THE FOREARM AS A WHOLE. 



299 



several small openings for the passage of vessels and nerves. Pressure transmitted 

 upward from the hand through the radius would tend to stretch the greater number 

 of the fibres, and thus distribute the strain through both bones. While the radius 



Fig. 313. 



Capsule of wrist-joint 



Head of ulna 



Styloid process of radius 



Capsule of inferior radio-ulnar 

 joint 



Ligament of triangular cartilage 



Triangular cartilage Styloid process of ulna 



Lower end of right radius in supination. 



can hardly be enough displaced to bring this about, it is conceivable that the bones 

 might bend sufficiently to make this action effective. 



The oblique ligament^ (Fig- 311), an inconstant little band, runs downward 

 and outward, partly closing the space above the membrane, from the tubercle of the 



Head of ulna 



Fig. 314. 



Capsule of inferior radio-ulnar joint 



Ligament of triangular cartilage 



Styloid process of ulna 



tyloid process of radius 



Triangular cartilage Capsule of wrist-joint 



Lower end of right radius in pronation. 



ulna to the beginning of the oblique line of the radius. It has been plausibly sug- 

 gested that it represents a part of the flexor longus pollicis muscle. 



THE FOREARM AS A WHOLE, AND ITS INTRINSIC MOVEMENTS. 



The two bones and the ligaments form an apparatus capable of being moved as 

 a whole on either the arm or the hand, and of greatly changing its own shape by 

 the movements of the radius on the ulna. As these latter are theoretically inde- 

 pendent of the position of the forearm with regard to the arm, it is best to consider 

 them here. 



The movement of the radius is a very simple one of rotation on an axis coincid- 

 ing with that of the neck of the bone, and then, owing to the outward bend of the 

 shaft, passing down between the bones and finally through the head of the ulna. 

 The amount of rotation probably rarely exceeds 160". Rotation is limited chiefly by 

 the anterior and posterior radio-ulnar ligaments the former being very tense at the 

 end of supination and the latter at the end of pronation. The oblique ligament 

 limits forced supination. As above stated, it is unlikely that the fibres of the 

 orbicular ligament to the radius become tense during life. The fact that the lower 

 end of the radius swings round the ulna in no way changes the character of the 

 movement. If the radius were throughout in continuation of the axis of the neck, 

 and the ulna enlarged below to support it, rotation on the axis of the neck w^ould 

 not change the position of the bone. The departure of the greater part of the radius 

 from that line necessitates the swinging round of the lower end, but does not aflect 

 the nature of the movement. 



The changes of relative position of the bones during rotation are very important. 

 It must be remembered that when the uhia is held so that the front of the middle of 

 the shaft is horizontal, the head of the radius is in a plane above that of the main 



^ Chorda obliqua. 



