THE FIRST CARPAL BONE — THE SECOND CARPAL BONE 



85 



The First Carpal Bone 

 The first carpal bone (Os carpale primum, 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 aliout 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 with l)oth the second carpal and the 

 second (inner) metacarpal bone, in other cases with the former only, 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- 2 I 

 terior or volar surface and articulates 

 with the posterior part 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 j||j^j|j^ __ , , ,^ 



tuberosity to which the lateral ligament 

 attached. The inferior or distal 



IS 



c3 



■J 



Mc. IV 



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 are 

 NOT Shown. 

 Cr, Radial carpal; Ct, intermefliate 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 exten.sor 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, (.\fter Schmaltz, Atlas, d. Anat. d. Pferdes.) 



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

 Ends of Radius .and Metacarpus; Anterior 

 View. The Accessory and First Carp.\l 

 Bones .are Not Shown. 



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. 



