Normal upper sacrum, sacroiliac joint and presacral soft tissues of a 29–year–old woman (same specimen as in Slide 13). The positioning of the cadaver during preparation of the cadaver has induced a slight malalignment of the sacroiliac joints. These joints carry a true diarthrodial synovial joint anteriorly and a complex array of dense ligaments posteriorly. The joints are covered anteriorly by the iliacus muscle which in turn is bounded anteriorly by the psoas major muscle. The iliolumbar nerve trunk and blood vessels are firmly stretched over the (anterior) convexity of the ala. The lower poles of the ganglia, arteries and veins are embedded in the fat filling the wide SI foramina, between them is the vestige of the SI disc. Posteriorly the sacral canal contains the S2 ganglia and the terminating thecal sac.