Avian skeletal system

1. Dr. Raja Samba Murthy. K
GVM/2018-20
MVSc Poultry Science
NTR CVSc, Gannavaram
AVIAN SKELETAL SYSTEM

3. • While fowls are not able to fly well, they still retain that
ability to some extent.
• These modifications include:
• Pneumatic bones where the air sacs of the respiratory
system connect with the cavity of some of the long bones.
• Fusion of some vertebral sections to provide the rigidity
required for flight.
• The sternum provides a large surface area for the strong
attachment of the main muscles used for flight.
• Compared to other animals, the size of the head has been
reduced significantly as a large head would be a hindrance
when flying.

4.  The neck is quite long in most bird species to enable:
 Protection of the delicate tissues of the brain from too much
jarring when landing.The flexibility of the neck acts as a shock
absorber.
 The bird to adjust its centre of gravity when the bird changes
from the upright position of walking or perching to the more
horizontal position of flight.
 The long tail of many other animals has been reduced to a very
short section of fused bones called the pygostyle.
 The ribs have been modified by the inclusion of the uncinate
process .

6. Vertebrae
 The vertebral column has 39 separate bones and is divided
into five sections or groups –
 Cervical vertebrae,
 Thoracic vertebrae,
 Lumbar vertebrae,
 Sacral vertebrae and
 Coccygeal vertebrae.
 The vertebral formula is C13,T7, L+S14 & C5 = 39

9. Atlas
 The atlas-axis at the base of the skull is quite different to
all other bones of the vertebral column.
 The atlas is small and ring-like with a deep cavity for
articulation (moving together) with a condyle at the base
of the skull.

10. Axis
 The axis, or epistropheus.
 It possess a small process, the dens, which allows the axis to
articulate with the occipital condyle.
 This joint allows the head to turn on the neck.

11.  The seven thoracic vertebrae carry the ribs and all except the
last have large ventral processes for the attachment of
muscles.
 It is very difficult to separate the seventh thoracic, lumber,
sacral and first coccygeal vertebrae because they are so
closely fused for strength. -Synsacrum
 Consequently, the lumbar and sacral vertebrae are usually
treated as one group.
 The last of the coccygeal are fused to form the pygostyle and
provides a strong foundation for the tail feathers.

12. Ribs
 There are seven pairs of ribs originating on the thoracic
vertebrae.
 All except the first and last have uncinate (meaning hooked or
bent)processes , which adds strength to the thoracic cavity and
it will not collapse during flight.

13. Sternum or breastbone
 The breast bone is attached to five or six pairs of ribs
which protect the chickens internal organs.

14. Metasternum
 The caudal medial projection, or metasternum, is the
longest projection and carries the tall plate like ridge or
sternal crest or keel or carina.
 This crest is more commonly called the keel bone and
provides a suitable attachment for the major muscles of
flight – the pectoralis and subcontractors muscles.

15. Skull
 The skull is divided into two regions:
 Rounded cranium
 Conical facial region
 Two very thin bones, the sphenoid and ethmoid bones,
together form the very thin septum that separates the
orbits.

17. The forelimb or wing
 They are essentially pentadactyl (five digits) limbs modified for the
special purpose of flight.
 The wing skeleton may be divided into:
 The shoulder girdle:
 Scapula
 Coracoid
 Clavicle
 The wing:
 The upper arm – or humerus
 The forearm – radius and ulna
 The manus or hand – carpus, metacarpus and digits

18. Scapula and coracoid
 The scapula is narrow, thin and slightly curved.
 At the cranial end a section of a cavity receives the head of the
humerus.
 The coracoid is the strongest bone of the shoulder girdle.
 One end carries a flattened articular surface to fit into the sternum.

19. Clavicle
 The clavicle or collarbone is thin, rod-like and slightly bent.
 Its upper, or dorsal, end is connected with the coracoid
bone.
 The other end is joined to that of the other wing to form the
“wish-bone”.
 The combined clavicles form a bone called the furcula.

21. Humerus, radius and ulna
 The humerus is a long bone with an ovoid head for articulation
with the scapula.
 It is a pneumatic bone with a connection with the clavicular air
sac.
 The two bones of the forearm are the ulna (the thicker and
longer), and the radius that lies laterally to the ulna.
 The large space between the ulna and radius is called the
interosseus space.

23. Manus (hand)
 The manus, consists of the carpus, metacarpus and the digits.
 The carpus of an adult contains only two bones – the ulnare and
radiale that represent the proximal row of mammalian carpal
bones.
 In the embryo, cartilaginous nodules represent the distal row but
these fuse with the metacarpus and disappear.
 In the adult, the metacarpus is in the form of a single bone that is
produced by the union of three elements that correspond to the
first, second and third metacarpal bones.
 Modification and fusion has reduced the number of recognisable
digits to

24. The leg
 The significant features of the skeleton of this limb are:
 The hipbone is firmly fixed to the vertebral column.
 There is no ventral union between the two hipbones.
 There is no independent tarsus.
 The leg skeleton may be divided into:
 The pelvic girdle or hip bones:
 Ileum
 Ischium
 Pubis or pin bones
 The leg:
 Femur or thighbone
 Tibia and fibia
 Pes or foot – tarsus, metatarsus and digits or toes

25. Femur, tibia and fibula
 The femur is a typical long bone and is quite thick and bent.
 The proximal end has a prominent head that fits loosely into the
acetabulum.
 The distal end carries the deep pulley shaped surface for the
patella (knee cap) and two convex condyles that articulate with
tibia.
 The tibia is a much longer bone than the fibula (greatly reduced)
 The proximal row of tarsal bones is fused to the distal end of the
tibia.

