chap, xviii.j THE PELVIS. 337 



Fractures of the pelvis. From what has 

 been already said, it may be surmised that the weakest 

 parts of the pelvis are at the symphysis and the sacro- 

 iliac joints. The bones of these parts, however, are so 

 very firmly knit together by powerful ligaments that 

 it is very rare for these articulations to give way, 

 fracture of the adjacent bones being more common. 

 The commonest fracture of the pelvis is in the weak 

 counter arch, and involves the rami of both the pubes 

 and the ischium. The* fracture is often associated with 

 some tearing of ligaments about the sacro-iliac synchon- 

 drosis, and is met with in accidents due to the most 

 varied forms of violence. This last remarkable cir- 

 cumstance is thus explained by Tillaux. If the pelvis 

 be compressed in (a) an antero-posterior direction, the 

 main brunt of the force comes upon the weak counter 

 arch, which fractures from direct violence. The force 

 continuing, tends to push asunder the two iliac bones, 

 and so causes rupture of the anterior ligaments at the 

 sacro-iliac joint. If the force be applied (6) trans- 

 versely, the two acetabula tend to be pressed towards 

 one another, the counter arch becomes more bent, and 

 ultimately gives way by indirect violence. The 

 violence continuing, forces the two ilia towards one 

 another, the strain then falls upon the sacro-iliac 

 synchondrosis, and the posterior ligaments of that joint 

 are apt to yield, or portions of the bone adjacent to 

 the joint are torn away. In cases of falls, when the 

 patient alights upon the feet or ischial tuberosities, it 

 can be understood how in many instances the main 

 arches will escape injury owing to their great strength, 

 while the counter arch becomes fractured. Any part 

 of the pelvis, including the sacrum, may be broken 

 by well-localised direct violence. More or less of the 

 iliac crest, the anterior superior and posterior superior 

 spines, have been knocked off. The first-named part 

 may be separated as an epiphysis. It joins the bone 

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