The proximal extremity presents a semi-oval articular surface, not quite at right angles with the shaft, which articulates with the condyles of the femur. The margin is raised toward the anterior of the bone. The _head of the tibia_, or _caput tibiæ_, extends into a rotular process which extends transversely, and is truncate. From the anterior of this process there descends two vertical ridges; one near the angle of the rotular process, the _procnemial ridge_; the other from the outer fibular angle, the _ectocnemial ridge_. On the outer side of the intercondylar tuberosity there is a surface for the ligamentous union with the head of the fibula; and a short distance below this there is a vertical ridge for the close attachment, almost a fusion, with the fibula, called the _fibular ridge_. The _shaft_, or _corpus tibiæ_ is straight and the upper two-thirds subtrihedral; the lower third oval. A _nutrient foramen_ occurs near the upper postero-internal portion of the middle third of the bone. The distal extremity is much smaller than the proximal one; it is quadrangular in form. The expanded inferior end of the tibia forms two articular condyles above which posteriorly there is the _epicondyloid fossa_. The inner condyle, the larger, has a groove near the lower end of the anterior part of the shaft, which deepens toward the intercondyloid space. This _intercondyloid fossa_ in young birds is covered by a strong ligament, which in older birds, becomes ossified. On the lateral side of each condyle, there is a depression for the attachment of ligaments. =The Patella.= _Location._—The patella, or knee-cap, thin and wide, articulates with the procnemial process of the tibia, and with the deep trochlea of the distal end of the femur. _Description._—The patella is irregular in shape with three faces and three borders. The posterior surface is articular. The other two surfaces are rough for tendinous attachment. The patellar ligaments in old birds may become ossified. In order to turn the foot in and out, the tibia not only turns around the inner condyle of the femur, but also around the patella, so that the posterior surface turns outward and thus turns outward the metatarsus and all the toes. =The Fibula.= _Location._—The fibula lies at the outer border of the tibia. It articulates superiorly with the outer condyle of the femur, and laterally with the tibia. _Description._—The fibula is rudimentary; it is largest superiorly and tapers to a slender point. The head is compressed laterally, and furnishes an upper and an inner articular surface. =The Tarsus.=—There exist, during fetal development of the chick, two rows of tarsal bones which later become fused. The upper row fuses with the tibia and the lower with the metatarsus. Therefore, in the adult, there is no tarsus.