338 THE ARTICULATIONS OE JOINTS. 



Lig. Arcuatum Pubis. The arcuate ligament of the pubis (O.T. inferior or 

 subpubic ligament) occupies the arch of the pubis, and is of considerable strength. 

 It gives roundness to the pubic arch and forms part of the inferior aperture of 

 the pelvis. It has considerable vertical thickness immediately below the interpubic 

 nbro-cartilage; to which it is attached. Laterally it is attached to adjacent sides 

 of the inferior rami of the pubis. Its inferior border is free, and separated from the 

 upper border of the fascia of the urogenital diaphragm by a transverse oval 

 interval, through which the dorsal vein of the penis passes backwards to the 

 interior of the pelvis. 



FASCIA DlAPHRAGMATIS UROGENITALIS INFERIOR. 



The inferior fascia of the urogenital diaphragm (O.T. superficial layer of the 

 triangular ligament) is a membranous structure which occupies the pubic arch below 

 and distinct from the arcuate ligament of the pubis. 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 morphological plane. The 

 fascia 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 fascia is truncated, free, and well defined, constituting 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 in the median plane one inch below the pubic arch, and trans- 

 mits the urethra. 



MEMBRANA OBTURATORIA. 



The obturator membrane (Fig. 316) occupies the obturator foramen. It ig 

 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 is formed. In this position the membrane is continuous with 

 the parietal pelvic fascia which clothes the medial side of the obturator internus 

 muscle, above the superior free margin of the muscle. From the lateral or femoral 

 aspect of the membrane some of its fibres are prolonged to the antero-inferioi 

 aspect of the capsule of the hip-joint. 



Mechanism and Movements of the Pelvis. The human pelvis presents a mechanism tin 

 principal requirement of which is stability and not movement, for, through the pelvis, the weigh j 

 of the trunk, superimposed upon the sacrum, is transmitted to the lower limbs. Moreover, it I 

 stability is largely concerned in the maintenance of the erect attitude. The movements of it 

 various parts are therefore merely such as are consistent with stability, without producing absolutl 

 rigidity. 



The two hip bones, being bound together by powerful ligaments at the pubic articulatior 

 constitute an inverted arch, of which the convexity is directed downwards and forwards, whil 

 its piers are turned upwards and backwards, and considerably expanded in relation to th 

 posterior parts of the iliac bones. Between the piers of this inverted arch the sacrum is situatec 

 This bone is in no sense a key -stone to an arch, because, as may readily be seen in antero-postericl 

 transverse section, the sacrum is wider in front than behind, and the superposed weight naturall j 

 tends to make the sacrum fall towards the pelvic cavity, and so fit less closely between tt| 

 hip bones. The sacrum is in reality an oblique platform, in contact with each hip bone througji 

 its articular auricular surfaces, and in this position it is suspended by the interosseous ar| 

 posterior sacro-iliac ligaments, and kept securely in place by the "grip" due to the irregularilk 

 of the opposed surfaces of the two sacro-iliac articulations. Since the weight of the trunk li 

 transmitted to the anterior and superior end of this sacral platform, there is a natural tenden< | 

 for the sacrum to revolve upon the transverse axis which passes through its sacro-iliac join' I 

 If this were permitted, the promontory of the sacrum would rotate downwards and forwar < 

 towards the pelvic cavity, as really does occur in certain deformities. This revolution or tiltii i 

 downwards of the anterior part of the sacrum is prevented by the action of the sacid 

 tuberous and sacro-spinous ligaments, extending from the ischial tuberosity to the poster! ::| 

 and inferior end of the suspended platform of the sacrum. Not only so, but these ligamen "1 

 acting on a rigid sacrum, tend to hold up the weight upon the sacral promontory. 



