AVIAN ANATOMY: TERMINOLOGIES ON STRUCTURAL ADAPTATION AND ASSOCIATED FUNCTIONS IN CHICKEN
P SENTHAMIL SELVAN and K RAJALAKSHMI
Assistant Professor,
Department of Veterinary Anatomy and Histology
RIVER, Puducherry
Email- drsenvet@gmail.com
Ph: 9566972413
INTRODUCTION
Structural adaptations to support locomotion and feeding are important for the survival, growth and reproduction of a species. Birds evolved from reptiles and has many special morphological adaptations in every body systems. Changes in its locomotor apparatus, notably the pectoral limb modified in to wings with a well built pectoral girdle and a large sternum to support the flight muscles, pneumatization of bones with the air sacs, modification of the integument with feathers, scales and claws, absence of teeth, lips, cheek, soft palate, urinary bladder, right ovary and oviduct in females etc., with required neuronal competence to coordinate vision, locomotion and balance in birds do exist in the present day chicken. Unlike other domestic animals and mammals, these adaptations in birds are unique and requires attention. In this chapter, some of the important terminologies that are associated with these structural variations are given below for the reference of practitioners in poultry medicine and other clinicians of pet and exotic birds.
TERMINOLOGIES
Air sacs
They are thin-walled, transparent sacs that extend from the mesobronchi or the lungs to the interior of some of the bones and different regions of the body. They are cervical, clavicular, axillary, anterior thoracic and posterior thoracic and abdominal airsacs. All airsacs are paired except clavicular air sac. They compensate for the small inelastic and fixed lung in chicken. With the help of respiratory muscles, the posterior thoracic and abdominal air sacs act as bellows to ventilate the lungs with a continous stream of air both during inspiration and expiration. All airsacs help reduce the specific gravity, regulate the body temperature, to distribute weight during flight and also in voice production.
Anterior larynx:
It is present at the level of pharynx and is supported by a single cricoid and paired arytenoid cartilages in fowl
Aditus laryngis
The median fissure in the floor of the mouth for the opening of the anterior larynx in chicken
Beak
A bird’s upper and lower jaws, including the external covering is called the beak/bill.
Bill tip organ
An aggregation of sensory cells at the tip of both the upper and lower beak. It is thought to sense tactile stimuli during feeding.
Bipedal
The ability to walk (or run) on two legs.
Body cavity
The cavity inside the body that holds the viscera in chicken. Unlike animals chicken has only a common body cavity. There is no separate thoracic and abdominal cavity as diaphragm that separates these cavities is rudimentary/ absent in chicken. There is no line of demarcation between abdominal and pelvic cavity. Pelvic girdle is present but pelvic symphysis is absent.
Breast
In chicken, the part of the lower (ventral) surface of the body, between the throat and belly is known as breast.
Bursa of fabricius or cloacal bursa
The lymphoid organ above the dorsal wall of the proctodeum. It is well developed at four months of age and disappears at one year of age in chicken. They are the source of B lymphocytes and are important for humoral response.
Carina/Keel
A midventral ridge of bone that projects outward from the metasternum that provides an attachment site for the pectoral muscles in birds.
Caeca
These are two blind tubes in chicken that extend forward from either side of the ileo-colic junction. They are parallel to the ileum and has a narrow proximal part that posses the caecal tonsil, a wide middle part and an expanded distal part with a pointed end. It helps in absorption of water and proteins and in microbial digestion of fiber
Carpals
The bones of the wrist. In fowl, the distal row is fused with metacarpal while the proximal row has just two bones—the radial and ulnar carpal.
Carpometacarpus
The largest bone of the manus of birds, formed by the fusion of distal row of carpals with the metacarpals.
Caudal / Coccygeal veretbrae
The vertebrae of the tail in birds. The first caudal vertebrae fuse with the last sacral while the posterior series are fused to form a pointed pygostyle to support the tail feathers.
Cervical vertebrae
The vertebrae of the neck region. Unlike other mammals they are 13-14 in number. They help the bird to turn its head more than 180 degrees.
