OF THE RADIUS WITH THE ULNA 229 



superior radio-iilnar joint, lining the orl)icular ligament. Outside the synovial 

 menil)rane, l)ut inside the capsule, are often seen some pads of fatty tissue; one is 

 situated on the inner side at the Ijase of the olecranon, another is seen on the out- 

 side projecting into the cavity between the radius and ulna; this latter, Avith a fold 

 of synovial membrane opposite the front of the outer lip of the trochlea, suggests 

 the division of the joint into two parts — one internally for the ulna, and another 

 externally for the radius. There are also pads of fatty tissue at tlie bottom of the 

 olecranon and coronoid foss?e, and at the tip of the olecranon process. 



The arterial supply is derived from each of the vessels forming the free anasto- 

 mosis around the elbow, and there is also a special branch to the front and outer 

 side of the joint, from the bracliial artery, and the arterial branch to tlie brachidlis 

 anticus also feeds the front of the joint. 



The nerve-supply comes from the musculo-cutaneous chiefly; the ulnar, 

 median, and musculo-spiral also give filaments to the joint. 



The movements permitted at the elbow are those of a true hinge joint, viz. 

 flexion and extension. These movements are oblique, so that the forearm is 

 inclined inwards in flexion, and outwards in extension; they are limited by the 

 contact respectively of the coronoid and olecranon processes of the ulna Avith their 

 corresponding fossae on the humerus, and their extent is determined by the rela- 

 tive proportion l)etween the length of the processes and depth of the fossje which 

 receive them, rather than by the tension of the ligaments, or the bulk of the soft 

 parts over them. The anterior and posterior portions of the capsule, together Avith 

 the corresponding portions of the lateral ligament, are not put on the stretch during 

 flexion and extension; but, although they may assist in checking the A-elocity, and 

 thus prevent undue force of impact, they do not control or determine the extent of 

 these movements. The limit of extension is reached when the ulna is nearly in a 

 straight line with the humerus; and the limit of flexion Avhenthe ulna describes an 

 angle of from 30° to 40° Avith the humerus. 



The ol )li(|uity of these moA^ements is due to the outAvard inclination of the upi)er 

 and Ijack j^art of the trochlear surface, and the greater prominence of tlie inner lip 

 of the trochlea below; thus the plane of motion is directed from behind iuAvards 

 and forwards, and carries the hand tOAvards the middle third of the clavicle. The 

 obliquity of the joint, the iuAvard tAvist of the shaft of the humerus, and the back- 

 ward direction of its head, all tend to bring the hand toAvards the mid line, under 

 the immediate observation of the eye, Avhether for defence, employment, or nourish- 

 ment. This is in striking contrast to the loAver liml), Avhere the direction of the 

 foot diverges from the median axis of the trunk, thus preventing aAvkAvardness in 

 locomotion. In flexion and extension, the cup-like depression of the radial head 

 glides upon the capitellum, and the inner margin of the radial head travels in the 

 groove betAveen the capitellum and the trochlea. This alloAvs the radius to rotate 

 upon the humerus Avhile following the ulna in all its moA^ements. In full exten- 

 sion and supination, the head of the radius is barel}^ in contact Avith the inferior 

 surface of the capitellum, and projects so much baclnvards that its posterior margin 

 can 1)e felt as a prominence at the back of the elboAV. In full flexion the anterior 

 edge of the radial head is received into, and checked against, the depression aboA^e 

 the capitellum; Avhile in mid-flexion the cup-like depression is fairly received upon 

 the ca})itellum, and in this position, the raclius being more completely steadied l)y 

 the humerus than in any other, })ronation and su})inati()n take ])lace most perfectly. 



5. THE UNION OF THE RADIUS WITH THE ULNA 



The radius is firmly united to the ulna by two joints, and an intermediate 

 fibrous union, viz. : — 



(a) The superior radio-ulnar, — Avhercat the head of the radius rotates Avithin 

 the lesser sigmoid cavity and orbicular ligament. 



(6) The union of the shafts, — the mid radio-ulnar union. 



(c) The inferior radio-ulnar, — Avhereat the lower end of the radius rolls round 

 the head of the ulna. 



