Part 28

The =fibular tarsal= bone (Os tarsi fibulare, calcaneum, os calcis) is the largest bone of the hock. It is elongated, flattened from side to side, and forms a lever for the muscles which extend the hock joint. It consists of a body and an inner process, the sustentaculum tali.

FIG. 77.—RIGHT TIBIAL TARSAL BONE OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

FIG. 78.—RIGHT TIBIAL TARSAL BONE OF HORSE, EXTERNAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =body= (Corpus calcanei) is enlarged at its proximal end to form the =tuber calcis= or “point of the hock.” The posterior part of this eminence gives attachment to the tendon of the gastrocnemius, while in front and laterally it furnishes insertion to tendons of the flexor perforatus, biceps, and semitendinosus muscles. The =inferior extremity= bears a concave facet for articulation with the fourth tarsal bone. The =internal surface= has on its lower part a strong process, the =sustentaculum tali=, which projects inward. The process has a large, oval, slightly concave facet in front for articulation with the tibial tarsal, and sometimes a small articular surface below for the central tarsal bone. Its plantar surface forms with the smooth inner surface of the body a groove for the deep flexor tendon (Sulcus musculi flexoris hallucis longi). Its inner surface has a prominence on the lower part for the attachment of the lateral ligament. The =external surface= of the body is flattened, except below, where there is a rough prominence for the attachment of the lateral ligament. The =anterior= or =dorsal border= is concave in its length, smooth and rounded in its upper part. About its middle is a blunt-pointed projection (Processus cochlearis) which bears facets on its inner and lower surfaces for articulation with the tibial tarsal bone, and is roughened outwardly for ligamentous attachment. Below this are two facets for the tibial tarsal, and an extensive rough fossa which concurs in the formation of the sinus tarsi. The =posterior= or =plantar border= is straight and thick, and widens a little at either end; it is rough, and gives attachment to the long plantar ligament.

FIG. 79.—RIGHT FIBULAR TARSAL BONE OF HORSE, ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

FIG. 80.—RIGHT FOURTH TARSAL BONE OF HORSE, UPPER SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

FIG. 81.—RIGHT CENTRAL TARSAL BONE OF HORSE, UPPER SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

FIG. 82.—RIGHT FIRST, SECOND, AND THIRD TARSAL BONES OF HORSE, UPPER SURFACE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

FIG. 83.—PROXIMAL ARTICULAR SURFACES OF LEFT METATARSAL BONES OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE CENTRAL TARSAL BONE

The =central tarsal= bone (Os tarsi centrale, scaphoid, or navicular) is irregularly quadrilateral, and is situated between the tibial tarsal above and the third tarsal below. It is flattened from above downward, and may be described as having two surfaces and four borders. The =superior surface= is concave from before backward, and almost all of it articulates with the tibial tarsal; a non-articular depression cuts into its outer part, and sometimes there is a facet for the fibular tarsal bone on the posterior angle. The =inferior surface= is convex, and is crossed by a non-articular groove, which separates facets for articulation with the third and the first and second (fused) tarsals. The =anterior= or =dorsal border= and the =internal border= are continuous, convex, and rough. The =posterior= or =plantar border= bears two prominences, separated by a notch. The =external border= is oblique, and bears anterior and posterior facets for articulation with the fourth tarsal, between which it is excavated and rough.

FIG. 84.—RIGHT TARSAL AND METATARSAL BONES OF HORSE. EXTERNAL VIEW. (After Schmaltz, Atlas d. d. Anat. d. Pferdes.) ]

FIRST AND SECOND TARSAL BONES

The =first= and =second tarsal= bones (Os tarsale primum et secundum, cuneiform parvum) are usually fused in the horse, forming a bone of very irregular shape, situated in the inner and posterior part of the lower row, below the central and behind the third tarsal. It is the smallest of the tarsal bones, and may be described as having two surfaces, two borders, and two extremities. The =internal surface= faces backward and inward, and is convex. Its anterior part is ridged, and gives attachment to the internal lateral ligament, and its posterior part bears an imprint where the inner tendon of the tibialis anterior is inserted. The =external surface= is marked by a deep notch which indicates the division between the first and second tarsal elements; it bears on its anterior part a large concave facet for the central tarsal. The =superior border= is convex. The =inferior border= is broad in front, where it articulates with the large and inner small metatarsal bones. The =anterior extremity= has a small facet for articulation with the third tarsal, and bears internally a ridge or tubercle. The =posterior extremity= is a blunt point.

