The =second carpal= bone (Os carpale secundum, trapezoid) is the
smallest constant bone of the lower row, and is irregularly
hemispherical in shape. The =superior= or =proximal surface= is a convex
facet which is continued upon the =posterior= or =volar surface= and
articulates with the posterior part of the radial carpal. The =external
surface= faces obliquely outward and forward, and bears three facets for
articulation with the third carpal bone. The =anterior= or =dorsal= and
the =internal surface= bear a tuberosity to which the lateral ligament
is attached. The =inferior= or =distal surface= is articular and
consists of a large flattened facet for the inner (second) metacarpal
bone, and a small one for the large (third) metacarpal bone. Some
specimens have a small facet on the lower part of the posterior surface
which articulates with the first carpal bone.
[Illustration:
FIG. 54.—CARPAL BONES OF HORSE, WITH ADJACENT ENDS OF RADIUS AND
METACARPUS; ANTERIOR VIEW. THE ACCESSORY AND FIRST CARPAL BONES ARE
NOT SHOWN.
]
[Illustration:
FIG. 55.—CARPAL ARTICULAR SURFACE OF RADIUS AND PROXIMAL ARTICULAR
SURFACES OF CARPAL AND METACARPAL BONES, LEFT SIDE. THE ACCESSORY
AND FIRST CARPAL BONES ARE NOT SHOWN.
_Cr_, Radial carpal; _Ci_, intermediate carpal; _Cu_, ulnar carpal;
_C2_, _C3_, _C4_, second, third, and fourth carpals; _Mc.II_, second
or inner small metacarpal bone; _Mc.III_, third or large metacarpal
bone; _Mc.IV_, fourth or outer small metacarpal bone; _1_, _2_,
grooves for tendons of anterior extensor and extensor carpi
radialis; _3_, metacarpal tuberosity. Arrows indicate relations of
facets. Short arrow points to facet on ulnar carpal for articulation
with accessory carpal. (After Schmaltz, Atlas. d. Anat. d. Pferdes.)
]