TRAPEZOID. 75 



surface, whicli slopes downwards and backwards, presents on its 

 inferior half a cup-shaped articular facet for the trapezium. 



The cuneiform articulates with four bones — viz., the radius 

 above, the lunar internally, the unciform below, and the trape- 

 zium behind. 



TRAPEZIUM.* 

 (Fig. 16. 4.^ 



Placed immediately behind the cuneiform bone, the trapezium 

 is broad and flattened, and presents two surfaces and a cir- 

 cumference. Both its surfaces are rough, the internal being 

 concave and the exiernal convex, the latter having a deep 

 groove for the passage of one of the tendons of the flexor meta- 

 carpi externus muscle. The circumference is roughened, the 

 superior part receiving the insertion of the tendon of the flexor 

 metacarpi medius, with the second tendon of the externus muscles, 

 while the remaining portion is rough for ligamentous attach- 

 ment. Anteriorly there are two small facets, an upper concave 

 one to articulate with the radius, and a lower convex one articu- 

 lating with the cuneiform bone. As the trapezium is the lever 

 bone of the carpal joint, its development and position are point* 

 of very great importance. 



TRAPEZOID 



(Fig. 16. 7.) 



This, the smallest of all the bones of the carpus, excepting the 

 pisiform, is placed on the lower row on the inside ; it presents 

 four surfaces, a base, and an apex. The superior surface, smooth 

 and rounded from before backwards, articulates with the scaphoid. 

 The inferior surface is smooth, with two united facets, one of 

 which, large, flat, and semicircular, articulates with the inner 

 metacarpal bone, and the other, small, square, and bevelled, with 

 the great metacarpal bone. The internal surface is convex and 

 rough, and the external rough, with the exception of three pro- 

 jecting facets, which articulate with the os magnum. The apex is 

 placed forwards, and the base backwards, both being roughened ; 

 a very small, cup-shaped articulation for the pisiform bone is 

 often found on the posterior part of the base. 



The trapezoid articulates with five bones — viz., the scaphoid 



* See note on page 77. 



