CHAPTER XIII.
THE FOOT.
Bony structure of the foot.—Union of bones of tarsus; astragalus and calcaneum (canalis tarsi, or tunnel of the tarsus); astragalar articulations, inferior calcaneo-scaphoid ligament; special functions of astragalus in the mechanism of the foot.—The other articulations of the tarsus, and their movements. Metatarsus and Metatarsal bones; importance of fifth metatarsal with regard to form.—The toes and their phalanges.—Skeleton of the foot as a whole; form of the foot.—Plantar arch.—Proportions of the lower limb; the foot as a common measure of the lower limb and the height.
We will now examine briefly the bones of the tarsus, especially with regard to the details which mark their articulations. The inferior surface of the astragalus presents two articular facets, separated by a deep, oblique groove. These two facets correspond with two similarly situated on the upper surface of the calcaneum. The astragalus thus rests upon the calcaneum, but it is supported on the inner side by a projection of the calcaneum, known by the name of the sustentaculum tali; and the anterior facet for the astragalus is to be looked for on the upper surface of this projection (Fig. 53, page 153). The two facets of the calcaneum are also separated by a deep groove. It follows, therefore, that when the astragalus is in its place, the groove of the astragalus and the calcaneum meet and form a kind of tunnel, which is called the cavity of the tarsus, or canalis tarsi. This cavity is filled up during life by a strong ligamentous band, which attaches the astragalus to the calcaneum, and is called the interosseous ligament. Placed between the two calcaneo-astragaloid articulations, one in front of it and the other behind, the ligament forms a sort of pivot, around which the movements between the astragalus and the calcaneum take place. It is round these joints as a centre that the movements take place by which the foot as a whole is turned inwards or outwards, and so that its outer and inner borders are elevated.
We have next to consider the articulations of the calcaneum and astragalus with the other bones of the tarsus. The articulation of the anterior extremity of the calcaneum with the posterior surface of the cuboid presents an interlocked arrangement which is surrounded by strong ligaments, especially on its lower plantar surface (the inferior calcaneo-cuboid ligaments, or long and short plantar ligaments), so that between the calcaneum and the cuboid only a slight gliding movement takes place, and to all intents and purposes these two bones form one elastic piece. The plantar ligaments from their strength have an important influence on the maintenance of the arch of the foot. The cuboid bone articulates in front with the metatarsal bones of the fourth and fifth toes, so helping to complete the outer part of the contour of the foot. On the inner side of the foot the bones completing the tarsus in front of the astragalus are the navicular (or scaphoid), and the three cuneiform bones, internal, middle, and external, which in turn are related to the inner three metatarsal bones. The astragalo-scaphoid articulation deserves the closest attention. The head of the astragalus, so called from its prominent rounded shape, articulates with the navicular bone, which is prolonged internally into a prominent tubercle. Between the sustentaculum tali and this tubercle a very powerful ligament passes, on which a part of the astragalus rests. The ligament is called the inferior calcaneo-scaphoid ligament, or spring ligament, and by its means an articulation is completed between the three bones—the astragalo-calcaneo-scaphoid joint. The whole weight of the body may be supported by the inferior calcaneo-scaphoid ligament, which thus serves an important purpose in supporting the arch of the foot. This is the ligament which gives way in flat-foot. By these articulations the foot possesses the power of lateral movement, by which the toes are carried inwards or outwards, and the outer border of the foot is elevated and the inner border is depressed, or the reverse. On the other hand, the movements of flexion and extension take place chiefly at the ankle-joint, the articulation, previously explained, of the astragalus with the tibia and fibula.
Fig. 54.
Right Foot (internal border).—1, 2, 3, 4, calcaneum;—5, 6, 7, astragalus;—8, posterior calcaneo-astragaloid articulation;—9, head and neck of the astragalus;—10, 11, navicular or scaphoid;—12, astragalo-scaphoid articulation;—13, 14, internal cuneiform;—15, articulation of the internal cuneiform with the scaphoid;—16, the articulation of the internal cuneiform with the metatarsal bone of the great toe;—17, the middle cuneiform;—18, its articulation with the scaphoid;—19, the first metatarsal bone;—20, the second metatarsal bone;—21, the articulation of the second metatarsal bone with the cuneiform bones;—22, sesamoid bone;—23, 24, the phalanges of the great toe; 25, 25, phalanges of the other toes.
The navicular or scaphoid bone articulates in front with the three cuneiform bones, and the cuneiform bones articulate with each other by facets which are flat, but present rough surfaces towards their inferior or plantar portions intended for interosseous ligaments. Similar articulations exist externally between the scaphoid and external cuneiform, and the cuboid bone. It is sufficient to say that in all these articulations a slight gliding movement takes place, just sufficient to give a certain elasticity to the plantar arch, which they contribute to form. We may point out the peculiarities that are to be noted in each of these bones: the scaphoid or navicular bone is concave behind, convex in front, and is prolonged into a prominent tubercle, which can be felt at the middle of the inner border of the foot; the cuboid is quadrilateral or pyramidal in form, and has an oblique groove (14, Fig. 55) on its inferior or plantar surface, which contains the tendon of the long peroneal muscle (see below); finally, the three cuneiform bones are distinguished, counting from within outwards, as the internal, middle, and external cuneiform (Fig. 53, page 153); the middle cuneiform (6) is smaller than the others, and does not extend so far forward, so that in relation to it the line of the tarso-metatarsal joints forms a notch opening into the tarsus, into which the posterior extremity of the second metatarsal bone is received.
In front of the tarsus is found the metatarsus, corresponding to the metacarpus of the hand, while the toes correspond to the fingers. We have only a few words to say about those portions of the skeleton which resemble the corresponding parts of the hand.
