338 



HUMAN ANATOMY. 



THE SACRO-ILIAC ARTICULATION. 



The sacro-iliac articulation, often improperly called the sacro-iliac synchondrosis, 

 partakes of the nature of both a true and a half-joint. The opposed surfaces of 

 the sacrum and ilium vary greatly in shape. The sacrum is broader in front than 

 behind, so that the line of the joint slants inward as well as backward ; but occasion- 



FiG. 358. 



Posterior sacro-iliac ligament 



Sacro-iliac joint 



Anterior sacro-iliac ligament 

 Horizontal section through right sacro-iliac joint. 



Fig. 359. 



Ilio-lumbar ligament 



ally in some part it is a little broader behind than in front. Often there is an out- 

 ward swelling between the borders, so that a part of the sacrum is received into a 

 hollow in the ilium, and a transverse cut of the joint shows a sinuous line. Perhaps 

 quite as often the ilium projects into the sacrum. In any case, as a rule, there is a 

 certain amount of interlocking. The opposed surfaces are covered with cartilage. 

 The layer on the sacrum, from one to two millimetres thick, is at least twice as thick 

 as the other, and, though generally reckoned fibro-cartilage, has much the appear- 

 ance of hyaline. The two are sepa- 

 rated by a synovial cavity, which 

 is enclosed by the sacro-iliac liga- 

 ments. The size of this cavity is 

 very uncertain. It may extend 

 backward beyond the auricular 

 surfaces, occupying on the ilium 

 a part of the space usually serving 

 for the origin of the posterior 

 sacro-iliac ligaments, or it may be 

 encroached upon by fibres. Some- 

 times, before old age, the joint is 

 replaced by bone. 



The fibres around the joint 

 are severally named according to 

 position. The posterior sacro- 

 iliac ligament (Fig. 358) is very 

 important. It comprises many 

 layers of strong fibres, filling up the depths of the cleft between the sacrum and the 

 overhanging ilium, extending from the rough area on the latter behind the auricular 

 surface to the back of the lateral masses of the sacrum, nearly or quite to the pos- 

 terior sacral foramina below the three upper sacral vertebrae. Those of both sides 



Sacro-lumba 

 ligament 



Sacro-iliac 

 ligament 



Fifth lumbar 

 vertebra 



Promontory 

 of sacrum 



Sacrum 



Anterior view of the sacroiliac joint and of the last lumbar ver- 

 tebra. 



