96 THE SKELETON OF THE HORSE 
three facets for articulation with the third carpal bone. The dorsal and 
medial surfaces are continuous and bear a tuberosity to which the collateral liga- 
ment is attached. The distal surface is articular and consists of a large flattened 
facet for the second or inner metacarpal bone, and a small one for the third or large 
metacarpal bone. Some specimens have a small facet on the lower part of the volar 
surface which articulates with the first carpal bone. 
THE THIRD CARPAL BONE 
The third carpal bone (Os carpale tertium)! is much the largest bone of the 
distal row, forming more than 
two-thirds of the width of the 
latter. It is flattened from 
above downward, and is twice as 
wide in front as behind. The 
proximal surface consists of two 
facets separated by an antero-pos- 
terior ridge; the medial facet is 
concave and articulates with the 
radial carpal; the lateral facet— 
for the intermediate carpal—is 
concave in front and convex be- 
hind, where it encroaches on the 
volar surface. The distal surface 
is slightly undulating, and articu- 
lates almost entirely with the third 
or large metacarpal bone, but it 
usually bears a small oblique facet 
at its medial side for the second 
metacarpal, and there is commonly 
a non-articular depression later- 
ally. The medial surface faces 
backward and inward, and bears 
three facets for articulation with 
the second carpal, between which 
it is excavated and rough. The 
lateral surface has two facets for 
articulation with the fourth car- 
Fic, 72—Lerr Carptus or Horse, with Parts oF ADJACENT q 
Bones; -Menie Winw: pal, and is depressed and rough 
Cr, Radial carpal; Ca, accessory carpal; CJ, first carpal; in its middle. The dorsal sur- 
C2, second carpal; McII, McIII, metacarpal bones; 1, tuber- face is convex and is_ erossed 
cy of radi fr aiachent of medal Hexneal of cau? by a rough transverse nilee 
val; 4, metacarpal tuberosity. volar surface is relatively small, 
and is rounded; its upper part 
is encroached upon by the proximal articular surface, below which it is rough. 
a 
Mcll 
MclII 
THE FOURTH CARPAL BONE 
The fourth carpal bone (Os carpale quartum)? is somewhat wedge-shaped, and 
is readily distinguished from the second by its greater size and its volar tubercle. 
The proximal surface articulates with the intermediate and ulnar; it is convex and 
curves outward, backward, and downward, encroaching on the lateral and volar sur- 
faces. The distal surface bears two medial facets for the third or large metacarpal 
1 Also known as the os magnum or capitatum. 
2 Also known as the unciform or hamatum. 
