CARPAL BONES. 85 



THE PISIFORM BONE. 



The pisiform bone, or os rotundum, so called from its resemblance to a 

 pea, lies on a plain anterior to the other bones of the carpus. Posteriorly 

 it possesses an articular surface, which rests on the cuneiform bone. The 

 mass of the bone is so inclined from this surface downwards and outwards, 

 that the pisiform bone of one hand is distinguishable from that of the other. 



Fig. 76. DORSAL VIEW OP THE CARPUS, WITH A 



PORTION OF EACH OF THE METACARPAL BONES. ^ Fig. 76. 



1, scaphoid; 2, serailunar; 3, cuneiform; 4, 

 pisiform; 5, trapezium; 7, trapezoid ; 8, os mag- 

 num ; 9, unciform. 



THE TRAPEZIUM. 



The trapezium, or os multangulum majus, 

 is the most external of the second row of 

 carpal bones. It presents a rhombic form 

 when seen from its dorsal or palmar aspect, 

 and has its most prominent angle directed 

 downwards. Its anterior surface is marked 

 by a vertical groove traversed by the tendon 



of the flexor carpi radialis muscle, and external to the groove by a ridge, 

 or tubercle, one of the four prominences which give attachment to the 

 anterior annular ligament. Of the internal sides of the rhomb, the 

 superior articulates with the scaphoid bone, the inferior with the trapezoid 

 bone, and by a small facet close to the inferior angle also with the second 

 metacarpal bone. Of the external sides the superior is rough, and the 

 inferior presents a smooth surface convex from behind forwards, and concave 

 from without inwards, which articulates with the metacarpal bone of the 

 thumb, and is separated by a rough line at the inferior angle from the surface 

 for the second metacarpal bone. The trapezium articulates with four bones, 

 viz., the scaphoid, trapezoid, and first and second metacarpals. 



THE TRAPEZOID BONE. 



The trapezoid bone, or os multangulum minus, is considerably smaller 

 than the trapezium. Its longest diameter is from before backwards. Its 

 posterior surface, which is pentagonal, is much larger than the anterior. 

 The external inferior angle of the anterior surface is distinguished by being 

 prolonged a little backwards between the trapezium and second metacarpal 

 bone. The superior surface articulates with the scaphoid bone ; the 

 external with the trapezium ; the internal with the os magnum ; and the 

 inferior by a large surface convex from side to side with the second meta- 

 ca.rpal bone. The trapezoid articulates with four bones, viz., the scaphoid, 

 trapezium, os magnum, and second metacarpal bone. 



THE OS MAGNUM. 



The os magnum, or os capitatum, is the largest of the carpal bones. In 

 form it is elongated vertically, rectangular inferiorly, rounded superiorly. 

 The articular surface of the superior extremity or head is prolonged on the 

 outer, but not on the inner side, and is continued further down behind 

 than in front. A neck is formed beneath by depressions on the anterior 

 and posterior surfaces. The anterior surface of the bone is much narrower 

 than the posterior. The posterior surface projects downwards at its 



