Irregular bone

Overview
The irregular bones are such as, from their peculiar form, cannot be grouped as long bone, short bone, flat bone or sesamoid bone. Irregular bones serve some unique purpose in the body of combining: (1) protection of nervous tissue (such as the vertebrae protect the spinal cord), (2) affording multiple anchor points for skeletal muscle attachment (as with the sacrum), and maintaining pharynx and trachea support, and tongue attachment (such as the hyoid bone).

They consist of cancellous tissue enclosed within a thin layer of compact bone.

The irregular bones are: the vertebræ, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, and hyoid.

Reference

 * Stedman's Online Medical Dictionary, 27th Edition

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