266 THE SKELETON. 



Bones of the Lower Row (Figs. 204, 205). 



The Trapezium (rpdxe^a, a table) is of very irregular form. It may be distin- 

 guished by a deep groove, for the tendon of the Flexor carpi radialis muscle. 

 It is situated at the external and inferior part of the carpus, between the scaphoid 

 and first metacarpal bone. The superior surface, concave and smooth, is directed 

 upward and inward, and articulates with the scaphoid. The inferior surface, 

 directed downward and inward, is oval, concave from side to side, convex from 

 before backward, so as to form a saddle-shaped surface, for articulation with the 

 base of the first metacarpal bone. The anterior or palmar surface is narrow and 

 rough. At its upper part is a deep groove running from above obliquely down- 

 ward and inward ; it transmits the tendon of the Flexor carpi radialis, and is 

 bounded externally by a prominent ridge, the oblique ridge 'of the trapezium. 

 This surface gives attachment to the Abductor pollicis, Flexor ossis metacarpi 

 pollicis, and Flexor brevis pollicis muscles, and the anterior annular ligament. 

 The posterior or dorsal surface is rough. The external surface is also broad and 

 rough, for the attachment of ligaments. The internal surface presents two 

 articular facets : the upper one, large and concave, articulates with the trapezoid ; 

 the lower one, narrow and flattened, with the base of the second metacarpal 

 bone. 



Hold the bone with the saddle-shaped surface downward and the grooved 

 surface away from you. The prominent, rough, non-articular surface points to 

 the side to which the bone belongs. 



Articulations. With four bones : the scaphoid above, the trapezoid and second 

 metacarpal bones internally, the first metacarpal below. 



Attachment of Muscles. Abductor pollicis, Flexor ossis metacarpi pollicis, and 

 part of the Flexor brevis pollicis. 



The Trapezoid is the smallest bone in the second row. It may be known by 

 its wedge-shaped form, the broad end of the wedge forming the dorsal, the narrow 

 end the palmar, surface, and by its having four articular surfaces touching each 

 other and separated by sharp edges. The superior surface, quadrilateral in form, 

 smooth, and slightly concave, articulates with the scaphoid. The inferior surface 

 articulates with the upper end of the second metacarpal bone ; it is convex from 

 side to side, concave from before backward, and subdivided by an elevated ridge 

 into two unequal lateral facets. The posterior or dorsal and anterior or palmar 

 surfaces are rough, for the attachment of ligaments, the former being the larger 

 of the two. The external surface, convex and smooth, articulates with the 

 trapezium. The internal surface is concave and smooth in front, for articulation 

 with the os magnum ; rough behind, for the attachment of an interosseous 

 ligament. 



Hold the bone with the larger, non-articular surface toward you, and the 

 smooth, quadrilateral articular surface upward. The convex, articular surface 

 will point to the side to which the bone belongs. 1 



Articulations. With four bones : the scaphoid above, second metacarpal bone 

 below, trapezium externally, os magnum internally. 



The Os Magnum is the largest bone of the carpus, and occupies the centre of 

 the wrist. It presents, above, a rounded portion or head, which is received into 

 the concavity formed by the scaphoid and semilunar bones ; a constricted portion 

 or neck ; and, below, the body. The superior surface is rounded, smooth, and 

 articulates with the semilunar. The inferior surface is divided by two ridges into 

 three facets, for articulation with the second, third, and fourth metacarpal bones, 

 that for the third (the middle facet) being the largest of the three. The posterior 

 or dorsal surface is broad and rough ; the anterior or palmar, narrow, rounded, 

 and also rough, for the attachment of ligaments. The external surface articulates 



1 Occasionally in a badly marked bone there is some difficulty in ascertaining to which sido the 

 bone belongs ; the following method will sometimes be found useful : Hold the bone with its broader, 

 non-articular surface upward, so that its sloping border is directed toward you. The border will slope 

 to the side to which the bone belongs. 



