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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
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.
 
6e. Radioulnar Articulation
 
  
(Articulatio Radioulnaris)


The articulation of the radius with the ulna is effected by ligaments which connect 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 radioulnar articulation; 2, the middle radioulnar ligaments; 3, those of the distal radioulnar 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.


FIG. 333– Annular ligament of radius, from above. The head of the radius has been sawn off and the bone dislodged from the ligament. (See enlarged image)


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.

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