The os femoris, the longest bone in the human skeleton, is the homotype of the os humeri. Of cylindrical form, it is surmounted by a convex head, directed obliquely upwards, inwards, and a little forwards, and connected to the shaft by a long constricted neck, which appears inserted between two tuberosities, the greater and the lesser trochanter; the former affording attachment to the gluteus me-dius and minimus muscles and to the external rotators of the thigh; the latter giving insertion to the conjoint tendon of the psoas magnus and the iliacus interims muscles. From the trochanters, which are connected at the base of the neck by an anterior and a posterior intertrochanteric line, two ridges extend obliquely down the back surface of the bone to unite in a rough prominent ridge, the linea aspera, which separates at the inferior third of the femur into two lines, which, leaving between them a triangular space, pass, one to a tubercle situated upon the outer condyle, and the other to a tubercle situated upon and a little above the inner condyle. The smooth convex articulating surfaces of the condyles extend much further posteriorly than anteriorly : the anterior surface mounts higher upon the outer than upon the inner condyle, and it offers a broad expanse upon which the patella plays. The plane of the condyle is oblique, so that if the bone be placed upright upon a flat surface, the axis passes obliquely from without downwards and inwards. In many animals, e. g. the elephant, the rhinoceros, the plane of the condyles is even, and the weight of the animal is thrown upon a perfectly upright column of support. The os femoris is not twisted upon itself as is the os humeri, but it is curved, so that the anterior surface is convex and the posterior concave.