STRUCTURE OF BIRDS. 31 



birds is united to the sacrum, forming a large mass of bone 

 indicated in the plate by the letters/, y. 



The pelvis, which is always open in front, excepting in the 

 Ostrich and American Rhea, is composed, as in the Mammalia, 

 of the ilium, ischium, and pubes, denoted by the Greek letters, 

 a, /5, 7. The ilium is anchylosed with the sacrum, and gene- 

 rally with, the two posterior dorsal vertebrae, as well as w^ith 

 the ischium, and frequently the pubes, which latter is a long 

 narrow bone curving backwards, and not meeting its fellow on 

 the opposite side, as in the mammalia. The ischiadic notch, 5, 

 is closed behind by the union of the ilium and ischium ; and the 

 obturator foramen, g, is generally completed by the pubes, but 

 sometimes merely forms a notch. The great size and solidity 

 of the pelvis were rendered necessary for the support of the 

 bird, which, when not flying, is essentially bipedal ; and its 

 separation in front, for facilitating the passage of the eggs. 



The os-femoris or thigh-bone, ^, is cylindrical, slightly curved, 

 with a single large trochanter, and a small hemispherical head, 

 standing off at right angles to the body of the bone, without a 

 neck. Its lower extremity has two condyles, >j, >j, to w^hich the 

 tibia, d, is articulated, but to the outer of which the fibula, /, is 

 also joined. Anterior to the joint is seen the small patella 

 connected as usual with the tibia by a ligament. The thigh- 

 bone is generally very short, and being concealed in the body, 

 leads superficial observers to consider the leg as the thigh. 



The leg, composed of the tibia ^, and the fibula, /, is always 

 much longer than the femur. The fibula is merely a slender, 

 gradually attenuated bone, generally adherent to the tibia, 

 w^hich is straight, of a triangular form above, and flattened below, 

 w^here it is articulated by two condyles. 



The tarsus and metatarsus are united so as to form a single 

 bone, generally named the tarsus, X, X, which varies greatly 

 in length and size, as well as in form, being roundish, square, 

 flattened transversely or compressed. At its upper extremity 

 it has two cavities corresponding to the pulley of the tibia, 

 generally with an intervening prominence or tubercle in front, 

 which being received into a depression of the tibia, tends to 

 strengthen the joint when the bird is standing. At its lower 



