The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position (i.e. when the arms are down at the sides of the body and the palms of the hands face forward) the ulna is located at the side of the forearm closest to the body (the medial side), the side of the little finger.
Articulations
The ulna articulates with:
Proximal and distal aspects
The ulna is broader proximally, and narrower distally.
Proximally, the ulna has a bony process, the olecranon process, a hook-like structure that fits into the olecranon fossa of the humerus. This prevents hyperextension and forms a hinge joint with the trochlea of the humerus. There is also a radial notch for the head of the radius, and the ulnar tuberosity to which muscles attach.
At the distal end of the ulna is a styloid process.
Structure
The long, narrow medullary cavity is enclosed in a strong wall of compact tissue which is thickest along the interosseous border and dorsal surface. At the extremities the compact layer thins. The compact layer is continued onto the back of the olecranon as a plate of close spongy bone with lamellæ parallel. From the inner surface of this plate and the compact layer below it trabeculæ arch forward toward the olecranon and coronoid and cross other trabeculæ, passing backward over the medullary cavity from the upper part of the shaft below the coronoid. Below the coronoid process there is a small area of compact bone from which trabeculæ curve upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of compact bone. The trabeculæ at the lower end have a more longitudinal direction. http://www.innerbody.com/image_skelfov/skel21_new.html
Muscle attachments
Fracture
Specific fracture types of the ulna include:
In other animals
In four-legged animals, the radius is the main load-bearing bone of the lower forelimb, and the ulna is important primarily for muscular attachment. In many mammals, the ulna is partially or wholly fused with the radius, and may therefore not exist as a separate bone. However, even in extreme cases of fusion, such as in horses, the olecranon process is still present, albeit as a projection from the upper radius.[1]
Gallery
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Ulna, olecranon process - posterior view
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Ulna, styloid process - anterior view
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Ulna, olecranon process - anterior view
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Right posterior human radius and ulna
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Bones of left forearm. Anterior aspect.
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The radius and ulna of the left forearm, posterior surface.
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Left elbow-joint, showing anterior and ulnar collateral ligaments.
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Cross-section through the middle of the forearm.
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Transverse section across distal ends of radius and ulna.
See also
References
- ^ Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. p. 200. ISBN 0-03-910284-X.
External links
This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.