446 THE BIRDS xvi. 6- 



The knee joint has some remarkable similarities to that of man. It is 

 stabilized by lateral, medial, and cruciate ligaments and contains a pair 

 of lunate cartilages or 'menisci'. The joint allows movements of flexion 

 and extension and the femur as it extends on the tibia in walking 

 rotates laterally because of the arrangement of the joint surfaces. The 

 bird thus balances in the medio-lateral plane by rotation at the knee, 

 somewhat as we do by abduction-adduction at the hip (Fig. 259). 

 When it makes a step forward the weight is brought by this rotation 

 at the knee over the leg that remains on the ground. 



Fig. 259. Drawings from photographs of a goose, A, standing; B, step- 

 ping. The centre of gravity S is brought over the foot on the ground by 

 lateral rotation of the femur on the tibia. Note the position of the tail 

 in B. (After Heinroth, from Stolpe.) 



The intertarsal joint allows mainly movements of flexion and exten- 

 sion. It is largely supported by ligaments and has a very strong capsule 

 and lateral and cruciate ligaments rather like those of the knee; there 

 is even a meniscus on the lateral side. The back of the tibia is occupied 

 by the gastrocnemius and the flexor muscles of the toes and at the front 

 there is a tibialis anterior acting across the inter-tarsal joint, and also 

 extensors of the toes. The calf muscles are mainly concerned with pro- 

 ducing flexion of the toes in the act of perching and they form an 

 elaborate system of tendons attached to the phalanges. These tendons 

 often act as a single unit, and there is an arrangement by which the 

 flexion is passively maintained by the weight of the body, even during 

 sleep. Many of the muscles are specially arranged to allow support of 

 the joint whether in the flexed or extended position. The ilioflbular 

 muscle passes through a conspicuous sling for this purpose (Figs. 255 

 and 257). The flexor muscles of the toes are inserted largely above the 

 knee and thus tend to tighten as the bird sinks. In this they may be 

 assisted by the ambiens, a muscle found in reptiles and some birds, 

 which takes origin from the ilium. The muscle belly lies on the medial 



