524 



SYNDESMOLOGY 



contact. Flexion and extension of the elbow-joint are limited by the tension of the structures 

 on the front and back of the joint; the limitation of flexion is also aided by the soft structures of 

 the arm and forearm coming into contact. 



In any position of flexion or extension, the radius, carrying the hand with it, can be rotated in 

 the proximal radioulnar joint. The hand is directly articulated to the lower surface of the radius 

 only, and the ulnar notch on the lower end of the radius travels around the lower end of the ulna. 

 The latter bone is excluded from the wrist-joint by the articular disk. Thus, rotation of the head 

 of the radius around an axis passing through the center of the radial head of the humerus imparts 

 circular movement to the hand through a very considerable arc. 



V. Radioulnar Articulations (Articulatio Radioulnaris). 



iptti ia 



The articulation of the radius with the ulna is effected by ligaments which con- 

 nect together the extremities as well as the bodies of these bones. The ligaments 

 may, consequently, be subdivided into three sets: 1, those of the proximal radio- 

 ulnar articulation; 2, the middle radioulnar ligaments; 3, those of the distal radio- 

 ulnar articulation. 



Proximal Radioulnar Articulation (articulatio radioulnaris proximalis; superior 

 radioulnar joint). This articulation is a trochoid or pivot-joint between the 

 circumference of the head of the radius and the ring formed by the radial notch 

 of the ulna and the annular ligament. 



Head of radius Quadrate Semilunar 



(cut) ligament notch 



Annular ligament 



Radial notch 



Olecranon (cut) 



FIG. 333. Annular ligament of radius, from above. The head of the radius has been sawn off and the bone 



dislodged from the ligament. 



The Annular Ligament (ligamentum annulare radii; orbicular ligament) (Fig. 333). 

 This ligament is a strong band of fibers, which encircles the head of the radius, 

 and retains it in contact with the radial notch of the ulna. It forms about four- 

 fifths of the osseo-fibrous ring, and is attached to the anterior and posterior margins 

 of the radial notch ; a few of its lower fibers are continued around below the cavity 

 and form at this level a complete fibrous ring. Its upper border blends with the 

 anterior and posterior ligaments of the elbow, while from its lower border a thin 

 loose membrane passes to be attached to the neck of the radius; a thickened band 

 which extends from the inferior border of the annular ligament below the radial 

 notch to the neck of the radius is known as the quadrate ligament. The superficial 

 surface of the annular ligament is strengthened by the radial collateral ligament 

 of the elbow, and affords origin to part of the Supinator. Its deep surface is smooth, 

 and lined by synovial membrane, which is continuous with that of the elbow-joint. 



