SACRO-COCCYGEAL ARTICULATION 



237 



Its front surface gives attachment to the coccygeus muscle, which overlies it. Behind, it is 

 connected with, and hidden by, the sacro-tuberous ligament, so that onlj' the lateral inch or 

 less (2 cm.) and a small part of its attachment to the coccyx can be seen; the internal pudic 

 nerve also passes over the posterior siirface. 



The arterial supply of the sacro-ihac joint comes from the superior gluteal, ilio-lumbar, and 

 lateral sacral. 



The nerve -supply is from the superior gluteal, sacral plexus, and external twigs of the 

 posterior divisions of the first and second sacral nerves. 



Movements. — Recent investigations have shown that in spite of the interlocking of the 

 articular surfaces and the strong ligaments connecting the bones together a slight amount of 

 movement, both a ghding and rotatory, does occur at the sacro-iliac joint. Tke gliding move- 

 ment. is both up and down, and forward and backward, and the latter is associated with a slight I 

 j:otation round a transverse axis which passes through the upper tubercles on the back of the 

 sacrum. The movement is but small in extent, nevertheless as the base of the sacrum moves 1 



Fig. 274. — Ligaments connecting Sacrum and Coccyx posteriorly. 



Superficial part of 

 the supraspinous 

 ligament, turned 

 up 



Deep part of the su- 

 praspinous ligament 

 turned up 



Lateral sacro-coccygeaL 

 ligament 



The deep posterior sacro- 

 cocygeal ligament, or the 

 lower end of the poste- 

 rior longitudinal liga- 

 ment 



The superficial posterior 

 sacro-coccygeal liga- 

 ment connecting the 

 cornus of the sacrum 

 and coccyx, cut and 

 turned down 



Idownward and forward the conjugate (antero-posterior) diameter of the pelvic inlet is diminished 

 and at the same time, as the coccyx moves up and back, the conjugate diameter of the outlet 

 is increased. This rotatory movement is limited principally by the sacro-sciatic (sacro- 

 tuberous and sacro-spinous) ligaments which prevent any extensive upward and backward 



I movement of the coccyx and lower part of the sacrum. 



Downward, displacement of the sacrum when the body is in the sitting posture is prevented 

 not only by the surrounding ligaments, but also by the wedge-like character of the sacrum, 

 which is broader above than below. Downward and forward displacement of the sacrum in 

 the erect posture is prevented by the ligaments and more particularly by the posterior sacro- 

 iliac bands, while backward displacement would be hindered by the breadth of the anterior as 

 contrasted with the posterior part of the sacrum as well as by the anterior ligaments. 



Relations. — The sacro-iliac joint is in relation above with psoas and iliacus. In front it 

 is in relation at its upper part with the hypogastric vessels and obturator nerve, and at its 

 lower part with the piriformis muscle. 



(b) The Sacro-coccygeal Articulation 

 Class. ^ — False Synchondrosis. 



The last piece of the sacrum and first piece of the coccyx enter into this union 

 [symphysis sacrococcygeal and are bound together by the following ligaments : — 

 Anterior sacro-coccygeal. Deep posterior sacro-coccygeal. 



Superficial posterior sacro-coccygeal. Lateral sacro-coccygeal. 

 Intervertebral substance. 



