Arthrology
Arthrology is a term that refers to the study and classification of joints of the body and how the joints contribute to the articulation (movement) of the various parts of the body.
Joints are located between bones. They can be thought of as the functional junction of the bones. For example, the position of the elbow and its ability to assume different positions makes possible a great range of motions in the arms. Without an elbow, an arm would be capable of only a rigid motion originating at the shoulder. With the articulation power of the elbow, however, an arm can be simultaneously lifted vertically and the lower arm can be turned at a right angle to the vertical movement.
Joints also allow for the alteration and expansion of bone during childhood development and growth, and permit the body to link muscular contraction to movement. Using the elbow as an example again, the contraction of the upper arm biceps muscle and the resulting upward movement of the lower arm would be impossible without the presence of the moveable elbow joint.
The classification of joints is based on the type of tissue present in the joint and on the degree of movement of the joint. With respect to tissue type, joints can be fibrous (containing fibrous connective tissue), cartilaginous (containing a structure of hyaline or fibrocartilage) or synovial (containing lubricating fluid). With respect to movement, joints can be completely immobile (synarthrotic), slightly movable (amphiarthrodic), or totally movable (diarthrodic).
There are several designs of diarthrodic joints in the human body. A ball and socket joint, in which a ball at the end of one bone fits into a cavity at the end of another bone, allows for up-down, side-to-side and rotational movement. Shoulder and hip joints are two examples of an enarthrodial joint. An ellipsoidal joint, such as that between carpals and metacarpals, can move in an up-down and side-to-side fashion. Both gliding (arthrodial) and hinged (ginglymoidal) joints are capable of movement in only one plane (up-down or side-to-side). The elbow and the knee are two examples of a hinged joint. The shoulder controls side-to-side motion of the arm, not the elbow. Another type of diarthrodic joint is also capable of only movement in one plane. But, the movement is rotational rather than up-down or side-to-side. Finally, as exemplified by the thumb, a saddle (or stellar) joint can move in two planes. The thumb joint is important from an evolutionary sense, as it allows the thumb to be used to grasp objects. This ability has set humans and primates apart from other life forms.
Diarthrodic joints vary in their construction. Some, like the shoulder, are relatively simple, with a ball meeting a cavity. Other joints can have three or more surfaces that are capable of motion. Still other joints, the spine for example, incorporate cartilaginous discs in the midst of the moveable joints. Some joints are packed closely together, while other joints, the knee for example, are arranged somewhat more loosely, with the bones being tied together via muscles and ligaments. The latter arrangement reduces friction in a joint that is frequently in motion.
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