THE RADIO-ULNAR JOINTS. 273 
notch. Both of the lateral ligaments are intimately associated with the muscles 
which take origin from the inner and outer condyles of the humerus. 
Synovial Pads of Fat (Fig. 213).—Internal to the capsule, there are several pads 
of fat situated be- 
tweenitand the sy- 
novial membrane. 
Small pads are so 
placed as to le im- 
- mediately in front . 
of the coronoid and 
supra-capitellar 
fosse, but a larger 
- one projects to- 
wards the olecra- 
non fossa. 
Synovial mem- 
brane (Fig. 213) 
lines the entire 
_ capsule and clothes 
_the pads of fat 
above referred to, 
as well as those 
portions of bone 
enclosed within the 
capsule which are 
not covered by 
articular cartilage. 
By its disposition 
the elbow and the Fic. 213.—VERTICAL SECTION THROUGH THE TROCHLEAR PART OF THE 
: : ELBOW-JOINT. 
superior radio- 
ulnar joints possess a common joint cavity. It should be specially noted that the 
upper part of the neck of the radius is surrounded by this synovial membrane, 
fF pad of fat 
ie / 
"/f ~Coronoid process 
Trochlea Olecranon process 
| Movements at the Elbow-Joint.—The movements of the radius and ulna upon the humerus 
have already been referred to as those characterising a uniaxial joint constructed on the plan of 
a hinge. In this case the axis of the joint is obliquely transverse, so that in the extended 
_ position the humerus and ulna form an obtuse angle open towards the radius, whereas in the 
_ flexed position the hand is carried inwards in the direction of the mouth. Extreme flexion is 
_ checked by the soft parts in front of the arm and of the fore-arm coming into contact, and 
extreme extension by the restraining effect of the ligaments and muscles. In each case the 
. movement is checked before either coronoid or olecranon processes come into contact with the 
humerus. The anterior and posterior bands of the internal lateral ligament are important factors 
in these results. Lateral movement of the ulna is not a characteristic movement, although it 
may occur to a slight extent, owing to a want of complete adaptation between the trochlear 
surface of the humerus and the sigmoid cavity of the ulna. This incongruence is noteworthy 
since the inner lip of the trochlea is prominent in front, and the outer lip is prominent behind. 
Consequently, this latter part is associated with a surface on the outer side of the olecranon 
which is only utilised in complete extension. 
The capitellum and the opposing surface upon the head of the radius are always in varying 
degrees of contact. The head of the radius participates in the movements of flexion and exten- 
sion, and is most closely and completely in contact with the humerus during the position of semi- 
flexion and semi-pronation. In complete extension a very considerable part of the capitellum is 
uncovered by the radius. 
THE RADIO-ULNAR JOINTS. 
These articulations, which are two in number, are situated at the proximal and 
distal ends of the radiusand ulna. They provide an adaptation whereby the radius 
rotates around a longitudinal axis in the movements of pronation and supination, 
and hence this form of uniaxial diarthrosis is termed lateral ginglymus. 
Superior Radio-ulnar Joint (articulatio radio-ulnaris proximalis).—In this 
joint the articular surfaces which enter into its formation are the lesser sigmoid 
cavity of the ulna and the lateral aspect of the head of the radius. In each case 
the articular cartilage is continuous with an articular surface entering into the 
18 
