134 Anatomy of Skeleton 



each side. The inner one of these surfaces presents two areas (Fig. 109), of which the 

 posterior is the larger, slightly convex, and gives origin to Erector spinae: it is separated 

 from the auricular surface by the ligamentous area of the " tuberosity. " Anterior 

 to this the Quadratus lumborum has an area of about the same length, but not so deep. 

 These two muscular areas should be definitely recognised on the bone, the Erector 

 slightly overlapping the Quadrate surface, and it can then be seen that the ligamentous 

 areas that lie below them on the inner side of the bone are distinctly in different planes : 

 that below the Erector is for the posterior sacro-iliac ligaments, whereas that below 

 the Quadratus is for the ilio-lumbar ligament. The muscles obtain part of their origin 

 from the ligaments. 



It is now easy to place the other attachments. The crest is described as having 

 inner, middle, and outer lips in its front part, made by the musculo-tendinous attach- 

 ments of the three large muscles of the abdominal wall. The Transversalis is on the 

 inner lip : traced back, it becomes aponeurotic just outside Quadratus lumborum, 

 and the anterior lamella runs in front of this muscle on to the ilio-lumbar ligament, 

 while the main lamella passes behind it and then between it and the Erector spinae 

 to reach the ligament also, and thus pass to the lumbar transverse processes (Fig. no) 

 (whence the ridge that separates the two muscle areas), and the posterior lamella comes 

 off the main sheet at the outer side of Erector spinae and is attached behind this to the 

 blunt edge, which brings it to the aponeuroses on the back of the sacral musculature 

 (see Fig. no and also Fig. 34). The posterior lamella is thickened by junction with 

 the aponeurotic origin of Latissimus dorsi, but this has a muscular origin further out, 

 behind the Quadratus. Internal oblique takes the middle lip as far back as Quadratus, 

 where it passes on to the Transversalis aponeurosis. External oblique is on the 

 outer lip and ends before reaching the level of Quadratus. Thus the External oblique 

 may be said to take more than the anterior half of the length of the outer lip of the crest, 

 and the other two take less than the anterior two-thirds of the middle and inner lips. 

 External oblique becomes aponeurotic at the anterior superior spine, and Poupart's 

 ligament, the lower edge of the aponeurosis, is attached to the prominent convexity 

 of the spine : its other attachments and the related structures are also shown in 

 Fig. no and a sufficient description given of them. 



The whole of the crest, including its two terminal spines, is a traction epiphysis 

 on the bone, developed from anterior and posterior centres that fuse and make a single 

 epiphysis. 



Anterior Border. -The Sartorius arises from the notch below the anterior superior 

 spine (Fig. 104). It has Tensor fasciae femoris outside it, and is separated from Iliacus 

 by a thin aponeurosis that passes to the bone from Poupart's ligament and holds up 

 the Iliacus here : the line of this aponeurosis may be picked up on the bone occasionally. 

 The remainder of the border has been already sufficiently considered. 



Crest of Pubis : presents from without inwards the spine for Poupart's ligament, 

 a groove for the conjoint tendon, and an area of origin for the Rectus abdominis. It 

 ends at the angle, which has a separate ossific centre that has been homologised with 

 the marsupial " epipubic bone." 



Symphysis is coated with hyaline cartilage, which is united with its fellow by an 

 intervening disc of fibre-cartilage : a small cavity, not lined by synovial mem- 

 brane, is found in the upper and back part of this after the first eight or ten years of 

 life. 



The ligamentous markings are weaker behind than in front, where they are 

 strengthened by the origin of the inner heads of the two Recti, which decussate on 

 them. 



