1 The upper extremity of the elbow as seen from the front. The inner surface of the coranoid process of the ulna is curved so as to clasp the pulleylike trochlea of the humerus.
2 The lower extremity of the humerus is somewhat flat. Projecting from each side are the internal and external condyles. Between the two is the rounded groove that receives the lip of the ulna.
3 Here the bones of the arm and forearm are connected. This is a view from the front. The humerus above shows the two condyles with a notch that receives the coranoid process of the ulna, when the arm is bent. The ulna at the elbow swings hinge-like on the bone of the upper arm. It moves backward and forward in one plane only. Just below the outer condyle of the humerus is a small and rounded bursa, called the radial head of the humerus, on the surface of which rolls the head of the radius.
The large bone, which carries the forearm, may be swinging upon its hinge at the elbow, at the same time that the lesser bone which carries the hand may be turning round it. Both these bones of the forearm, the radius and ulna, have prominent ridges and grooves. They are directed obliquely from above, downward and inward. The radius turns round the ulna in these grooves and on the tubercles at the heads of both bones.
The lower extremity of the humerus gives a key to the movements of the elbow joint. Above, the shaft of the humerus is completely covered by the muscles of the upper arm. Below, the inner and outer condyles come to the surface near the elbow. The inner condyle is more in evidence. The outer one is hidden by muscle, when the arm is straightened out. When the arm is bent, it becomes more prominent and easier to locate.
1 The humerus at the elbow is flattened in front and back, terminating in two condyles. Between these is placed the trochlea, a rounded spool-like form that is clasped by the olecranon process of the ulna.
2 This is a diagram of the spool-like form of the trochlea with the embracing condyles at the sides.
3 From the back, the olecranon process of the ulna is lodged into the hollowed-out portion of the back of the humerus, forming the elbow point.
4 This shows the bony structure of the hinge joint at the elbow.
1 The ulna swings on the pulley of the humerus. The articulation is known as a hinge joint.
2 Shows the mechanical device used in straightening the forearm, on the arm, at the elbow. The common tendon of the triceps grasps the olecranon of the ulna, which in turn clasps round the spool-like trochlea of the humerus.
3 When the forearm is flexed on the arm, the ulna hooks round the pulleylike device of the humerus. The triceps in this position is opposed by the biceps and brachialis anticus in front, which becomes the power that raises the forearm upward. The triceps in reverse is inert and somewhat flattened out.
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