TRAPEZOID. 75 



posterior surface, which slopes downwards and backwards, pre- 

 sents 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 circum- 

 ference. Both its surfaces are rough, the internal being concave 

 and the external convex, the latter having a deep groove for the 

 passage of one of the tendons of the flexor metacarpi 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 attachment. 

 Anteriorly there are two small facets, an upper concave one to 

 articulate with the radius, and a lower convex one articulating 

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

 the carpal joint, its development and position are points of very 

 great importance. 



TRAPEZOID. 



(Fig. 16. 7.) 



This, the smallest of all the hones 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 surf ace, 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 

 sometimes found on the posterior part of the base. 



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

 * See note on page 77. 