Choana
A single slit in the roof of the oropharynx in fowl. It connects the oral cavity with the nasal cavity in birds.
Cloaca
The common tubular opening for digestive, urinary and genital systems at the terminal part of intestinal tract in chicken. It posses three compartments namely, Coprodeum that receives and temporarily stores feces from the colon/colorectum, Urodeum is the next compartment that receives uric acid from the kidneys through ureters (urinary bladder is absent), and eggs or sperm from the gonads, The last portion is the proctodeum which is short and opens at the vent.
Cloacal phallus
The copulatory organ of cock. It is present on the caudal end of the cloaca at its ventral aspect. It is formed of an elongate, spiral, ridged structure that erects by lymphatic pressure during copulation. Sperm travel along its surface to reach the cloaca of the female.
Colorectum:
In chicken the colon and rectum is short without any demarcation. This short straight segment of the large intestine that extends backwards from the cecal orifice to terminate in the coprodeum of the cloaca
Columella
Small, thin bone extending across the middle ear of birds, It is the only single auditory ossicle which corresponds to the stapes in mammals. It is attached at one end to the inner surface of the eardrum and at the other end to the vestibular window of the inner ear; it transmits sound waves from the eardrum to the fluid-filled cochlea.
Cranial kinesis
The relative movement of the upper jaw aided by the flexible craniofacial hinge in birds
Crop
A large ventral dilatation of the esophagus located just before its entry into the body cavity on the right side of the base of the neck. The crop temporarily holds the food. This capacity permits the fowl to eat its food at time intervals but helps to supply continuously to the proventriculus in fowl.
Egg tooth
A short, pointed, calcareous structure on the tip of the upper beak that develops in bird embryos shortly before hatching to break the egg shell to hatch. The egg tooth disappears within a few days post hatch.
Feather follicle
A small pit in the skin of the fowl from which a feather grows and to which it remains attached.
Feather tracts/Pterylae.
The specific pattern on the bird’s skin where feathers are attached.
Filoplumes
Hairlike but relatively stiff feathers with a rachis but few or no barbs. These follicles have sensory receptors, which allow them to monitor the movement within the feather coat.
Foramen triosseum
The foramen formed by the three bones of the shoulder girdle in fowl. This hole acts as part of a pulley system that allows the force of the supracoracoideus muscle to be redirected
Gizzard
The lower part of the stomach in chicken that is rounded, thick walled and muscular with internal ridges. It grinds and softens foods. It is often found with grits or small stones in the lumen that is taken by the bird to aid in grinding (mechanical digestion). Koilin is the innermost tough lining of gizzard and is formed by the dense horny substances secreted by the glands of gizzard.
Hepatic ducts
The ducts that collect the bile synthesized in liver. They are two in number one for the right lobe and one for the left lobe. The Hepato-cystic duct collects the bile from the right lobe of the liver and joins the gall bladder to form the ductus cysticus in chicken. The Hepato enteric duct collects the bile from the left lobe of the liver in chicken. It opens independently in the duodenum.
Hyoid apparatus
A V-shaped unit formed of bones and cartilage that supports the tongue and lingual muscles in fowl. They help regulate the movement of the tongue in birds.
Keratin
A hard protein that forms scales and claws in fowl and is the primary structural component of mature feathers. Keratin of fowl differs from keratin of mammals.
Leg spur/calcar
The bony outgrowths near the distal end of the tarsometatarsus that is covered by a pointed horny sheath and is used as weapons by male chickens
Mandible
In birds, the lower jaw is modified in to lower beak and is supported by mandible.
Manus
The portion of the wing distal to the radius and ulna that is supported by carpals, carpometacarpals and digits.. The primary feathers attach to the manus
Meckels diverticulum
Meckel’s Diverticulum is a remanent of the yolksac found in the free surface of the mid jejunum.
Metacarpals
In chicken, they are three in number and are fused with the distal row of the carpals to make carpometacarpus.
