The leg bone's connected to …

There are three large major bones in the leg, the femur (or 'thigh bone') which connects to the socket of the pelvis, the tibia, which engages with the femur at the knee, and the fibula, which runs roughly parallel to the tibia from the knee down to the ankle, and whose purpose mainly seems to be getting broken on skiing trips (my father, working as a radiologist in the ski resort town of Tahoe, said that broken fibulas put me through art school). The femur and the tibia are the largest, longest bones of the body, with large knobs at either end, such as make good weapons for cavemen in cartoons (note especially the hammer shape of the femur where it engages with the pelvis).

The knee joint is protected, if that is the word (again, it seems to get injured all the time) by the bony cap of the patella, which connects to the tibia with a large wedge of ligament.

The important thing to note about the bones of the leg is that none of them are truly vertical - the femur, from the front, is held out and away from the body's centerline by its engagement with the pelvis, and then is angled back towards the body's centerline as it moves down towards the knee. From the side we also see the femur travel down and back as it moves towards the knee - it's at a one o'clock position rather than noon, if you can picture that. Likewise the tibia and fibula, while roughly parallel to one another, echo the femur's movement towards the body's centerline as it travels down to the ankle, and tilts slightly back when viewed from the side (though closer to vertical than the femur - maybe 12:30 rather than noon).

Be sensitive to the beautiful, changing angles and rhythms of the leg as it travels from hips to ground, and avoid the straight vertical shaft that is appropriate in drawing a robot (but only a pre- '50's robot) but undermines a figure drawing.