The Tarsus

The tarsus or hock of the horse usually comprises six short bones (Ossa tarsi), but exceptionally seven are present.

The Tibial Tarsal Bone

The tibial tarsal bone (Os tarsi tibiale, astragalus, or talus) is the inner bone of the proximal row. It is extremely irregular in form, but may be considered as offering six surfaces for description.

The superior and the anterior or dorsal surface are continuous, and form a trochlea for articulation with the distal end of the tibia. The trochlea consists of two oblique ridges with a deep groove between them, which curve spirally forward, downward, and outward. There is usually a shallow synovial fossa in the groove. The inferior surface is convex from before backward, and most of it articulates with the central tarsal; externally it has an oblique facet for the fourth tarsal, and a non-articular groove cuts into the surface to its middle. The posterior or plantar surface is oblique and extremely irregular; it presents four facets for articulation with the fibular tarsal bone; the facets are separated by rough excavated areas, and the largest fossa (Sulcus tali) forms with a corresponding one on the fibular tarsal a cavity termed the sinus tarsi. The internal surface bears on its lower part a large tuberosity, and on its upper part a small one for the attachment of the internal lateral ligament. The external surface is smaller than the internal, and is marked by a wide rough fossa in which the external ligament is attached.

The Fibular Tarsal Bone

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.

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.

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.

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.

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.