The toes are fastened by means of well adapted joints to the lower end of the tarsus, and form what is popularly regarded as the bird's foot. When spoken of separately, these toes are called digits, and when spoken of collectively, they are called the podium. They are composed of small bones called phalanges or internodes, which are jointed upon one another like the several parts of the human fingers. The digits can be spread out for walking purposes, or bent around so as to clasp an object. The outer bone of each digit almost always bears a nail or claw, which is sometimes very strong and hooked, as is the case with the birds of prey, while in other species it is only slightly curved and is not meant as a weapon of offense or defense, but chiefly to enable the bird to "scratch for a living." How do the birds, in perching and roosting, retain their hold so long on a limb without becoming weary? They do not need to make a conscious effort to do this, but are held by the mechanical action of certain muscles and tendons in the leg and foot. Of course, the bird can also control these muscles by an act of its will, but a large part of their action is automatic. In some species there is a muscle called the ambiens, which has its rise in the pelvis, passes along the inner side of the thigh, whence its tendon runs over the apex of the angle of the knee joint, and down the leg till it joins the muscles that flex the toes. Now when the bird's leg is bent at the joints, as is the case in perching, the tendons of this muscle are stretched over the knee and ankle joints, thus pulling the digits together, and causing them of their own accord to grasp the perch more or less tightly. When a bird wishes to unloose its hold, it simply rises on its feet and relaxes the tendons. All birds by no means possess this particular muscle, but all the perchers have some muscular arrangement in the legs and toes that practically answers the same purpose. If you will bend your wrist backward as far as you can, you will observe that your fingers will have a tendency to curve slightly forward. This is caused by the stretching of the tendons over the convex part of your bent wrist joints. The typical bird has four digits, three in front and one reaching backward. The hind toe is called the hallux, and corresponds to the thumb of the human hand, so that in grasping an object it can be made to meet any of the other toes. But many birds are not provided with a quartet of digits. The ostrich has only two, the inner and hinder toes being wanting. However, this great fowl does not experience any lack, for its feet are almost solid like hoofs, and quite flat, and hence are especially adapted for traveling across the sandy desert.