THE FIRST CARPAL BONE THE SECOND CARPAL BONE 



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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 prirnum)! is a small inconstant bone, com- 

 monly about the size and shape of a pea, which is situated in the distal part of 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. 



Fig. 71. — Left Carpus of Hohse, with Parts of Adjacen-t Bones; Lateral View. 

 a. Intermediate carpal; Cu, ulnar carpal; Ca, accessory carpal; C3, third carpal; CJ,, fourth carpal; McIII, 

 Mcl}, metacarpal bones; 1, groove for common extensor tendon; 2, groove for lateral extensor tendon; 3, groove 

 for long 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. 



