| THE RADIO-CARPAL JOINT. 275 
The interosseous membrane (Fig. 212) of the forearm (membrana interossea inter- 
brachii) is a strong fibrous membrane which stretches across the interval between 
the radius and ulna, and is firmly attached to the interosseous border of each. Below 
it extends downwards to the lower limit of the space between the bones, whilst 
above it does not reach higher than a point about one inch below the tuberosity 
of the radius. A gap, called the hiatus interosseus, is thus left above its upper 
margin, and through this the posterior interosseous artery passes backwards between 
the bones to reach the dorsal aspect of the forearm. ‘This gap is bounded above 
by the oblique hgament. The fibres which compose the interosseous membrane 
run for the most part downwards and inwards from the radius to the ulna, although 
on its dorsal aspect several bands may be observed stretching in an opposite direc- 
tion. The interosseous membrane augments the surface for the origin of the muscles 
of the forearm; it braces the radius and ulna together ; and when shocks are com- 
municated from the hand to the radius, owing to the direction of its fibres, the 
interosseous Membrane transmits these, to some slight extent, to the ulna. 
The oblique ligament (lig. 212) is a slender tendinous band of very varying 
strength which springs from the outer part of the coronoid process of the ulna, and 
stretches obliquely downwards and outwards to the radius where it is attached 
immediately below the bicipital tuberosity. 
Movements of the Radius on the Ulna.—The axis about which the radius moves is a 
longitudinal one, having one end passing through the centre of the head of the radius and 
the other through the styloid process of the ulna and the line of the ring-finger. In this axis 
the head of the radius is so secured that it can only rotate upon the lesser sigmoid cavity 
of the ulna within the orbicular ligament, and consequently the radial head remains upon the 
same plane as the ulna; but the lower end of the radius being merely restrained by the 
triangular fibro-cartilage, is able to describe nearly a half-circle, of which the apex of this 
ligament is the centre. In this movement the radius carries the hand from a position in which 
the palm is directed forwards, and in which the radius and ulna lie parallel to each other 
(supination), to one in which the palm is directed backwards, and the radius lies diagonally across 
the front of the ulna (pronation). 
The ulna is unable to rotate upon a long axis, but while the radius is travelling through the 
are of a circle from without inwards in front of the ulna, it will usually be seen that the ulna 
appears to move through the are of a smaller circle in the reverse direction, viz. from within 
_ outwards. If the humerus be prevented from moving at the shoulder-joint, a very large propor- 
tion, if not the entire amount, of this apparent movement of the ulna will disappear. At the 
saine time some observers maintain that it really occurs at the elbow-joint, associated with lateral 
_ movement during slight degrees of flexion and extension at that joint. 
THE RADIO-CARPAL JOINT. 
This joint (articulatio radiocarpea) is a bi-axial diarthrosis, frequently called a 
— condyloid joint. 
The articular elements which enter into its formation are: on its provimal side, the 
| inferior surface of the lower end of the radius, together with the inferior surface of 
; the triangular fibro-cartilage ; on its distal side, the superior articular surfaces of the 
_ scaphoid, semilunar, and cuneiform bones. The articular surface of the radius is 
concave both in its antero-posterior and transverse diameters, in order to adapt 
itself to the opposing surfaces of the scaphoid and semilunar, which are convex in 
the two axes named. In the ordinary straight position of the hand the triangular 
fibro-cartilage is in contact with the semilunar bone, and the upper articular surface 
of the cuneiform bone is in contact with the capsule of the joint. When, however, 
the hand is bent towards the ulna, the cuneiform bone is carried outwards as well 
as the semilunar and scaphoid, and the triangular fibro-cartilage comes into contact 
with the cuneiform. The articular surface of the radius is subdivided by an antero- 
posterior, slightly elevated ridge, into an outer triangular facet which usually arti- 
culates with the scaphoid, and an inner quadrilateral facet for articulation with a 
portion of the semilunar bone. 
In the intervals between the scaphoid, semilunar, and cuneiform bones, the con- 
tinuity of the articular surfaces is usually maintained by the presence of interosseous 
ligaments which are situated upon the same level as the articular cartilage. 
Ligaments.—A capsular ligament completely surrounds the joint. It is some- 
what loosely arranged, and permits of subdivision into the following portions :— 
The external lateral ligament (Fig. 216) is a well-defined band which is attached 
