286 THE ARTICULATIONS OR JOINTS. 
The acetabulum is a deep cup-shaped cavity which presents an interruption or 
notch on its antero-inferior margin. The interior of the cup is lined by a ribbon- 
like band of articular cartilage which extends to the brim of the cavity, but 
does not cover the floor of the cup. This articular ribbon-shaped band is widest on 
its supero-posterior aspect, and narrowest at the anterior margin of the notch. 
The transverse ligament (lig. transversum acetabuli, Fig. 221) bridges the ace- 
tabular notch, and consists of strong transverse fibres which are attached to both 
of its margins, but more extensively to the postero-inferior. This ligament does 
not entirely fill the notch, but leaves an open interval between its lower border and 
the bottom of the notch through which vessels and nerves enter the cup. The 
acetabular aspect of this ligament constitutes an articular surface. 
The acetabulum is deepened by the cotyloid ligament (labrum glenoidale, 
Figs. 221 and 222). This ligament consists of a strong ring of fibro-cartilaginous 
tissue attached to the entire rim of the cup. The attached surface of the ring is 
broader than its free edge, and, moreover, the latter is somewhat contracted, so 
that the hgament grasps the head of the femur which it encircles. Its fibres are 
partly oblique and partly circular in their direction. By the former it is firmly 
implanted on the rim of the acetabulum and the transverse ligament of the notch; 
by the latter the depth of the cup is increased through the elevation of its edge, 
and its mouth shehtly narrowed. By one surface this hgament is also articular. 
A capsule (capsula articularis, Figs. 220 and 222) completely invests the joint- 
cavity. This is a fibrous membrane of great strength, although it is not of equal 
thickness throughout, being considerably thicker on the supero-anterior aspect than 
at any other part. Unlike the corresponding structure of the shoulder- joint, it does 
not permit of the withdrawal of the head of the femur from contact with the aceta- 
bular articular surfaces, except to a very linited extent. Its fibres are arranged 
both in the circular and in the longitudinal direction, the former, known as the 
zona orbicularis, being best marked posteriorly, while the longitudinal fibres stand 
out more distinctly in front, where they constitute special ligaments. Looked at 
as a whole, the capsule has the following attachments :—superiorly it surrounds 
the acetabulum, on the upper and hinder aspects of which it is attached directly 
to the innominate bone, while on the front and lower aspects it is attached to the 
non -articular surfaces of the cotyloid and transverse hgaments; inferiorly it 
encircles the neck of the femur, where it is attached in front to the anterior inter- 
trochanteric line; above, to the inner aspect of the root of the great trochanter ; 
below, to the lower part of the neck of the femur, in close proximity to the small 
trochanter; behind, to the line of junction of the outer and middle thirds of the 
neck of the femur. It is a matter of some importance to note that only part 
of the posterior surface of the neck of the femur is enclosed within the capsule. 
The femoral attachments of the capsule vary considerably in their strength, being 
particularly firm above and in front, but much weaker below and behind, where 
the orbicular fibres are well seen. Many fibres of the capsule are reflected from 
its deep aspect upwards upon the neck of the femur, where they form ridges, and 
to these the term rectinacula (Fig. 221) is applied. 
The longitudinal fibres of the capsule are arranged so as to form certain definite 
bands, viz. :— 
(1) The ilio-femoral ligament (lig. ilio- femorale, Fig. 222) consists of a tri- 
angular set of fibres attached above, by their apex, to the lower part of the anterior 
inferior iac spine and the immediately adjoining part of the rim of the aceta- 
bulum, and below, by their base, to the anterior intertrochanteric line of the femur. 
This ligament is the thickest part of the capsule, but its sides are more pronounced 
than its centre, especially towards its base. Consequently the ilio-femoral band 
presents some resemblance to an inverted Y (A), and therefore it is very generally 
known as the Y-shaped ligament of Bigelow. 
The outer or upper limb of the ilio-femoral ligament may be somewhat extended by the 
inclusion of additional longitudinal fibres, and described as the ilio-trochanteric ligament (lig. 
ilio-trochantericum). This band arises from the anterior part of the dorsum of the acetabulum, 
ar eager. to the femoral neck, close to the anterior end of the inner surface of the great 
rochanter, 
