60 



ARBOREAL MAN 



segment ; but with the gradual loss of this mobility, and 

 its substitution by stability, the muscle shifted its origin 

 as its function changed, and, ascending via the capsule 

 of the knee-joint, it joined the external condyle of the 



Fig. 21. — The Back of the Human Knee-Joint, to show the 



PoPLiTEus Muscle. 



femur. It then exchanged its rotating function for that 

 of flexion of the knee-joint. This is a mere repetition of 

 the story of the ulnar head of the M. pronator radii teres 

 in those quadrupedal animals in which the fore-limb has 

 suffered changes identical with those in the hind-limb 



