Classification of Bones According to Gross Appearance
Any bone may be classified in one of the following groups: long, short, flat, sesamoid, pneumatic, or irregular.
Long bones are greater in one dimension than any other. Each consists of a relatively cylindrical shaft (the diaphysis) and two extremities, the epiphyses, with a metaphysis between each epiphysis and the diaphysis.
Long bones function chiefly as levers and aid in support, locomotion, and prehension. The best examples of long bones are in the extremities. in the thoracic limb the long bones include the humerus, radius, ulna, metacarpals, and phalanges. in the pelvic limb, the long bones are the femur, tibia, fibula, metatarsals, and phalanges.Short bones are cuboid, or approximately equal in all dimensions. There is no single marrow cavity, but the interior is composed of spongy bone filled with marrow spaces. The exterior is formed by a thin layer of compact bone. short bones absorb concussion, and they are found in complex joints such as the carpus (the “knee” of the thoracic limb) and tarsus (hock), where a variety of movements as well as absorption of shock occur.
Flat bones are relatively thin and expanded in two dimensions. They consist of two plates of compact bone, the lamina externa and lamina interna, separated by spongy material called diploe.
Flat bones function chiefly for protection of vital organs such as the brain (skull), the heart and lungs (scapulae and ribs), and the pelvic viscera (pelvis), but many, notably the scapulae and pelvis, provide large areas for attachment of muscles.
Sesamoid bones are so called because of their fancied resemblance to a sesame seed, although many sesamoid bones of domestic animals have decidedly un-seedlike shapes. sesamoid bones occur along the course of tendons to reduce friction, increase leverage, or change the direction of pull. The patella (kneecap) is the largest sesamoid bone in the body.
Pneumatic bones contain air spaces or sinuses that communicate with the atmosphere. The frontal bones and maxillary bones of the skull are examples of this type of bone among mammals. Many avian bones are “pneumatized” as well (see Chapter 30).
Irregular bones are unpaired bones on the median plane; they include the vertebrae and some of the unpaired bones of the skull. These bones do not fit well into any other descriptive classification. They feature prominent processes and offer protection, support, and muscular attachment.