THE ELBOW-JOINT. 



303 



th'*- t. There is a thick pad of fat, which, when large, may bear well-marked 

 synovial folds at the notch on the inner side of the ulna where the olecranon joins 

 the coronoid. 



Movements. — These are of two orders : that of flexion and extension, and 

 those which occur in twisting the forearm. For practical purposes the former may 

 be reduced to those of the ulna, which the radius is forced to follow. The move- 

 ments of the ulna are not far from turning on an oblique axis, which cuts the long 

 axis of the humerus at an angle of approximately 80° externally. When the forearm 

 i^ fully extended, it therefore forms externally an obtuse angle with the humerus. 

 Were the long axis of the ulna perpendicular to the axis of the joint, the forearm in 

 flexion would cross the humerus, as indeed is often erroneously stated ; in fact, 

 however, the long axis of the ulna also forms an angle of about 80° with the axis of 

 the joint, and, as these angles equal each other, in flexion the forearm is parallel 

 with the humerus. A simple demonstration of this is gained by cutting out a copy 

 of Fig. 320.' On folding it at the line of the joint (a <J) the two parts will lie one on 



Fig. 318. 



Olecranon fossa 



Internal condyle 



Posterior part of capsule — ^ 

 Bursa 



Radius 



External part of capsule con- 

 cealing orbicular ligament 



Ulna 



Right elbow-joint, posterior and outer aspect. 



the Other. If then another model be made with the axis of the lower piece at right 

 angles to the joint, it will show that the lower piece crosses the upper. When 

 extension is complete, the tip of the olecranon can go no farther into the fossa on 

 the back of the humerus, and the front of the capsule is tense. In complete flexion 

 of the dissected arm, the tip of the coronoid is in contact with the humerus in front ; 

 but in life the motion may be checked by the soft parts before it has reached its 

 limit. Morris has shown that there is much variation in the range of movement, 

 depending on differences in the upper end of the ulna. The lateral ligaments of a 

 theoretically perfect hinge-joint should always be tense ; in the elbow they are not 

 quite tense in semiflexion. Moreover, the imaginary axis does not remain fixed 

 throughout the motion. 



Motions of the Forearm on the Humerus in twisting the Hand. — 

 The articulation between the concave head of the radius and the convex capitellum 

 of the humerus is practically a ball-and-socket joint ; the radius may glide on the 

 humerus, following the ulna, or it may rotate on a fixed axis, as described abbve. 

 It is easily shown, however, that the swinging of the lower end of the radius round 



* Potter : Journal cf Anatomy and Physiology, vol. xxix., 1895. 



