on the Conformation of Bones 379 



The knee extensors show in cursorials and saltatorials a con- 

 siderable development in size, the vastus externus being particu- 

 larly affected. Its method of attachment to the femur is influenced 

 by its internal structure, the penniform pattern, as in man, being 

 relatively more powerful than the simple form of the hedgehog; 

 it is largely determined by the attachments of the neighbouring 

 femorococcygeus and gluteus superficialis. Where these lose their 

 attachment to the lateral margin of the femur, the vastus externus 

 and crureus are able to spread around the outer aspect (as in 

 the baboon, man, and sheep) of the shaft. In primitive types, 

 however, it is unable so to spread backwards on account of the 



Fig. 2. Left femur of Priodonies giganieus, showing the descending extensor ridge 

 ("frontal pilaster") and the flange-like third trochanter. M. vastus externus 

 envelops the extensor ridge and occupies the hollow in front of the third 

 trochanter. 



femorococcygeus and the third trochanter. Now the vasti muscles 

 become large in other animals beside cursorials and saltatorials. 

 In edentates, which present a primitive type of femorococcygeus, 

 gluteus, and adductor, the vasti are, compared with the rectus 

 femoris, of unusually large size. As will be seen below, a special 

 outgrowth of bone, the "descending extensor ridge" (Fig. 2), 

 appears on the front below the great trochanter for the attachment 

 and accommodation of the vastus externus. This long outgrowth 

 would be unnecessary and does not occur when the superficial 

 gluteus (and femorococcygeus) have no attachment to the shaft as 

 in the speciahsed runners and jumpers; in them the vastus externus 