27. Feet
 In the adult fowl there is no independent
tarsus.
 In the embryo, the two rows exist but the
proximal fuses with the tibia.
 The adult tarsometatarsus is a long bone
actually formed by the union of the second,
third and fourth metatarsal bones and distal
tarsus
In male chickens a small conical projection
on the medial side supports the spur.
 Most fowl breeds have four digits – three
facing forwards and one facing back.

28. Bone formation
 The skeleton provides support and protection for the remainder
of the systems and tissue.
 The chemical composition is also quite variable although it
mainly consists of calcium and phosphorus in the form
of hydroxyapetite [3Ca3(PO4)2.Ca(OH)2] crystals deposited
on a fine matrix of collagen fibres, along with other cell types.

29. Deposition and adsorption of bone
 The metabolic activity in bones is continuous and the
microscopic structure is constantly changing.
 Small cells, called osteoblasts, are responsible for depositing
new bone tissue, while large polynucleate cells, called
osteoclasts, resorb existing bone.
 Other cells, called osteocytes, found in the calcified mass of
bone, help maintain the bone structure.
 Thus the skeleton is a major reservoir of calcium and
phosphorus.

30.  The microstructure of the bone changes continuously as bone is a
target for a number of influences.
 Other influences include:
 The level of certain hormones e.g. growth hormone,
parathyroid hormone, calcitonin, oestrogenic and androgenic
hormones in the blood
 The level of vitamin D in the diet
 Young chickens are very sensitive to vitamin D deficiency.
 In the layer hen, the skeleton is particularly vulnerable to the
demands made for calcium for eggshell formation

31. Stages of bone development
 It is usual to find that bones pass through three stages as
they develop:
 Prechondral or membranous stage
 Chondral or cartilaginous stage
 Ossification stage (bone formation)

32.  Most of the bones of the fowl go through the cartilaginous stage.
A few such as the bones of the skull omit this stage.
 The membranous stage takes place in the egg during embryo
development.
 Only the cartilaginous stage and the ossification stages are easily
identified.
 The secretion of special cells called chondroblasts, brings about
cartilage formation.
 The ossification process then hardens the cartilage when the bone
takes up minerals, mainly calcium carbonate.
 Long bones are usually hollow with the hollow filled with bone
marrow and extensions of the air sacs.

35.  Compact bone is modified by the formation of special cavities that
eventually mineralise by depositing concentric layers of new bone.
 The new structure is called the haversian system.
 If a transverse section (slice) of bone is examined under a
microscope, a large number of small canals that run more or less
parallel to the long axis will be seen.
 These canals are called haversian canals and carry the blood vessels
and nerve fibres.
 Surrounding these canals are small spaces called lacunae.
 A special bone cell called an osteocyte is found in each lacunae.
 Nutrients pass from the blood vessels in the haversian canals to the
bone through small canals called canaliculi.

37. Medullary bone (layer fatigue)
 A very unique feature of the female avian skeleton is the way the
bird lays down a supply of highly unstable secondary bone in the
marrow cavities of bones during the reproductive period.
 This bone is called medullary bone and because of its instability,
provides a very ready source of calcium for eggshells.
 Without this medullary bone the eggshells would be very thin and
weak.
 Examples:– tibia, femur, pubic bones, ribs, ulna, toes, and
scapula.

38.  Medullary bone starts to develop about 10-14 days before the
first egg is laid as a result of the presence of oestrogen and
androgen in the blood as the pullet reaches sexual maturity.
 This function remains for the length of her laying life.
Approximately two weeks before egg production starts the
pullet flock should be changed from the growing to the layer
diet which is higher in calcium.
 If pullets come into production at too young an age, they may
deplete body reserves of calcium that may result in thinner
shelled eggs and /or a drop in production.
 This should not occur if production is delayed to an age best
suited to the strain of layer

39.  If a calcium deficient diet is provided to a layer, it will deplete
her skeleton of calcium and thus make the bird significantly
weaker.
 Eggshells will also become thinner and production will decline.
 A condition of paralysis, called cage layer fatigue, may be seen
in layers housed in laying cages.
 When seen, it is always associated with birds in very high
production and takes the form of muscular paralysis and
osteoporosis (weak bones).
 While the cause is not fully understood, the birds usually
recover quite quickly when taken out of the cages and placed on
the floor for a short period.