THE FIRST CARPAL BONE — THE SECOND CARPAL BONE 



85 



The First Carpal Bone 

 The first carpal bone (Os carpale primuni, trapezium) is a small inconstant 

 bone, commonly about the size and shape of a pea, which is situated in the lower 

 part of the internal lateral ligament behind the second carpal bone. 



This bone appears to be absent on both sides in about half of the cases; in a good many 

 subjects it is present on one side only. In size it varies from a minute nodule to a discoid mass 

 10 to 12 mm. in length. In exceptional cases it articulates ^dth both the second carpal and the 

 second (inner) metacarpal bone, in other cases with the former onlj-, but in the majority of 

 specimens no articular facet is present. 



The Second Carpal Bone 

 The second carpal bone (Os carpale secundum, trapezoid) is the smallest con- 

 stant bone of the lower row, and is irregularly hemispherical in shape. The su- 

 perior or proximal surface is a convex 

 facet which is continued upon the pos- 

 terior or volar surface and articulates 

 with the posterior ])art of the radial 

 carpal. The external surface faces ob- 

 liquely 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 



% 



Mc. IV 



Mc. Ill 



Fig. 54. — Carpal Boxes of Horse, with Adjacent 

 Ends of Radius .\nd Metacarpus; Anterior 

 View. The Accessory and First Carpal 

 Bones are Not Shown. 



Fig. 



55. — Carpal Articular Surface of Radius 

 and Proximal Articular Surfaces of Car- 

 pal and Metacarpal Bones, Left Side. 

 The Accessory and First Carpal Bones ark 

 not Shown. 

 Cr, Radial carpal; Ci, intermediate carpal; Cu, ulnar carpal; C2, C3, C4, second, third, and fourth car- 

 pals; 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, meta- 

 carpal tuberosity. Arrows indicate relations of facets. Short arrow points to facet on ulnar carpal for articu- 

 lation with accessory carpal. (After Schmaltz, Atlas, d. Anat. d. Pferdes.) 



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. 