Metatarsals
In chicken they are three in number and are fused with the distal row of tarsals to form the tarsometatarsus.
Monocular vision
Type of vision that produces flat, two-dimensional images, in contrast to binocular vision, which produces three-dimensional images. Monocular vision is a result when the eyes are positioned on the lateral sides of the head such that an object can be seen by one eye or the other, but not by both eyes at the same time.
Occipital condyle
A prominent single cylindrical articular surface on the basioccipital of the skull with which the atlas (the first cervical vertebra) articulates. In contrast, mammals have two.
Oil gland/Preen gland/Uropygeal gland
The gland, located at the dorsal aspect of the base of the tail that secretes oils that the birds spread over their feathers during preening. The oils prevent the feathers and scales from becoming brittle, and to waterproof the bird. Its secretion attracts male birds for mating in some species.
Operculum
It is a flap of horny skin on the dorsal border of the cartilaginous nostrils in chicken. . It partially cover the nares and it may help to keep out debris.
Orbit
Cavity in the skull that houses the eye. It is very big in birds to accommodate the larger eyeballs. The space available for the intraorbital movement of the eyeball is still limited in birds.
Ovary
The female gonad in fowl. It matures and releases egg cells (ova) periodically throughout the breeding season in a process called ovulation. In most birds only the left ovary is functional.
Oviduct/ Fallopian tube
The tube that transports the egg from the ovary to the urodeum of cloaca in chicken. Only left oviduct is developed to support the left ovary in fowl. it is suspended between two layers of the peritoneum that form its dorsal and ventral ligaments. It consists of five parts such as:
- Infundibulum a funnel shaped structure with delicate walls to capture the ovum as it is released from the ovary.
- Magnum that forms the major part of the tube, thick walled and secretes about 40% of the egg albumen
- Isthmus with narrow lumen that connects magnum with uterus. It secretes albumen and shell membrane.
- Uterus or shell gland that secretes rest 40% of egg albumen and the shell of the egg is formed in this segment. It ends in a sphincter in front of vagina
- Vagina is where the muscular cuticle of the egg is formed. Oviposition for laying occurs here. It terminates in the urodeum of cloaca.
Parabronchi
Tiny (microscopic) air tubes formed by the branching of secondary bronchi within the avian lung. It is the functional unit of the avian lung for gas exchange.
Pecten
It is a pigmented highly vascularized structure similar to the choroid layer in chicken and other birds. It posses number of folds that projects from the optic disc into the vitreous body towards the lens where the optic nerve exits the eyeball. The pecten is believed to nourish the retina and to control the pH of the vitreous body.
Pectoral girdle/ shoulder girdle
The part of the appendicular skeleton formed by the union of Clavicle, coracoids and scapula on each side of the avian body to support the wings. A functional and fully developed pectoral girdle is absent in domestic animals.
Pectoralis
They are large, powerful flight muscle of birds that attaches to the sternum. The pectoralis has two portions; pectoralis major and pectoralis minor (tender loin in chicken). They are considered as white muscle as they are rich in white muscle fibers.
Pelvic girdle
The part of the appendicular skeleton that is formed by the ilium, ischium, and pubis on each side of the body to support the muscles of the pelvic limb, tail, and abdomen in birds. They also provide protection for the abdominal organs. These bones do not form symphysis as in other domestic animals..
Pin feathers
Developing feathers that are still surrounded by a feather sheath
Plumage
Refers to a bird’s entire feather coat with regard to its color, pattern and arrangement.
Pneumatic bones
The bones that are filled with air spaces and may contain the extensions of air sacs. These bones are hollow and posses one or more pneumatic foramen on their surface. Examples include the skull, humerus (arm bone), pelvis, and collar bones.
Proventriculus
The upper part of the stomach in chicken; that is elongated, fusiform and glandular. The proventricular glands secrete mucus, hydrochloric acid, and an inactive precursor to pepsin, an enzyme that digests protein.
