• biological joints

Biological joints come in many types and forms. Some allow movement to different degrees, some do not. Synovial joints like the one shown in section alongside allow free movement. The ends of the bones are covered with articular cartilage (purple), a smooth resilient material which facilitates movement at the joint. The joint surfaces are lubricated by synovial fluid (blue). A capsule surrounds the joint and retains the synovial fluid.

Movement at the joint is partially determined by the shapes of the articular surfaces and controlled and limited by ligaments and muscles.

• joint surfaces

Joint surfaces are adapted for particular ranges of movement. In the example modelled alongside, a pulley-shaped surface articulates with a notch with a complementary shape. Both surfaces are concave in one plane and convex in another at right angles. This is the arrangement seen in a hinge joint such as the elbow. It allows free flexion and extension, and yet is surprisingly stable when twisting forces are applied.

• ligaments

The ligament shown in the diagram to the left is the ulnar collateral ligament on the inside (medial aspect) of the elbow joint. Although the joint between the humerus and ulna is a hinge joint allowing rotation around only one axis, the collagen bundles within the ligament have a complex three-dimensional arrangement. This is to ensure that the required degree of tension is achieved at each position of the joint during flexion and extension. A simplified arrangement has been used for the joints of the adaptable mast.

Reference
Fuss, F.K. (1991) The ulnar collateral ligament of the human elbow joint. Anatomy, function and biomechanics. Journal of Anatomy, 175, 203-212.

• parallels...

And now for something completely different... This is for lovers of fractal geometry - there is a remarkable resemblance between the patterns on the leaves of the outdoor cyclamen and the Mandelbrot set. (Click on the thumbnails for enlarged images.)