Skeleton and Bone

The skeletal system provides the chicken with structural support. In addition, the skeletal system assists in respiration and calcium transport. The system is compact and lightweight, but incredibly strong. The pneumatic and medullary bones make up the system. Pneumatic bones contain the air sacs of the respiratory system that connect with the cavity of some of the long bones. Medullary bones consist of interlacing bony spinicles with blood sinuses and marrow between the spinicles. This type of bone provides calcium that can be used for eggshell production.

Vertebral Column

The vertebral column is divided into cervical, thoracic, lumbar, sacral and caudal regions. Vertebrae in the thoracic portion are partially fused into the notarium. In the lumbrosacral region, they are fused into the synsacrum. The last few caudal vertebrae are fused into the pygostyle.  These three areas are fused to give the body sufficient strength to support the wings and tail feathers. The flexibility of the chicken’s neck is due to the cervical vertebrae. Most mammals have seven, whereas chickens, turkeys, and ducks have fourteen. The vertebral formula of the chicken. At the base of the skull, the atlas provides support for movement of the head. It is small and ring-like with a deep cavity for articulation with the single projection or condyle of the base of the skull. The axis, also called the epistropheus, is short and possesses a small process. This process is the dens, which projects from the cranial end. It passes through the ring-like atlas and articulates with the occipital condyle. It allows the head to turn on the neck.

Image of the vertebral column of the chicken

Source: http://svpow.files.wordpress.com/2009/02/wedel-2009-fig-4.jpg

Skull

             The head of the chicken is largely reduced because a large head would hinder flight. It is divided into two regions: the rounded cranium and the conical facial region. The sphenoid bone and the ethmoid bone are small, thin bones that come together to form thin septum that separate the orbits into which the eyes fit. Openings at the base of the cranium provide for the direct connection between the brain and the spinal cord. Openings in each lateral part of the occipital bone provide for the carotid arteries and jugular veins. The incisive bone is a portion of the maxilla that is adjacent to the incisors, in animals with teeth. However, chickens lack teeth because their strong beak and hard tongue are all they need for their diet. The mandible makes up the bottom part of the beak. However, the equivalent of the maxilla in mammals, is often called the “upper mandible” in birds. The upper mandible is usually longer than the lower mandible and sometimes has to be filed or cut back to prevent injury.

Image of the skull of a chicken

Source: http://rowntreestudio.com/wp-content/uploads/catablog/originals/chicken_skull_ersmith.jpg

Thoracic limb

            The scapula, the coracoid and the clavicle make up the shoulder girdle. The shoulder girdle is the attachment to the axial skeleton. The right and left clavicles are fused. This is called the wishbone, but is formally known as the furcula. It acts as a bony strut to maintain distance between the two shoulders. The humerus is a large long bone with an ovoid head for articulation with the scapula, coracoid and clavicle. It is a pneumatic bone with a connection to the clavicular air sac. In the chicken, the radius is smaller than the ulna. They both articulate with the carpal bones. The three carpometacarpal bones are the result of fusion between the distal row of carpal bones and the proximal end of the metacarpals. There are three digits. The first is the pollux and possess two phalanges. The second has two developed phalanges and one vestigial phalanx. It is robust and strong. The third digit is small with one phalanx.

Image of the thoracic limb of a chicken

Source: http://arscpart2.wikispaces.com/file/view/wing4labels.jpg/215931050/wing4labels.jpg

Pelvic limb

Making up the pelvis of the chicken, as in most mammals, is the ilium, the ischium and the pubis. These three bones also make up the acetabulum, which articulates with the femur. The femur is a long bone that is thick and bent. The distal end provides a pulley-like articulation with the patella and two convex condyles that articulate with the bones of the lower leg. The pelvic limb is similar to the thoracic limb in that it shows a reduction in the number of individual bones of the tarsus. The tibiotarsus is formed by the fusion of the proximal row of tarsal bones with the tibia. The tibia is longer than the fibula and is much thicker at the proximal end. The tibiotarsus is fused with the fibula as well. The tarsometatarsus is formed by the fusion of the distal tarsal bones to produce a single weight-bearing metatarsal bone. Moving further downwards, poultry have four digits. One is in the medioplantar direction and the other three are aimed forward.

Image of the pelvic limb of a chicken

Source: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZ0Hk8d13IT_yQpBzIHMva-2YALTKPX4pjNSejvA9XvLrcAFS_AdgLjB4s9Nvzi-rxxjxe9jfeBqroLPvSOQ06Pz-J_ka3D0v9zkRQhP_nHHkc7o8EvptFfZF216yRS_AytYIdgrmFfC47/s1600/HumbirdlegCCS11.jpg

Sternum

            Poultry need drastic modification to ready their body for flight, as opposed to flightless mammals. The sternum is one of the most modified bones in the chicken’s body for this purpose. The sternum is the primary attachment site for the pectoral muscles. The m. pectoralis major and m. pectoralis minor are responsible for the down and upbeat of the wings during flight. The sternum is enlarged and shield-like for the purpose of flight. To enhance muscle attachment, there is a large midline keel or sternal crest. The cranial portion of the sternum provides many projections for attachment sites of muscles.

Image of the skeleton of the chicken (featuring the sternum between the coracoid and fibula)

Source: http://www2.ca.uky.edu/poultryprofitability/Production_manual/Chapter3_Anatomy_and_Physiology/chicken_skeleton.png