1. Femur, thigh bone 2 2. Tibia, shin bone 2 3. Fibula 2 4. Patella, kneepan 2 5. Tarsal, ankle bones 14 6. Metatarsal, instep bones 10 7. Phalanges of toes 28 ARTICULATIONS Any place in the body where two or more bones meet is called an articulation, or joint. At the place of meeting the bones are firmly attached to each other, thereby securing the necessary coherence of the skeleton. The large number of bones, and consequently of articulations, are necessary for the different movements of the body and also on account of the manner in which the skeleton develops, or grows. Articulations are classed with reference to their freedom of motion, as _movable_, _slightly movable_, and _immovable_ articulations. Most of the _immovable_ articulations are found in the skull. Here irregular, tooth-like projections from the different bones enable them to interlock with one another, while they are held firmly together by a thin layer of connective tissue. The wavy lines formed by articulations of this kind are called _sutures_ (Fig. 100). The best examples of joints that are _slightly_, but not freely, _movable_ are found in the front of the spinal column. The cartilaginous pads between the vertebræ permit, by their elasticity, of a slight bending of the column in different directions. These movements are caused, not by one bone gliding over another, but by compressions and extensions of the cartilage. Between the vertebræ in the back of the spinal column, however, there is a slight movement of the bone surfaces upon one another. *Structure of the Movable Joints.*—By far the most numerous and important of the joints are those that are freely movable. Such joints are strongly constructed and endure great strain without dislocation, and yet their parts move over each other easily and without friction. The ends of the bones are usually enlarged and have specially formed projections or depressions which fit into corresponding depressions or elevations on the bones with which they articulate. In addition to this the articular surfaces are quite smooth and dense, having no Haversian canals, and they are covered with a layer of cartilage. Strong ligaments pass from one bone to the other to hold each in its place (_A, _Fig. 103). Some of these consist simply of bands, connecting the joint on its different sides, while others form continuous sheaths around the joint. [Fig. 103] Fig. 103—*Outside and inside view of knee joint.* 1. Tendons. 2. Ligaments. 3. Cartilage. 4. Space containing synovial fluid. This space is lined, except upon the articular surfaces, by the synovial membrane.