902 A MANUAL OF ANATOMY 



this latter point it sends forwards an expansion, called the falci- 

 form process, which is attached to a sharp ridge on the lower 

 part of the inner surface of the ramus of the ischium, close to 

 its internal border. The ligament is broad at its attached ends, 

 especially the upper, but in crossing towards the tuber ischii 

 it becomes narrow. Its direction is downwards and forwards, 

 and its ischial fibres are continuous with the tendinous origin 

 of the long head of the biceps femoris. By its superficial surface 

 it gives origin to part of the gluteus maximus, and resting 

 on this surface are the plexiform loops formed by the external 

 branches of the posterior primary divisions of the first three sacral 

 nerves. Its deep surface gives origin to some fibres of the pyri- 

 formis, and lower down is intimately connected with the small 

 sacro-sciatic ligament. The falciform process affords attachment 

 to the lower part of the parietal pelvic fascia. The great sacro- 

 sciatic ligament is pierced by the coccygeal branch of the sciatic 

 artery, the sacral branch of the internal pudic artery, and the 

 perforating cutaneous nerve from the sacral plexus. The ligament 

 assists in the formation of the great and small sacro-sciatic foramina. 



The great sacro-sciatic ligament is to be regarded as the separated tendon 

 of the long head of the biceps femoris muscle. 



The small or anterior sacro-sciatic ligament, which is triangular, 

 is attached by its base to the sides of the last two sacral and first 

 coccygeal vertebrae, where it is intimately connected with the more 

 superficially placed great sacro-sciatic ligament. Its apex is 

 attached to the tip of the spine of the ischium. Its deep surface 

 is incorporated with the coccygeus muscle, and, along with the 

 spine of the ischium, it forms the separation between the great and. 

 smaU sciatic foramina. 



The small sacro-sciatic ligament is to be regarded as resulting from the 

 fibrous degeneration of the superficial part of the coccygeus muscle. 



Arterial Supply of the Sacro-iliac Articulation. — Ilio-lurnbar, 

 superior lateral sacral, and gluteal arteries. 



Nerve-supply. — Superior gluteal, and anterior primary divisions, 

 and external branches of the posterior primary divisions, of the 

 first two sacral nerves. 



Movements. — Stability being required at this joint, it is almost 

 immovable. The two ossa innominata by their union at the pubic 

 articulation form an arch, the convexity of which is directed down- 

 wards and forwards. The piers of this arch are separated by a wide 

 interval, into which the sacrum fits in the form of a wedge. The 

 sacrum being narrower behind than in front, the superincumbent 

 weight of the trunk has a tendency to displace it downwards into 

 the pelvic cavity, but this is resisted partly by the powerful pos- 

 terior sacro-iliac ligaments, which suspend the bone, and partly 

 by the strong hold which the sacrum has upon the ilium, in virtue 

 of the irregularities of the opposed surfaces. Under the influence 



