the tarsus 109 



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, situ- 

 ated 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 l)orders, and two extremities. The internal surface faces back- 

 ward and inward, and is convex. Its anterior part is ridged, and gives attach- 

 ment 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. . » „ 



A yiV External 



In some cases the first and second tarsal bones remain V\i^3^%T T' "^'^'^''"' 



separate — a remarkable reversion to the condition in the early iwmm.A. one 



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, y External 



equivalent to the thick anterior part of the bone as described C\ metatarsal 



above, and overlapped by the anterior part of the first tarsal. /^ /|j fto/ig 



The Third Tarsal Bone 

 The third tarsal bone (Os tarsale tertium, third or 

 great cuneiform) resembles the central, but is smaller 



, . . , . ,,. Ti • -i ill, ii Fig. 85. — Cross-sections of 



and triangular in outline. It is situated between the Metacarp.^l and Met.a.- 



central above and the large metatarsal bone below. It tarsal Bones op Horse. 



possesses two surfaces and three borders. y^^Trf^pf^d*^') ^*'^^ ^ 



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. 



