5S2 COLLEGE ZOOLOGY 



is known as the bastard wing (BW); a middle digit with three 

 phalanges (O1-O2); and a postaxial digit (Ri) containing a 

 single phalanx. 



The pelvic girdle consists of the ilia (Fig. 474, 16), the ischia (30), 

 and the pubes (2Q), as in nearly all of the vertebrates above the 

 fishes. These bones are firmly fused together and united with 

 the posterior part of the vertebral column in the trunk which 

 is called the sacrum. At their junction on either side is a con- 

 cavity, the acetabulum, in which the head of the thigh-bone fits. 



The hind limbs are used for bipedal locomotion. The thigh 

 is concealed beneath the feathers. The femur (Fig. 474, ig) is 

 the short, thick, thigh-bone. In the leg are the slender fibula 

 (21), and the long, stout tibiotarsus (20) which consists of the 

 tibia fused with the proximal row of tarsal bones. The ankle- 

 joint is between the tibiotarsus (26) and the tar so -metatarsus 

 {23) ; the latter represents the distal row of tarsal bones and the 

 second, third, fourth, and fifth metatarsals fused together. The 

 foot possesses, besides the tarso-metatarsus, four digits ; the first 

 is directed backwards and is called the hallux (24) ; and the other 

 three (25, 26, 27) are directed forwards. Each digit bears a 

 terminal claw. The tarso-metatarsus of the fowl bears a back- 

 wardly directed spur (28). 



The Muscular System. — The muscles of the neck, tail, wings, 

 and legs are especially well developed. Those that produce the 

 downward stroke of the wings, the pectoral muscles, are the largest; 

 they weigh about one fifth as much as the entire body; they 

 take their origin from the sternum and its keel, and constitute 

 what is popularly known as the " breast " of the bird. Con- 

 nected with the leg muscles is a mechanism which enables the bird 

 to maintain itself upon a perch even while asleep. If the hind 

 limb is bent, a pull is exerted on a tendon which flexes all of the 

 toes and bends them automatically round the perch. When 

 resting, the mere weight of the body bends the hind limb and con- 

 sequently causes the toes to grasp the perch and hold the bird 

 firmly in place. 



