284 THE ARTICULATIONS OR JOINTS. 
The anterior pubic ligament (lig. pubicum anterius, Fig. 219) is a structure of 
considerable thickness and strength, Its superficial fibres, which are derived very 
largely from the tendons and aponeuroses of adjoining muscles, are oblique, and 
form an interlaced decussation. The deeper fibres are short, and extend trans- 
versely from one pubic bone to the other. 
The posterior pubic ligament (lig. pubicum posterius, Fig. 220) is very weak, 
and consists of scattered fibres which extend transversely between contiguous 
pubic surfaces posterior to the articulation. 
The superior pubic ligament (lig. pubicum superius, Fig. 219) is likewise weak, 
and consists of transverse fibres passing between the two pubic crests. 
The inferior or subpubic ligament (lig. pubicum inferius vel hg. arcuatum pubis, 
Fig. 220) occupies the arch of the pubis, and is of considerable strength. It gives 
roundness to the pubic arch and forms part of the pelvic outlet. It has con- 
siderable vertical thickness immediately below the interpubic dise to which it is 
attached. Laterally it is attached to adjacent sides of the descending rami of the 
pubis. Its lower border is free, and separated from the triangular ligament of 
the perineum by a transverse oval interval through which the dorsal vein of the 
penis passes backwards to the interior of the pelvis. 
THE TRIANGULAR LIGAMENT OF THE PERINEUM. 
The triangular ligament of the perineum is a membranous structure which 
occupies the pubic arch below, and is distinct from the subpubic hgament. It assists 
in completing the pelvic walls anteriorly in the same manner that the obturator 
membrane does laterally. Indeed, these two structures occupy the same mor- 
phological plane. The triangular ligament presents two surfaces—one superficial 
or perineal; the other deep, or pelvic, and both of these surfaces are associated 
with muscles. Its lateral borders are attached to the sides of the pubic arch, 
while its base is somewhat ill-defined, by reason of its fusion with the fascia of 
Colles in the urethral region of the perineum. 
The apex of the triangular ligament is truncated, free, and well-defined, constitut- 
ing the transverse perineal ligament, above which there is the interval for the dorsal 
vein of the penis. It is pierced by a number of vessels and nerves, but the 
principal opening is situated mesially one inch below the pubic arch, and transmits 
the urethra. 
THE OBTURATOR MEMBRANE. 
The obturator membrane (membrana obturatoria, Fig. 219) occupies the 
obturator or thyroid foramen. It is attached to the pelvic aspect of the 
circumference of this foramen. It consists of fibres irregularly arranged and of 
varying strength, so that sometimes it almost appears fenestrated. At the highest 
part of the foramen it is incomplete and forms a U-shaped border, between which 
and the bony circumference of the foramen, the obturator canal (canalis obtura- 
torius) is formed. In this position the membrane is continuous with the parietal 
pelvic fascia which clothes the inner side of the obturator internus muscle, above 
the upper free margin of the muscle. From the outer or crural aspect of the 
membrane some of its fibres are prolonged to the antero-inferior aspect of the 
capsule of the hip-joint. 
Mechanism and Movements of the Pelvis.—The human pelvis presents a mechanism the 
principal requirement of which is stability and not movement, for, through the pelvis, the weight 
of the trunk, superimposed upon the sacrum, is transmitted to the lower limbs. Moreover, its 
stability is largely concerned in the maintenance of the erect attitude. The movements of its 
various parts are therefore merely such as are consistent with stability, without producing absolute 
rigidity. 
The two innominate bones, being bound together by powerful ligaments at the pubic articula- 
tion, constitute an inverted arch, of which the convexity is directed downwards and forwards, 
while its piers are turned upwards and backwards, and considerably expanded in relation to the 
hinder parts of the iliac bones. Between the piers of this inverted arch the sacrum is situated. 
This bone is in no sense a key-stone to an arch, because, as may readily be seen in antero-posterior 
transverse section, the sacrum is wider in front than behind, and the superposed weight naturally 
tends to make the sacrum fall towards the pelvic cavity, and so fit less closely between the 
innominate bones. The sacrum is in reality an oblique platform, in contact with each innominate 
