xvi. 6 LEGS 443 



this arrangement. The ischium and pubis are directed backwards and 

 do not meet in a symphysis, which would prevent the underslinging 

 of the viscera. 



The legs are used for balance and walking or hopping in ways that 

 show interesting similarities and differences from those of man. The 

 femur is turned under the body and articulates with the acetabulum 

 in such a way that movement is almost restricted to the antero- 

 posterior direction. The bird balances on its hips only in the sagittal 

 plane; there are no movements of abduction and adduction such as are 

 found in man. Abduction of the leg, or the falling medially of the 

 bird's body when standing on one leg, is prevented by the fact that 

 besides the ball and socket articulation of the femoral head there is 

 also a second joint surface between the trochanter and an anti- 

 trochanter of the ilium. The ligaments across the top of this joint are 

 very strong and they limit abduction movements, while movements 

 of adduction are restricted by a strong ligamentum teres attached to 

 the femoral head. 



In life the femur is held nearly horizontal, bringing the legs well 

 forward. The bird replaces the movements of abduction and adduc- 

 tion, which we make at the hip during walking in order to prevent 

 falling over while only one leg is on the ground, by movements of 

 rotation at the knee. The muscles around the hip joint form a system 

 of braces allowing balancing and locomotion much as in man, but they 

 are well developed only anteriorly and posteriorly; the lateral and 

 medial (abductor and adductor) elements are weak (Figs. 255-8). The 

 anterior group (protractors) includes a sartorius (ilio-tibialis internus) 

 running from the ilium to the tibia, an ilio-femoral, and a large 

 anterior ilio-tibial inserted through a patella to a ridge on the front of 

 the tibia. Associated with this muscle, w*hich crosses both hip and 

 knee joints, there are also, as in man, femoro-tibial muscles, making 

 up with the longer muscles, the extensor system of the knee. 



The lateral side of the hip joint is supported by rather small ab- 

 ductor braces, the ilio-trochanteric muscles, corresponding to our 

 glutei, and acting mainly as medial rotators, opposed by obturator and 

 ischio-femoral muscles, which work as lateral rotators. The main loco- 

 motor muscles are the posterior braces or retractors, lying behind the 

 hip joint and including muscles know r n as the posterior ilio-tibial, 

 ilio-fibular, caud-ilio-flexorius, pub-ischio-femoral, ischio-femoral, 

 and caud-ischio-femoral. Some of these also act with the obturator 

 muscle as lateral rotators, and those placed more medially function 

 as adductors or medial braces, so far as such are required. 



