. Bulletin. Natural history. THE POSTCRANIAL SKELETON OF AFRICAN CYNODONTS 185 sal migration of the ilio-psoas group from its primitive attachment to the pubis (as the pubo-ischio-femoralis internus) is explicable in terms of introducing an upward com- ponent to femoral flexion. The shift of ilio-psoas insertion onto the anteroventral tro- chanter appears to be related to the development of an inflected femoral head and the resultant repositioning of the mechanical axis. In mammals the mechanical axis passes through the femoral head and between the tibial condyles (Fig. 55Ai). If the ilio- pso
. Bulletin. Natural history. THE POSTCRANIAL SKELETON OF AFRICAN CYNODONTS 185 sal migration of the ilio-psoas group from its primitive attachment to the pubis (as the pubo-ischio-femoralis internus) is explicable in terms of introducing an upward com- ponent to femoral flexion. The shift of ilio-psoas insertion onto the anteroventral tro- chanter appears to be related to the development of an inflected femoral head and the resultant repositioning of the mechanical axis. In mammals the mechanical axis passes through the femoral head and between the tibial condyles (Fig. 55Ai). If the ilio- psoas had remained in a position analogous to that in pelycosaurs—on the anterior and dorsal aspect of the proximal shaft (Fig. 55B)—the insertion would lie lateral to the mechanical axis. Such an arrangement would engender a substantial rotational component to ilio-psoas action and the result of both flexion and rotation might ap- pear as in Figure 55B. However, the migration of ilio-psoas insertion onto the apex of the lesser trochanter eliminates any major rotational component because the insertion lies approximately on the mechanical axis. Flexion alone results. Although there has been longstanding controversy over the possible rotational actions of the ilio-psoas, elec- tromyography in humans, at least, has demonstrated that no rotational components exist (Basmajian, 1967: 206-209). Insertion of the ilio-psoas onto the lesser trochanter thus appears to be an arrangement to produce flexion of the thigh without rotation— the simplest movement to advance the femur in a nearly parasagittal plane. In conclusion, it seems likely that there has always been an anteroventral trochan- ter (an internal trochanter or a lesser trochanter) in synapsids that were on or close to the line of mammalian ancestry. In the more primitive forms the trochanter was asso- ciated primarily with the pubo-ischio-femoralis externus and to a lesser extent with the pubo-ischio-femoralis internus. In l
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