Part 9

=The Femur= (Fig. 12, A and B). Location.—The femur, or os femoris, the first bone of the hind extremity, extends obliquely downward and forward, articulating with the acetabulum above and the tibia, the fibula, and the patella below.

Description.—The femur, one of the largest, thickest, and strongest bones of the body, belongs to the class of cylindrical bones, and presents for examination a shaft and two extremities.

The superior, or proximal extremity, is provided with a head, caput femoris; the neck, collum femoris; and the trochanter. The head is relatively small, and is marked by a depression above for the round ligament which fills the cavity in the acetabular wall. Its axis is nearly at right angles with the shaft. A neck joins it to the body at the proximal end. External to the head of the femur is the trochanter. The trochanter presents an outer convexity over which the tendon of the gluteus maximus extends to become inserted below. The trochanteric ridge, which is opposite the articular head, presents an outer flattened surface which possesses impressions for muscular attachments. The trochanteric fossa, or fossa trochanterica, is shallow.

The shaft, or corpus femoris, is shorter than the tibia, is in general cylindrical, bent forward, and the lower half is flattened and expanded transversely. A nutrient foramen is located in the median portion of the shaft. The shaft shows muscular linear ridges.

The distal extremity is large in both directions and comprises the trochlea in front and two condyles behind, one internal and one external. The condyles are separated by the intercondyloid fossa, or fossa intercondyloidea, which is marked with pits for the attachment of ligaments; and above this is the epitrochlear fossa.

FIG. 12.—Bones of the hind extremity.

A. Posterior view of femur.

B. Anterior view. 1, Shaft. 2, Proximal extremity. 3, Distal extremity. 4, Articular head. 5, Trochanter major. 6, Shallow trochanteric fossa. 7, Convexity over which the tendon of the gluteus maximus glides. 8, External condyle. 9, Internal condyle. 10, Nutrient foramen. 11, Intercondyloid fossa. 12, Muscular linear ridges. 13, Epitrochlear fossa.

C. Internal view of tibia.

D. External view. 1, Shaft. 2, Articular head. 3, Distal end. 4, The rotular process. 5, The fibular ridge. 6, External condyle. 7, Internal condyle. 8, Intercondyloid space. 9, Nutrient foramen.

E. Fibula, lateral view. 1, Articular head. 2, Its attenuated portion.

F. Posterior view of metatarsus. 1, Trochlea for inner or second digit. 2, Trochlea for middle or third toe. 3, Trochlea for outer digit. 4, Bony core for spur. 5, Bony canal for tendons. 6, Groove for tendons. 7, Fossa intercondyloidea. ]

The inner condyle begins anteriorly as a ridge, and expands into a convexity which attains its greatest breadth posteriorly where it becomes more flattened. The inner side of the inner condyle is flattened and is provided with a tuberosity at its mid-part and a second just above the posterior part of the condyle.

The outer condyle is formed in the same manner as the inner condyle. It is indented at its broad, lower end by an angular groove, which, winding divides the posterior part of the condyle into two convexities. The more external convex ridge and the groove dividing it from the outer condyle are adapted to the head of the fibula. There is in this part a fibular ridge and above this ridge a tuberosity.

=The Tibia.= Location.—The tibia (Fig. 12, D and C) extends obliquely downward and backward from the knee-joint to the hock. It articulates above with the femur and, by its procnemial process with the patella, below with the metatarsus, and laterally with the fibula.

Description.—The tibia is the longest bone in the posterior limb, and possesses a shaft and two extremities. It is largest at the proximal end and presents three faces.

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.

=The Metatarsus.= Location.—The metatarsal bone extends downward and forward. In birds it consists of one bone, which articulates superiorly with the inferior extremity of the tibia. On the distal end it has a threefold trochlear arrangement which articulates with the three principal digits.

Description.—The proximal extremity posteriorly has a process which may be considered as a consolidation of originally separate metatarsal bones. The process at the supero-posterior part of the metatarsus is called the hypotarsus of the tarso-metatarsus, through which extends a canal called the hypotarsal canal, and which gives passage to flexor tendons.

The proximal end of the metatarsus is ossified from one center of ossification forming an epiphysis which caps the ends of the three original metatarsal bones that coalesce, first with one another, then with the epiphysis, thus forming a single compound bone. Above and just to the inside of the metatarsus there may occur a small bone which is imbedded in ligaments and articulates with the inner proximal surface of the metatarsus. This has been called by Gadow a sesamoid bone.