294 DISSECTION OF THE UPPER LIMB. 



Articular surfaces. The humerus presents interiorly two distinct ar- 

 ticular faces for the bones of the forearm. The one for the radius, on the 

 outer side, consists of a rounded eminence (capitellum) on the front of the 

 bone, which is covered with cartilage only on the interior aspect. The 

 surface in contact with the ulna is limited internally and externally by a 

 prominence, and hollowed out in the centre (trochlea). On the front of 

 the humerus above the articular surface are two depressions which receive 

 the coronoid process and the head of the radius during flexure of the 

 joint ; and on the posterior aspect is a large fossa for the reception of the 

 olecranon in extension of the joint. 



On the end of the ulna the large sigmoid cavity is narrowed in the cen- 

 tre, but expanded in front and behind (fig. 91). A slightly raised line 

 extends from front to back, and is received into the hollow of the trochlea 

 of the humerus. Across the bottom of the cavity the cartilage is wanting 

 completely, or for a greater or smaller distance. 



The end of the radius presents a circular depression with a raised mar- 

 gin. In the bent state of the joint the hollow of the radius fits on the 

 outer eminence of the humerus, and the bone is supported during rotation 

 of the limb. 



Movement. This joint is like a hinge in its movements, and permits 

 only flexion and extension. 



In flexion the bones of the forearm move forwards, each on its own 

 articular surface, so as to leave the back of the humerus uncovered. The 

 extent is checked by the meeting of the bones of the arm and forearm. 

 Owing to the slanting surface of the humerus the hand falls inside the 

 limb when the joint is fully bent. 



The ligaments are relaxed, with the exception of the following, viz., 

 the posterior, and the hinder part of the internal lateral. 



In extension the ulna and radius are carried back over the articular sur- 

 face of the humerus until they come into a line with the arm-bone. This 

 movement is checked by the olecranon touching the humerus. 



The anterior ligament, and the fore part of the internal lateral ligament 

 are made tight, but the hinder fibres of the internal lateral are relaxed. 



I'XION OF THE RADIUS AND ULNA. The radius is connected with the 

 ulna at both ends by means of distinct ligaments and synovial membrane; 

 and the shafts of the bones are united by interosseous ligaments. 



Upper radio-ulnar articulation. In this joint the head of the radius is 

 received into the small sigmoid cavity of the ulna, and is kept in place by 

 the following ligamentous band: 



The annular or orbicular ligament (fig. 91, a) is about one-third of an 

 inch wide, arid is stronger behind than before; it is placed around the 

 prominence of the head of the radius, and is attached to the anterior and 

 posterior edges of the small sigmoid cavity of the ulna. Its upper border, 

 the widest, is connected with the ligaments of the elbow joint; but the 

 lower is free, and is applied around the neck of the radius. In the socket 

 formed by this ligament and the cavity of the ulna, the radius moves 

 freely. 



The synovial membrane is a prolongation of that lining the elbow joint; 

 it projects inferiorly between the neck of the radius and the lower margin 

 of the annular ligament. 



Ligaments of the shafts of the bones. The aponeurotic stratum con- 

 necting together the bones in nearly their whole length consists of the two 

 following parts: 



