Joint task in the human body
- Ball-and-Socket Joints body
The most range of movement by the joints is provided by a "ball-and- socket" joint, in which the spherical head of one bone lodges in the spherical cavity of another. In the shoulder joint, the humerus (upper arm bone) fits into the socket of the shoulder blade. Because the socket is shallow and the joint loose, the shoulder is the body's most mobile joint. The hip joint is less mobile than the shoulder, but it is more stable. The ball of the femur's head fits tightly into a deep socket in the hip bone. A rim of cartilage lining the socket helps grip the femur firmly; the ligament binding the two bones is among the strongest in the human body.
Hinge Joints
The simplest type of joint is the "hinge," as found in the elbows and the joints of the fingers and toes. Hinge joints allow movement in only one direction. The hinge joint of the knee, the body's largest joint, is unusual because it can swivel on its axis, allowing the foot to turn from side to side. Thus, the knee is constantly rolling and gliding during walking.
Gliding Joints
"Gliding" joints permit a wide range of mostly sideways movements - as well as movements in one direction - a pivot joint near the top of the spine allows the head to swivel and bend. Other pivot joints, in the forearm and lower leg, allow the wrist and ankle to twist.
Ligaments of The Hand and Wrist
On the radial shaft of the smaller forearm bone, just below its head, is a process called the "radial tuberosity." It serves to attach the biceps brachi muscle, which bends the arm at the elbow. At the lower end of the radius, a lateral "styloid process" provides attachments for the "palmar radiocarpal ligament" (on the palm of the hand) and the "dorsal radiocarpal ligament" (on the back of the hand) from the radius into the wrist. At the lower end of the larger forearm bone (ulna), its knob-like head articulates with a notch of the radius (ulnar notch) laterally and with a disk of fibrocartilage below. This disk, in turn, joins a wrist bone (the triquetrum). A medial "styloid process" at the lower end of the ulna provides attachments for ligaments ("palmar ulnocarpal ligament" and "dorsal ulnocarpal ligament") into the wrist. The skeleton of the wrist is made up of eight small "carpal bones" that are firmly bound in two rows of four bones each. The mass that results from these bones is called the "carpus." The carpus is rounded on its nearest surface, where it articulates with the radius and with the fibrocartilaginous disk on the ulnar side. The carpus is rounded convexly in the front, forming a canal (retinaculum) through which tendons, ligaments and nerves extend to the palm. Its distal surface articulates with the metacarpal bones, which are joined to the carpus by the "palmar carpometacarpal ligaments."
Saddle Joints
A "saddle" joint is more versatile than either a hinge joint or a gliding joint. It allows movement in two directions. The saddle joint gives the human thumb the ability to "cross over" the palm of the hand.
Spine, Vertebra and Disk
The spine is a column of bone and cartilage that extends from the base of the skull to the pelvis. It encloses and protects the spinal cord and supports the trunk of the body and the head. The spine is made up of approximately thirty-three bones called "vertebrae." Each pair of vertebrae is connected by a joint which stabilizes the vertebral column and allows it to move. Between each pair of vertebrae is a disk-shaped pad of fibrous cartilage with a jelly-like core, which is called the "intervertebral" disk - or usually just the "disk". These disks cushion the vertebrae during movement. The entire spine encloses and protects the spinal cord, which is a column of nerve tracts running from every area of the body to the brain. The vertebrae are bound together by two long, thick ligaments running the entire length of the spine and by smaller ligaments between each pair of vertebrae. The anterior longitudinal ligament consists of strong, dense fibers, located inside the bodies of the vertebrae. They span nearly the whole length of the spine, beginning with the second vertebrae (or "axis"), and extending to the sacrum. The ligament is thicker in the middle (or "thoracic" region). Some of the shorter fibers are separated by circular openings, which allow for the passage of blood vessels. Several groups of muscles are also attached to the vertebrae, and these control movements of the spine as well as to support it. Quasimodo, the central character of Victor Hugo's novel, "The Hunchback of Notre Dame," is probably the most famous of all real or fictional sufferers of "kyphosis," an abnormal, backward curvature of the spine.
