Avian Skeletal System

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Avian Skeletal System

Prepared for the 2012 MacFarlane Pheasant Symposium

By: Rob Porter, DVM, PhD
Minnesota Veterinary Diagnostic Laboratory

pheasant in flight

How does avian skeleton differ from mammalian skeleton?

When animals adapt to particular environments their physical structures will change (natural selection) to take advantage of that environment (eating and reproduction). For example, humans are adapted to walking and running with an upright skeletal structure. Birds are adapted to flight and that is reflected in the structure of the skeleton and individual bones.

Ossification

Bird bones are relatively thin, but are stiff and dense compared to mammal bone. One adaptation is fusion of vertebrae to form a rigid spinal column to support flight.

Bone mass

Birds have reduced absolute bone mass compared to mammals, but the avian bone is generally more dense, indicating a higher strength-to-weight ratio in birds.

Pneumatic bone

Avian bones are often pneumatic (hollow; infiltrated by air sacs). Pneumatic bone is much more extensive in birds that fly for long distances, e.g. songbirds. Penguins and ostriches are ground dwelling birds that have very little pneumatic bone. Gallinaceous birds, such as chickens, turkeys, pheasants, and partridges, have a moderate amount of pneumatic bone. Pneumatic bone has a honeycomb structure on cross-section. The medullary cavity of pneumatic bone in the avian skeleton contains diverticula of internal air sacs. In the chicken, and likely other gallinaceous birds, the sternum, humerus, pelvis, cervical and thoracic vertebrae are pneumatic.

Cross-section of an avian pneumatic vertebral bone

Cross-section of pneumatic vertebral bone reveals lacey, interconnected air channels and cross-branching trabeculae for bone support.

Bone Function

The skeletal system consists of bone and has the following functions:

  1. Protection and support of internal organs, e.g. heart and lungs
  2. Source of calcium for egg production
  3. Articulated, jointed system for movement- walking and flight
  4. Supportive matrix for stem cells that form cells of the peripheral blood- granulopoiesis(leukocytes) and hematopoiesis (red blood cells)

Bone Composition

Inorganic matrix: Bone mineral content = bone ash is approximately 55-65% of bone by dry weight. Consists of hydroxyapatite, which has the chemical formula Ca5(PO4)3(OH) and is a crystal produced by calcium and phosphorus. It is the major component of bones and teeth and provides strength and hardness. Organic matrix (35%) = protein which is mostly type 1 collagen, which gives bone flexibility.

Skeletal Anatomy

The skeleton can be divided into axial and appendicular segments:

  1. Axial skeleton: central axis of body = skull, vertebral column, ribs and sternum
  2. Appendicular skeleton: Pectoral and pelvic girdles, wings and legs

Skull

Only few bones in the skull are movable. These are the lower jaw bones, roof of the mouth and bones of the tongue. Orbits encase the eyes and are large. Ridges or hard edges of the beak substitute for teeth.

full chicken skull with an arrow pointing to the sceral ossicle

Skull (chicken): Note the scleral ossicle (arrow), which is a ring of bone that surrounds and supports the eyeball within the orbit.

Cross-section of an avian pneumatic vertebral bone

 

Vertebral (spinal) column

Provides base for the bones of the trunk and limbs and is main support for skull. Spinal column is composed of chain of bones referred to as vertebrae. These vertebrae provide passage and protection for the spinal cord and nerve roots.

  1. Cervical vertebrae (C) - extend from the skull to the first thoracic vertebral body
  2. Thoracic vertebrae (T) - each is connected to two ribs, one on each side. The first three thoracic vertebrae are fused in chickens.
  3. Lumbar and sacral vertebrae (L and S) - these are fused together into one long synsacrum that also includes several fused caudal vertebrae.
  4. Caudal vertebrae: The last 3-4 caudal vertebrae are fused into single, flattened bone called the pygostyle, which provides attachment for several tail feathers (rectrices).
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Oval outlines the fused thoracic vertebrae that give the spine rigidity for flight

 

 

Ribs

A flattened arch of bone that is attached to the last cervical or thoracic vertebrae and the sternum. Uncinate processes project from one rib to an adjacent rib to form a firm ribcage.

Thoracic ribs (arrows) have uncinate processes (stars), which are projections overlapping and fused to adjacent ribs to create a more rigid thoracic cage.

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Sternum

Breastbone is flattened in birds that do not fly, e.g. ostrich. Most birds, including gallinaceous birds, have a carinate or keeled sternum that resembles the keel of a boat. This provides greater surface area for the flight muscles to attach.

Sternum of gallinaceous birds has a broad, flat, but narrow surface for attachment of flight muscles on either side.

 

Pectoral Girdle

The pectoral or shoulder girdle consists of three bones (“tripod”) to support the wing bones. The girdle consists of 1) coracoid bone, 2) scapula and 3) furcula = clavicle. The coracoid and scapula form a pocket (glenoid cavity) for the head of the humerus to attach. The coracoids, scapula and calvicle also form the foramen triosseum, which is a channel for the tendon of an important flight muscle, the supracoracoideus muscle.

Sternum of gallinaceous birds has a broad, flat, but narrow surface for attachment of flight muscles on either side.

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The bones of the pelvic girdle are the ilium, ischium and pubis. These provide a socket (acetabulum) for the head of the femur.

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The space between the ends of the pubic bones are greatest in birds that are currently in egg production.

 

Wing

The skeleton of the wing consists of the humerus, radius and ulna, carpal bones, a carpometacarpus and digits. The humerus lies close to the thoracic cavity when the bird is at rest and connects with the glenoid cavity of the pectoral girdle. Tendons of two large breast muscles, the supracoracoideus and superficial pectoral muscle, attach to opposite sides of the humerus to raise and lower the wing during flight.

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