Quadrate bone
Bone on each side of the skull, between the upper and lower jaw that helps in wide opening of the mouth in birds. The four articulations are with the temporal, mandible, pterygoid and quadratojugal.
Red fibers
A muscle fiber that contains large amount of myoglobin and is permeated by massive capillary beds. These muscles can sustain actions for long periods of time and will not build up lactic acid that cause fatigue. They are often called as “dark meat” eg. Muscles of the thigh in chicken.
Renal portal system
This system collects venous blood from the lower portion of the digestive tract via the caudal mesenteric vein, and conveys it to the venous ring within the kidney, where some of it passes through a capillary bed before being conveyed back to the heart. The system of blood flow, is found in birds but not in mammals
Scleral ossicles
The sclera of chicken at its anterior margin posses a small ring of bone called the sclera ossicles. they are not present in mammals.
Sternal rib
The ventral portion of the thoracic rib that articulates with the sternum ventrally and with the vertebral rib dorsally. Sternal ribs are absent in 1st, 2nd and 7th rib in chicken and hence they are floating.
Supracoracoideus
The powerful upstroke flight muscle that raises a bird’s wing. The tendon of it passes through the foramen triosseum and is fixed with the humerus. It also slows down the wing at the end of the downstroke and accelerates it at the beginning of the upstroke
Synsacrum
The segment of the vertebral column of birds that is formed by the fusion of last thoracic vertebrae with all of the lumbar, all of the sacral, and the first coccygeal vertebrae in birds. It is in turn fused on either side with the ilium of the pelvis.
Syrinx/ posterior larynx
It is the organ of voice in chicken and other birds. It is at the level of tracheal bifurcation . It is a cartilaginous compartment with internal and external tympaniform membranes (homologous to vocal cords in mammals) that produce a slit like opening similar to the glottis in mammals.
Tarsals
Tarsals are present in birds but, the proximal tarsals fuse with the tibia to form the tibiotarsus, and the distal tarsals fuse with the metatarsals to form the tarsometatarsus.
Taste buds
The receptors for taste sensations in all vertebrates. Humans have numerous taste buds located on the tongue, but birds have few taste buds, which are located primarily on the roof of the mouth or deep in the oral cavity, with none on the tongue.
Testicles
Testicles are paired, bean shaped and are intra abdominal unlike other animals. Scrotum is absent. The size of it increases about ten fold during breeding season. Spermatogenesis occur even at higher body temperature in fowl.
Thoracic ducts
These are paired lymph ducts in chicken that collect the products of fat digestion from the intestinal lymph trunk they run along the surface of the aorta and eventually deliver their contents to the venous system at the cranial vena cavae.
Thoracic vertebrae
The vertebrae of the thorax are seven in number. The body of each articulate with the vertebral ribs and forms part of the rib cage. In chicken the 2nd, 3rd, 4th and 5th thoracic vertebrae are fused together, to form a single mass known as Notarium.
Thymus
It is a lymphoid organ well developed in chicks. They are present as chain of lobules on either side of the entire length of the neck. They produce T lymphocytes.
Ultimobranchial glands
Two small, light-colored glands located near the parathyroid glands, they secrete calcitonin, which lowers the blood calcium concentration.
Uncinate process
A flattened, hook-shaped extension of bone that projects caudally from the vertebral segment of the ribs in chicken. It helps to strengthen the rib cage and is absent in the 1st and 7th rib in chicken.
Urinary bladder
Sac that stores urine before it is excreted; It is absent in chicken.
White fibers
Muscle fibers that appear lighter in color than red fibers because they have fewer capillaries and less of myoglobin. White fibers also are larger in diameter than red fibers. Muscles with many white fibers are used for quick bursts of action, but they cannot carry out sustained activity as lactic acid builds up quickly and causes fatigue. Muscles with many white fibers are called as “white meat” Eg. Pectorals in fowl
Wing vein
Cutaneous ulnar vein that extends as brachial vein in domestic fowl; preferred site for venipuncture
REFERENCE
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4th edition. Philadelphia: Saunders publications ,784-813.
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