IV.] 



SKELETOX OF UPPER LIMB. 



149 



of the humerus, while by a similar joint its margin plays 

 into a concavity on the adjacent surface of the ulna. 



At its lower end the radius becomes much broadened out, 

 and its posterior surface is grooved for the passage of tendons. 

 Its outer side is prolonged into what is called the styloid 

 process. The lower end of the bone articulates with the wrist 

 by a concave surface mainly supporting the hand, which is 

 carried round with the radius in pronation and supination. 



These motions have been explained in the Seventh Lesson 

 of " Physiology/' and are pro- 

 duced by a rotation of the radius 

 on the capitellum. 



When the arm and hand hang 

 down, the palm being directed 

 forwards, the position is that of 

 supination, and the bones of the 

 fore-arm are situate side by side. 



When the arm and hand hang 

 downwards, but the back of the 

 hand is turned forwards, the po- 

 sition is that of pronation, and 

 the radius crosses over the ulna. 



When we rest on the hands 

 and knees, with the palms to 

 the ground, the fore-arms are in 

 pronation. 



6. The ULNA is larger than 

 the radius, and while the latter 

 is broader below than above, the 

 reverse condition obtains in the 

 ulna. The shaft is flattened in 

 front, with a sharp outer (or 

 radial) margin, to which an inter- 

 osseous membrane is attached 

 which connects the ulna with 

 the shaft of the radius. 



Its upper end presents a deep 

 concavity for articulation (by a 

 hinge joint) with the trochlea of 



the humerus. This fossa is called, from its shape, the great 

 sigmoid cavity? and is divided unequally by a vertical ridge 

 which extends between the two processes which bound the 

 fossa above and below respectively. 



1 From 2 and c*3og. 



FIG. 125. FRONT-VIEW OF MAN'S 

 RIGHT RADIUS AND ULNA. 



1. The Radius: /*, the head; /, 

 tubercle ; st, styloid process. 



2. The Ulna: c, coronoid pro- 

 cess ; gs, greater sigmpid cavity ; 

 Is, lesser sigmoid cavity ; o, ole- 

 cranon ; st, styioid process. 



