122 THE SKELETON OF THE HORSE 
FIRST AND SECOND TARSAL BONES 
The first and second tarsal bones (Os tarsale primum et secundum)! are usually 
fused in the horse, forming a bone of very irregular shape, situated in the medio- 
plantar part of the distal row, below the central and behind the third tarsal. It is 
the smallest of the tarsal bones, and may be described as having four surfaces and 
two extremities. The medial surface faces backward and inward, and is convex. 
Its anterior part is ridged, and gives attachment to the medial ligament, and its 
posterior part bears an imprint where the medial tendon of the tibialis anterior is 
inserted. The lateral surface is marked by a deep notch which indicates the divi- 
sion between the first and second tarsal elements; it bears on its anterior part a 
facet for the third tarsal. The proximal surface is concave and has two facets for 
articulation with the central tarsal; it is separated from the medial surface by a 
prominent border. The distal surface is broad in front, where it articulates with 
Fic. 109.—Ricuar Frrst anp Sxc- Fre. 110.—Ricur Turrp Tarsanu Fie. 111.—Rieur Fourtse Tarsab 
onp (Fusep) TarsaLt Bones Bonet or Horse; PROXIMAL Bone or Horse; Mepian Sur- 
or Horse; LATERAL SURFACE. SURFACE. FACE. 
T1, T2, First and second tarsal 1, 2, Facets for central tarsal; 1, 1’, Facets for central tarsal; 
bones; 1, 1’, articular surface for 3, 4, facets for fourth tarsal; 5, non- 2, 2’, facets for third tarsal; 3, facet 
central tarsal; 2, facet for third tar- articular depression; 6, dorsal ridge. for tibial tarsal; 4, 4’, facets for 
sal; 3, facet for medial small meta- fibular tarsal; 5, facet for large 
tarsal bone. Separation between metatarsal bone; 6, groove which 
two bones (when present) is indi- concurs with central and third tar- 
cated so far as visible by dotted sals in formation of vascular canal 
line between 1 and 1’. of tarsus. 
the large and medial small metatarsal bones. The dorsal extremity bears a ridge 
or tubercle. The plantar 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 in part by the first tarsal. 
THE THIRD TARSAL BONE 
2 
The third tarsal bone (Os tarsale tertium)? 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 proximal 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 distal surface is slightly convex, and rests on the large metatarsal bone; it 
has an extensive central rough excavation. The dorsal border is convex and bears 
a rounded ridge on its medial part. The medial border is deeply notched and has 
a small facet for the second tarsal on its anterior part. The lateral border is also 
divided by a notch into two parts, and bears two diagonally opposite facets for 
1 Also termed the cuneiform parvum. 2 Also termed the cuneiform magnum s. tertium. 
