8. THE JOINTS.--The point of union of two or more bones forms a joint or _articulation_, the connection being made in various ways according to the kind and amount of motion desired. The movable joints are compacted together by certain strong fibrous bands, called ligaments. These ligaments are of a shining, silvery whiteness, and very unyielding; so much so, that when sudden violence is brought to bear in the vicinity of a joint, the bone to which a ligament is attached may be broken, while the ligament itself remains uninjured. When this connecting material of the joints is strained or lacerated by an {20} accident, a "sprain" is the consequence. An injury of this sort may be, and frequently is, quite as serious as the breaking of a bone. [Illustration: FIG. 4.--CELLS OF CARTILAGE.] [Sidenote: 9. Office of the ligament? What must it have? How accomplished? Describe it. Synovia?] 9. The ligament, then, secures firmness to the joint; it must also have flexibility and smoothness of motion. This is accomplished by a beautiful mechanism the perfection of which is only feebly imitated by the most ingenious contrivance of man. The ends of the bones are covered by a thin layer of _cartilage_, which being smooth and elastic, renders all the movements of the joint very easy. In addition to this, there is an arrangement introduced for "lubricating" the joint, by means of a delicate sac containing fluid. This fluid is constantly supplied in small quantities, but only so fast as it is used up in exercise. In appearance, it is not unlike the white of an egg, and hence its name _synovia_, or egg-like. [Sidenote: 10. What do we observe as regards the composition of a joint? The ligament and cartilage? What varies? Example of the skull? Other examples? The ball-and-socket joint?] 10. Thus, we observe, that two very different substances enter into the composition of a joint. The ligament, very unyielding, affords strength, while the cartilage, elastic and moist, gives ease and smoothness of motion. The amount of motion provided for varies greatly in different joints. In some there is none at all, as in the skull, where one bone is dove-tailed into another by what are termed _sutures_. Others have a hinge-like motion, such as those of the elbow, wrist, ankle, and knee; the most complete of these being the elbow-joint (Fig. 5). Belonging to another class, the {21} ball-and-socket joint, is that at the shoulder, possessing a freedom of motion greater than any other in the body. [Illustration: FIG. 5.--ELBOW JOINT. A, Bone of the arm; B, C, Bones of the fore-arm.] [Sidenote: 11. What is the spinal column? What does it connect and form? Joints of the vertebræ? Amount of motion? Result?]