THE ELBOW-JOINT. 



301 



the humerus ; and when it is strongly flexed, the top of the olecranon corresponds 

 to the vertical plane of the internal condyle. The head of the radius and the furrow 

 above it opening into the joint are easily felt at the outside and behind. In the lower 

 third of the forearm the bones can easily be felt. The ulna here is posterior and 

 best felt at the back. In supination the styloid process is distinct. It is hidden by 

 the soft parts in pronation, and the head is exposed. The forward sweep of the 

 lower end of the radius is evident. The inferior expansion can be felt both before 

 and behind ; the styloid process is examined best from the outer side. It extends 

 nearly one centimetre lower than that of the ulna. The inequalities on the back can 

 be felt vaguely ; the most evident js the ridge bounding the groove for the long 

 extensor of the thumb. 



THE ELBOW JOINT.' 



This is a considerably modified hinge-joint, the axis of rotation being oblique to 

 the long axis of both the humerus and the ulna, and the course of the latter at the 

 joint being also a spiral one. It is to be understood that the radius follows the ulna, 

 which is the directing bone of the forearm in the motions of the elbow. 



The Articular Surfaces. — These have been described with the bones ; it 

 remains only to give here a summary. The motions between the forearm and the 

 humerus depend essentially on the trochlea and on the surfaces of the greater sigmoid 

 cavity. This is a modified hinge-joint. As has been shown, the transverse axis of 



Fig. 316. 



Triceps 



Brachialis anticus 



Capsule 



Trochlea 



Bursa 



Subcutaneous bursa — ^ 



Olecranon 



Coronoid process 



Sagittal section of right elbow-joint through the trochlea. 



the trochlea is not at right angles to the shaft, and it may be added that the same is 

 true of the sigmoid cavity and the axis of the ulna. The effect of this will be noticed 

 later. Again, as already pointed out, the trochlea is not only oblique, but is so 

 shaped that the ulna in turning on it describes a spiral line. It has also been shown 

 that the trochlea is not equally broad throughout, and that there are curious differ- 

 ences of curve in the sigmoid cavity. Finally, the lateral ligaments are not quite 

 tense, especially when the joint is half flexed. It follows from these facts that the 

 motion is a very complicated one, and that a certain lateral motion of the ulna on 



^ Articulatio cubiti. 



