The Lower Extremity and Pelvis 



125 



backward and outward, whereas the lower and front parts looks forward and outward. 

 The two planes cut each other nearly at a right angle, and the upper one roughly 

 corresponds with the Ilium, the lower with the Ischium and pubis. 



The scheme in Fig. 103 illustrates this : a is the upper and back plane and b the 

 lower and front one. The projection forwards of a could be taken to Represent the 

 schematic position of the anterior superior spine, while the hinder end of b would 

 represent the projection of the tuberosity of the Ischium. The transition from one 

 plane to the other is comparatively abrupt on the outer side, and occurs about a line 

 drawn from below the Ischial spine to the anterior superior spine : this places the 

 acetabulum on the lower segment, but, as can be seen on the other side of the scheme 

 and is apparent on the bone, the part of the wall of this cavity that rests on the head of 

 the femur and transmits weight to it is supported by the front edge of the upper plane, 

 which carries the weight to it from the sacrum. Accordingly we find that that portion 

 of the Ilium between the auricular surface and the top of the acetabulum is very thick 

 and strong, and forms the acetabular 

 rim along the line of transition from 

 one plane to the other. 



On the inner side, however, the 

 transition is modified by the curved 

 bar c of the ilio-pectineal line, whose 

 mechanical function is concerned 

 with the mode of transmission of the 

 weight from the sacrum to the ilium. 

 The appearance of a section through 

 the pelvis along the ilio-pectineal line 

 would be in its essentials like the 

 scheme in the figure : notice from 

 the direction of the articular surfaces 

 that the sacrum is not wedged down 

 between the two hip bones, thus 

 tending to drive them apart, but is 

 suspended from them by posterior 

 sacro-iliac ligaments (x), and therefore would draw them together as it throws weight 

 on them. The bracket c counteracts the inward bending of a on b and makes the 

 whole structure strong and rigid, dispensing with the necessity for strong anterior 

 sacro-iliac ligaments to resist the separation of sacrum and ilium in front. 



In this way we see why there must be such a large ligamentous area above and 

 behind the auricular surface, separating it from the post-vertebral muscle area, with a 

 very ill-marked ligamentous line below and in front of it, and at the same time it 

 becomes clear that the pubic symphysis is not concerned in carrying or transmitting 

 the weight of the trunk, and is on this account not provided with excessively strong 

 ligaments. 



The Acetabular Region (Fig. 104). It has a'ready been said that the three main 

 elements of the bone are all represented in the acetabulum. The Ischium is credited 

 with forming rather more than two-fifths, the Ilium with rather less than this, while 

 the pubis accounts for the remaining fifth. But as a matter of fact the triradiate 

 cartilage that separates these elements, almost till puberty, develops a variable number 

 of small ossifying centres in the floor of the acetabulum : these may fuse to form a small 

 separate bone, the os acetabidi, but in any case there is ultimate junction of the various 



FIG. 103. Scheme to show the mechanical build of the 

 pelvis ; it practically represents a frontal section. 

 Explanation in text. 