In some cases the first and second tarsal bones remain separate—a remarkable reversion to the condition in the early ancestors of the horse. In such specimens the first tarsal is a discoid bone, articulating above with the central, below with the small metacarpal bone. The second tarsal is quadrangular, equivalent to the thick anterior part of the bone as described above, and overlapped by the anterior part of the first tarsal.

THE THIRD TARSAL BONE

The =third tarsal= bone (Os tarsale tertium, third or great cuneiform) resembles the central, but is smaller and triangular in outline. It is situated between the central above and the large metatarsal bone below. It possesses two surfaces and three borders.

The =superior surface= is concave, and is crossed by a non-articular depression which divides it into two unequal facets; it articulates with the central tarsal. The =inferior surface= is slightly convex, and rests on the large metatarsal bone; it has an extensive central rough excavation. The =anterior= or =dorsal border= is convex and bears a rounded ridge on its inner part. The =internal border= is deeply notched and has a small facet for the second tarsal on its anterior part. The =external border= is also divided by a notch into two parts, and bears two diagonally opposite facets for articulation with the fourth tarsal. In some cases there is a facet for the inner small metatarsal bone.

FIG. 85.—CROSS-SECTIONS OF METACARPAL AND METATARSAL BONES OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE FOURTH TARSAL BONE

The =fourth tarsal= bone (Os tarsale quartum, cuboid) is the outer bone of the lower row, and is equal in height to the central and third together. It is cuboid in shape and presents six surfaces.

The =superior surface= is convex from side to side, and articulates chiefly with the fibular tarsal, but to a small extent with the tibial tarsal also. The =inferior surface= rests on the large and external small metatarsal bones. The =internal surface= bears four facets for articulation with the central and third tarsal bones. It is crossed from before backward by a smooth groove, which by apposition with the adjacent bones forms the =canal of the tarsus= (Canalis tarsi) for the passage of the perforating tarsal vessels. The =anterior= or =dorsal=, =external=, and =posterior= or =plantar surfaces= are continuous and rough. A tuberosity behind gives attachment to the plantar ligament.

=Development.=—The fibular tarsal bone has two centers of ossification, one for the main mass and the other for the tuber calcis; the latter fuses with the rest of the bone at about three years of age. The first and second tarsals have separate centers, but fusion usually occurs before birth. Each of the other bones ossifies from a single center.

FIG. 86.—DIGITAL BONES OF FORE LIMB OF HORSE, VOLAR ASPECT. ]

FIG. 87.—DIGITAL BONES OF HIND LIMB OF HORSE, VOLAR ASPECT. ]

THE METATARSUS

The =metatarsal= bones (Ossa metatarsalia), three in number, have the same general arrangement as the metacarpal bones, but present some important differences. Their direction is slightly oblique, downward and a little forward.

The =large= or =third metatarsal= bone is about one-sixth longer than the corresponding metacarpal; in an animal of medium size the difference is about two inches. The shaft is more cylindrical, and is almost circular on cross-section, except in its lower part. At the upper part of its external surface there is a groove, which is directed obliquely downward and backward, and is continued by the furrow formed by the apposition of the external metatarsal bone; it indicates the course of the great metatarsal artery. A faint impression in a similar place on the inner side marks the position of the corresponding vein. The nutrient foramen is relatively higher than on the metacarpal bone. The proximal extremity is much wider from before backward than that of the metacarpal bone. Its articular surface is slightly concave, and is marked by a large central non-articular depression, continued outward by a deep notch. The greater part of the surface articulates with the third tarsal, but there is an outer facet for the fourth, and usually a small facet posterointernally for the second tarsal bone. Posteriorly there are two pairs of facets for articulation with the small metatarsal bones. The front is crossed by a rough ridge for insertion, which becomes larger and turns downward on the outer side behind the vascular groove. The distal extremity closely resembles that of the corresponding metacarpal bone.