The hinder extremities consist of a nearly straight, long, cylindrical bone, the femur, which has a hemispherical joint head, and a large trochanter, which reaches above the joint; of two leg bones--the tibia, which is prismatic above and cylindrical below, but with only a slight inner ankle projection, and the fibula, which is distinct but thinned and concave in its lower half, where it is close and attached to the other bone, and forms the outer ankle projection. To these are added the bones of the ankle-joint and the clawed toes. The Kangaroo being a great jumper, and having a great tendon, the analogue of the tendo achillis of man, has a powerful projecting process of the hinder ankle bone for its attachment. But the great length of the foot is produced by the size of the fourth and fifth or two outer toes, and especially of the fourth, which often reaches a foot in length, including the metatarsal bone behind, and the pointed claw in front. The great claw looks like a long hoof, is three-sided and sharp-pointed like a bayonet, and with it the animal stabs and rips open the body of its opponent.[86] The outer claw is very small, and there is no great toe (or first), but the second and third are long and slender, and are united in a common skin, so as to look like a single toe with a double nail, the hair coming to the roots of it. The long narrow foot is nearly as long as the leg bones, and is admirably adapted for jumping forwards, as well as sideways, and for supporting, when the legs are widely separated, the weight of the erect body. The body in that position has the leg bones straight, the thigh bones oblique, and the pelvis and spine erect, the great tail being a prop behind. Owen remarks that in man it is the massive and expanded muscles of the back of the pelvis and upper part of the thigh, or the gluteal muscles,[87] which are the chief structures in maintaining the erect posture. But in the Kangaroo, the narrow bones of the haunch could not afford attachment to great gluteal muscles; so a muscle which is but slightly formed in man, and is called the little psoas, is greatly developed in the Kangaroo, and has evidently the power of maintaining the erect posture, although it is situate within the body and in front of the spine. The great jumping power is due to the leverage of the ankle and long toes, and the muscles which supply the tendon already mentioned, and others which have the same office. These are of great strength and size, and there are some accessory muscles to the thigh and leg. The long spine of the back has powerful processes, and the jar of the great jumps is received by two vertebræ which, anchylosed or united together, form the sacrum. The tail is made up of many vertebræ, and covered with muscles. The great blood-vessels running underneath it have many chevron, or V-shaped, bones, to protect them from pressure. The marsupial bones, one on each side, are long, and broad below; they