi-plantar vascular anastomoses. The terminal portion is the most movable, and its separate elements enjoy a varied range of movements, the chief of which are flexion and extension.
The function of the hand and foot are, however, very different, and the general similarity between them is greatly modified to meet these requirements. Thus the foot forms a firm basis of support for the body in the erect posture, and is therefore more solidly built up and its component parts are less movable on each other than those of the hand. In the case of the phalanges the difference is readily noticeable; those of the foot are smaller and their movements are more limited than those of the hand. Very much more marked is the difference between the metacarpal bone of the thumb and the metatarsal bone of the great toe. The metacarpal bone of the thumb is constructed to permit of great mobility, is directed at an acute angle from that of the index finger, and is capable of a considerable range
1 As was noted in the 6rat metacarpal (see footnote, page 231), so in the first metatarsal, there ia often a second epiphysis for its head.
Comparison of the Bones of the Hand and Foot.
of movements at its articulation with the carpus. The metatarsal bone of the great toe assists in supporting the weight of the body, is constructed with great solidity, lies parallel with the other metatarsals, and has a very limited degree of mobility. The carpus is small in proportion to the rest of the hand, is placed in line with the forearm, and forms a transverse arch, the concavity of which constitutes a bed for the Flexor tendons and the palmar vessels and nerves. The tarsus forms a considerable part of the foot, and is placed at right angles to the leg, a position which is almost peculiar to man, and has relation to his erect posture. In order to allow of their supporting the weight of the body with the least expenditure of material the tarsus and a part of the metatarsus are constructed in a series of arches (Figs. 290, 291), the disposition of which will be considered after the articulations of the foot have been described.
The Sesamoid Bones (Ossa Sesamoidea).
Sesamoid bones are small more or less rounded masses embedded in certain tendons and usually related to joint surfaces. Their functions probably are to modify pressure, to diminish friction, and occasionally to alter the direction of a muscle pull. That they are not developed to meet certain physical requirements in the adult is evidenced by the fact that they are present as cartilaginous nodules in the fetus, and in greater numbers than in the adult. They must be regarded, according to Thilenius, as integral parts of the skeleton phylogenetically inherited.1 Physical necessities probably come into play in selecting and in regulating the degree of development of the original cartilaginous nodules. Nevertheless, irregular nodules of bone may appear as the result of intermittent pressure in certain regions, e. g., the "rider's bone," which is occasionally developed in the Adductor muscles of the thigh.
Sesamoid bones are invested by the fibrous tissue of the tendons, except on the surfaces in contact with the parts over which they glide, where they present smooth articular facets.
In the upper extremity the sesamoid bones of the joints are found only on the palmar surface of the hand. Two, of which the medial is the the larger, are constant at the metacarpophalangeal joint of the thumb; one is frequently present in the corresponding joint of the little finger, and one (or two) in the same joint of the index finger. Sesamoid bones are also found occasionally at the metacarpophalangeal joints of the middle and ring fingers, at the interphalangeal joint of the thumb and at the distal interphalangeal joint of the index finger.
In the lower extremity the largest sesamoid bone of the joints is the patella, developed in the tendon of the Quadriceps femoris. On the plantar aspect of the foot, two, of which the medial is the larger, are always present at the metatar
1 Morpholog. Arbeiten, 1906, v, 309.
sophalangeal joint of the great toe; one sometimes at the metatarsophalangeal joints of the second and fifth toes, one occasionally at the corresponding joint of the third and fourth toes, and one at the interphalangeal joint of the great toe.
Sesamoid bones apart from joints are seldom found in the tendons of the upper limb; one is sometimes seen in the tendon of the Biceps brachii opposite the radial tuberosity. They are, however, present in several of the tendons of the lower limb, viz., one in the tendon of the Peronseus longus, where it glides on the cuboid; one, appearing late in life, in the tendon of the Tibialis anterior, opposite the smooth facet of the first cuneiform bone; one in the tendon of the Tibialis posterior, opposite the medial side of the head of the talus; one in the lateral head of the Gastrocnemius, behind the lateral condyle of the femur; and one in the tendon of the Psoas major, where it glides over the pubis. Sesamoid bones are found occasionally in the tendon of the Glutseus maximus, as it passes over the greater trochanter, and in the tendons which wind around the medial and lateral malleoli.
THE bones of the skeleton are joined to one another at different parts of their surfaces, and such connections are termed Joints or Articulations. Where the joints are immovable, as in the articulations between practically all the bones of the skull, the adjacent margins of the bones are almost in contact, being separated merely by a thin layer of fibrous membrane, named the sutural ligament. In certain regions at the base of the skull this fibrous membrane is replaced by a layer of cartilage. WTiere slight movement combined with great strength is required, the osseous surfaces are united by tough and elastic fibrocartilages, as in the joints between the vertebral bodies, and in the interpubic articulation. In the f reely movable joints the surfaces are completely separated; the bones forming the articulation are expanded for greater convenience of mutual connection, covered by cartilage and enveloped by capsules of fibrous tissue. The cells lining the interior of the fibrous capsule form an imperfect membrane—the synovial membrane—which secretes a lubricating fluid. The joints are strengthened by strong fibrous bands called ligaments, which extend between the bones forming the joint.
Bone.—Bone constitutes the fundamental element of all the joints. In the long bones, the extremities are the parts which form the articulations; they are generally somewhat enlarged; and consist of spongy cancellous tissue with a thin coating of compact substance. In the flat bones, the articulations usually take place at the edges; and in the short bones at various parts of their surfaces. The layer of compact bone which forms the joint surface, and to which the articular cartilage is attached, is called the articular lamella. It differs from ordinary bone tissue in that it contains no Haversian canals, and its lacunae are larger and have no canaliculi. The vessels of the cancellous tissue, as they approach the articular lamella, turn back in loops, and do not perforate it; this layer is consequently denser and firmer than ordinary bone, and is evidently designed to form an unyielding support for the articular cartilage.
Cartilage.—Cartilage is a non-vascular structure which is found in various parts of the body—in adult life chiefly in the joints, in the parietes of the thorax, and in various tubes, such as the trachea and bronchi, nose, and ears, which require to be kept permanently open. In the fetus, at an early period, the greater part of the skeleton is cartilaginous; as this cartilage is afterward replaced by bone, it is called temporary, in contradistinction to that which remains unossified during the whole of life, and is called permanent.
Cartilage is divided, according to its minute structure, into hyaline cartilage, white fibrocartilage, and yellow or elastic fibrocartilage.
Hyaline Cartilage.—Hyaline cartilage consists of a gristly mass of a firm consistence, but of considerable elasticity and pearly bluish color. Except where it coats the articular ends of bones, it is covered externally by a fibrous membrane, the perichondrium, from the vessels of which it imbibes its nutritive fluids, being itself destitute of bloodvessels. It contains no nerves. Its intimate structure is very simple. If a thin slice be examined under the microscope, it will be found to consist of cells of a rounded or bluntly angular form, lying in groups of two or more in a granular or almost homogeneous matrix (Fig. 292). The cells, when arranged in groups of two or more, have generally straight outlines where they are in contact