The formation of the joints requires not only bones, but also cartilages, ligaments, and the synovial membrane, to complete the articulation. _Cartilage_ is a smooth, elastic substance, softer than bone, and invested with a thin membrane, called _perichondrium_. When cartilage is placed upon convex surfaces, the reverse is true. The _Ligaments_ are white, inelastic, tendinous substances, softer than cartilage, but harder than membrane. Their function is to bind together the bones. The _Synovial Membrane_ covers the cartilages, and is then reflected upon the ligaments, thus forming a thin, closed sac, called the _synovial capsule._ All the synovial membranes secrete a lubricating fluid, termed _synovia_, which enables the surfaces of the bones and ligaments to move freely upon one another. When this fluid is secreted in excessive quantities, it produces a disease known as "dropsy of the joints." There are numerous smaller sacs besides the synovial, called _bursæ mucosæ_, which in structure are analogous to them, and secrete a similar fluid. Some joints permit motion in every direction, as the shoulders, some in two directions only, as the elbows, while others do not admit of any movement. The bones, ligaments, cartilages, and synovial membrane, are supplied with nerves, arteries, and veins. When an animal is provided with an internal bony structure, it indicates a high rank in the scale of organization. An elaborate texture of bone is found in no class below the vertebrates. Even in the lower order of this sub-kingdom, which is the highest of animals, bone does not exist, as is the case in some tribes of fishes, such as sharks, etc., and in all classes below that of the cartilaginous fishes, the inflexible substance which sustains the soft parts is either shell or some modification of bone, and is usually found on the outside of the body. True bone, on the contrary, is found in the interior, and, therefore, in higher animals, the skeleton is always internal, while the soft parts are placed external to the bony frame. While many animals of the lowest species, being composed of soft gelatinous matter, are buoyant in water, the highest type of animals requires not only a bony skeleton, but also a flexible, muscular system, for locomotion in the water or upon the land. Each species of the animal kingdom is thus organically adapted to its condition and sphere of life. * * * * * CHAPTER III. PHYSIOLOGICAL ANATOMY. THE MUSCLES. [Illustration: Fig. 19. Muscular fillers highly magnified.]