122 



THE SKELETON OF THE HOHSE 



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 



Fig. 109. — Right First and Sec- 

 ond (Fused) Tarsal Bones 

 OF Horse; Lateral Surf.\ce. 

 Tl, T2, First and second tarsal 

 bones; 1, 1', articular surface for 

 central tarsal ; 2, facet for third tar- 

 sal; 3, facet for medial small meta- 

 tarsal bone. Separation between 

 two bones (when present) is indi- 

 cated so far as visible by dotted 

 line between 1 and 1'. 



Fig. 110. — Right Third Tarsal 

 Bone of Horse; Proximal 

 Surface. 



1, 2, Facets for central tarsal; 

 3, 4, facets for fourth tarsal; 5, non- 

 articular depression; 6, dorsal ridge. 



Fig. 111. — Right Fourth Tarsal 

 Bone of Horse; Medial Sur- 

 face. 



1, 1', Facets for central tarsal; 

 2, 2' , facets for third tarsal; 3, facet 

 for tibial tarsal; 4, 4', facets for 

 fibular tarsal; 5, facet for large 

 metatarsal bone; 6, groove which 

 concurs with central and third tar- 

 sals in formation of vascular canal 

 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 



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. ^ Also termed the cuneiform magnum s. tertium. 