Fig. 55.
Right Foot (external border).—1 to 6, the calcaneum;—7, the tibial surface of the astragalus;—8, the lateral facet of the astragalus for the external malleolus;—9, the tunnel of the tarsus (canalis tarsi);—10, the posterior calcaneo-astragaloid articulations;—11, head and neck of the astragalus;—12, the scaphoid;—13, cuboid;—14, commencement of the groove on the inferior surface of the cuboid (for the tendon of the long peroneus muscle);—16, external cuneiform;—17, middle cuneiform;—18, the five metatarsal bones with their posterior extremities (19);—20, 21, 22, 23, fourth, third, second, and first metatarsal bones;—24 to 25, phalanges of the toes.
The metatarsal bones, five in number, are long bones, each composed of a prismatic shaft and two extremities, one posterior or tarsal, more or less wedge-shaped; the other anterior or digital, forming a rounded, globular head which articulates with the base of a phalanx. The metatarsal bones are placed parallel side by side, but that of the great toe is not markedly shorter than its fellows like the metacarpal bone of the thumb, and the great toe does not possess anything like the same amount of mobility as the thumb. It is further directed straight forward so as to complete the straight line of the inner border of the foot. With regard to particular details, it is necessary to note in the first metatarsal (19, Fig. 54) its large size, in the second (22, Fig. 55), its greater length, for it passes behind the others (in entering into the notch which corresponds to the middle cuneiform), and it also exceeds them in length, so that the second toe is as a rule a little longer than the rest. In the fifth we must note the form of its tarsal extremity, prolonged behind into a process (19, Fig. 55) which appears as a prominence at the middle of the outer border of the foot, and gives insertion to the short peroneal muscle. Lastly, it may be observed that all the metatarsal bones except the first are sloped inwards and forwards, to point as it were towards the great toe.
The phalanges of the toes (Figs. 54 and 55) resemble in number and arrangement the corresponding bones of the hand, only they are much shorter, especially those of the little toe, the two last of which are reduced to small osseous nodules, so that the fifth toe is always much shorter than the rest. The names of the phalanges are the same as in the hand (page 93).
The Foot as a whole.—The skeleton of the foot forms an arch which presents two curves or concavities, one antero-posterior, the other transverse. The sole of the foot forms a hollow which extends from the posterior extremity of the calcaneum to the anterior extremity of the metatarsal bones; but this plantar hollow is much more elevated on the inner side (Fig. 54) than on the outer side (Fig. 55). It is necessary, in other words, in order to understand the general form of the foot, to consider that its dorsal surface looks upwards and outwards, and the sole downwards and inwards. The outer border is thin, and comes almost in contact with the ground, its inner border is thick and is raised from the ground.
The skeleton of a well-articulated foot placed upon a horizontal surface comes in contact with the flat surface only by the posterior extremity (tuberosity) of the calcaneum (heel), and by the heads of the metatarsal bones (the balls of the toes, Fig. 52, page 151). When the foot is covered with its soft parts these points of contact are scarcely changed; for, except at the heel and the balls of the toes, we see that the greater part of the outer border of the foot touches the ground but lightly, unless the subject is carrying a heavy load, which, pressing upon the plantar arch, brings its elasticity into play and slightly flattens it. We shall mention later on the special structures (ligaments and tendons) which act as cords which bend the bow and maintain the plantar arch.
Proportions of the Lower Limb.—As we have already inquired into the ratio of proportions of the hand and upper limb, we shall now see if the foot can furnish any measurements relative to the proportions of the body. We find that we can make the foot, no more than the hand, a common measure for the body in general and for the inferior limb in particular. We must confine ourselves to such measurements as will apply to the average subject. Thus it is easy to perceive upon the skeleton that the distance from the upper margin of the head of the femur to the inferior border of the internal condyle is equal to twice the length of the foot; but this has no practical value—it cannot be used on the living body, as it is difficult to recognise the level of the upper part of the head of the femur. If, instead of the head of this bone, we take the superior border of the great trochanter (a part easily felt beneath the skin), we find that the length from the superior border of the great trochanter to the inferior border of the external condyle scarcely ever measures the length of two feet, as the great trochanter is upon a considerably lower level than the head of the femur.
The leg, including the thickness of the foot, is not as much as twice the length of the foot—that is, the distance from the lower border of the internal condyle of the femur to the ground (or the sole of the foot); but it is interesting to observe that in general the length of the leg, plus the thickness of the foot, is equal to the distance from the great trochanter to the lower border of the external condyle; in other words, the centre point of the lower limb (starting from the great trochanter) corresponds exactly to the line of the knee.
When we compare the length of the foot with the leg, beginning from below upwards, we find a regular proportion, and one of practical interest—viz. that the line from the ground to the middle of the patella usually measures twice the length of the foot.
As a common measure of the height of the body, the foot does not give us a result that can be expressed by an even number. From the numerous researches of Leger on this question, the length of the foot is generally contained 6⅓ times in the total height. However, this number presents an interesting fact when we express this proportion by taking the third part of the foot for a unit; 6⅓ feet forming nineteen thirds of the foot, we see that the height of the body contains nineteen thirds of the foot. It is interesting to observe that the number 19 is precisely that which expresses the proportion that the middle finger bears to the height in the Egyptian canon, according to Charles Blanc.
With regard to the foot itself, we need only say that the tarso-metatarsal line offers, on the skeleton, a simple means of dividing the foot. This line is oblique from within outwards and backwards; its inner extremity at the base of the first metatarsal bone divides the foot into an anterior and posterior half, while its outer extremity, at the base of the fifth metatarsal bone, divides it into a posterior one-third and anterior two-thirds.