THE FIRST CARPAL BONE—THE SECOND CARPAL BONE 95 
The accessory does not directly bear weight, and may be regarded as a sesamoid bone 
interposed in the course of the tendons of the middle and lateral flexors of the carpus, which it 
enables to act at a mechanical advantage. The posterior border furnishes attachment to the 
transverse carpal ligament, which completes the carpal canal for the flexors of the digit. 
THE First CARPAL BONE 
The first carpal bone (Os carpale primum)! is a small inconstant bone, com- 
monly about the size and shape of a pea, which is situated in the distal part ot the 
medial ligament of the carpus, 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 or cylin- 
drical mass 12-15 mm. in length. In exceptional cases it articulates with both the second carpal 
and the second metacarpal bone; in other cases with the former only, but in the majority of speci- 
mens no articular facet is present. 
Ci 
MclV 
MelII 
Fic. 71—Lerr Carpvus or Horse, with Parts oF ADJACENT Bones; LATERAL VIEW. 
Ci, Intermediate carpal; Cu, ulnar carpal; Ca, accessory carpal; C3, third carpal; C4, fourth carpal; MceJIT, 
McIV, metacarpal bones; 1, groove for common extensor tendon; 2, groove for lateral extensor tendon; 3, groove 
for iong tendon of ulnaris lateralis; 4, metacarpal tuberosity; 5, original distal end of ulna, which is fused with the 
radius and regarded as part of the latter. 
THE SECOND CARPAL BONE 
The second carpal bone (Os carpale secundum)? is the smallest constant bone 
of the distal row, and is irregularly hemispherical in shape. The proximal surface 
is a convex facet, which is continued upon the volar surface and articulates with the 
radial carpal. The lateral surface faces obliquely outward and forward, and bears 
1 Also known as the trapezium, and often erroneously called the pisiform. 
? Also known as the trapezoid. 
