The Lower Extremity and Pelvis 137 



scapula and innominate bone, and it has been assumed that the dorsal elements of the 

 girdles have undergone a rotation on their long axes in opposite directions, so that the 

 iliacus represents the infraspinatus and the ilio-pectineal line the spine of the scapula, 

 with other parts to correspond. This seems to be a pure assumption without any 

 evidence or ground of support, and it would probably be safer merely to look on the 

 dorsal and ventral parts as corresponding in that relation without endeavouring to 

 establish definite homologies (see pp. 63 and 80). 



It is of course a different and justifiable thing to establish homology between the 

 human and other animal bones, and in the case of the pelvis the general correspondence 

 is very evident. Some details, however, require a little consideration. In reptiles 

 and a few birds, and in some mammals, the ischia meet as well as the pubis, the 

 " hypoischium " being a prolongation of the connecting bar between them (Fig. 113). 

 This is nbro-cartilaginous, and as the pubic arch opens out in response to the demand 

 for space for parturition, the cartilage is stranded, so to speak, on the ischial processes 

 as the epiphysis there. The homology between the angular epiphysis and the 

 marsupial bone, itself the homologue of the epipubis of lower vertebrates, has already 

 been mentioned. This must not be confounded with the prepubic process found in 

 many reptiles (Fig. 113), which, if it has any 

 homologous counterpart in the human bone, 

 is represented by the spine of the pubis or 



ui i- ii. ! i i 



possibly by the ilio-pectineal eminence. 



In structure the bone consists, like other 

 flat bones, of a layer of cancellous tissue 

 enclosed between two planes of compact 

 bone. Where the weight of the trunk is 

 transmitted through it that is, in the ilio- 

 pectineal line and its immediate neighbour- FIG. 1 13. Peiv^onizard^seen from below, 

 hood, between the auricular surface and the 



upper and back part of the acetabulum the strong compact layers are very thick, 

 with corresponding decrease in the cancellous layer. The density of the cancellous 

 tissue is increased especially in the tuberosity of the ischium. 



The iliac crest can be felt directly in its whole length, and the anterior superior 

 spine is a very useful point for many measurements during life. Of the posterior spines, 

 the upper can be felt easily, and a transverse line joining it with its fellow goes across 

 the second sacral spine ; the lower is felt less readily, and gives the general level of the 

 lower limit of the sacro-iliac joint and the upper margin of the great sciatic notch. 

 The ischial tuberosity is an important landmark. It is sometimes difficult to place 

 definitely in subjects with well-developed buttocks when lying on their backs, but it 

 can be found by reaching it from below ; this is because it is covered in extension by the 

 thick lower edge of the Gluteus maximus, and by nbro-fatty tissue round this. When the 

 leg is flexed, the edge of the muscle is drawn up and exposes the tuberosity. The ischio- 

 pubic ramus can be followed from the tuberosity forwards. The body of the pubis 

 can be felt, but its parts are defined with difficulty. In women, when the bladder is 

 empty, the bone can be examined bimanually on the surface and through the vagina. 

 The bone in the neighbourhood of the obturator foramen can be more or less examined 

 also through the vagina. 



Development of Ossification. 



The innominate bone is represented in the early embryonic stages by a mass of 

 mesenchyme which is situated in the base of the limb bud : about the beginning of 



