Avian Anatomy and Physiology
All the notes that I’ve taken on avian anatomy and physiology compiled in one place.
Birds don’t have sweat glands. The dispel heat through the skin and blood shunts.
Galliformes dilate the vascular plexus venus intracutaneous collaris
During stress, a large portion of blood from the left ventricle is pumped to the legs to increase heat loss.
Some aquatic and wading birds have countercurrent arteriovenous tibiotarsal retes in the proximal, feathered part of the leg.
Many species have fused vertebrae to confer rigidity during flight.
In some species, the first 3-5 thoracic vertebrae fuse into a single bone called the notarium.
Budgies have mobile thoracic vertebrae.
The notarium is followed by the only mobile vertebra of the trunk. (When ventrally displaced, this can cause spondylolisthesis
Thoracic vertebrae vary in number from 3-10. Some cranial and sternal ribs do not articulate with the sternum and are instead attached by a ligament.
A unique feature of avian ribs is the backwards-facing uncinate process which extends caudodorsally from every rib. This provides attachment for muscles which extend ventrocaudally to the rob behind, adding strength to the thoracic wall.
Synsacrum contains 10-23 vertebrae and is the fusion of the caudal, thoracic, lumbar, and sacral vertebrae. It supports the pelvic girdle.
The final vertebrae of the tail fuse into the pygostyle which supports the tail feathers.
Primary feathers insert on the manus and secondary feathers insert on the caudal ulna.
No diaphragm = coelom extending from the first thoracic vertebra to the vent. The cavity is no lined by serosae but instead contains 16 separate cavities. 8 cavities are air sacs, 2 pleura, 1 pericardial, and 5 peritoneal cavities unique to birds
Birds lack an omentum and instead have a double-layered peritoneal sheet, the post-hepatic septum which divides the caudal cavity into three parts. (Middle intestinal cavity, and 2 lateral hepatic cavities)
The hepatic cavities are divided into the left & right ventral and left & right dorsal hepatic cavities.
Lungs lie dorsal to the heart.
Liver lobes cover the base of the heart dorsally and laterally.
Ascending aorta curves to the right in some species.
In some species (ex: ostrich and chicken) the cranial and caudal vena cavae enter a sinus venosus before entering the right atrium.
Like reptiles, birds have a renal portal system.
The right atrioventricular valve is structually unique to birds and has no chorda tendinae.
The left atrioventricular valve is tricuspid.
The aortic arch gives rise almost immediately to the brachiocephalic trunks. These branch into subclavian arteries which supply blood to the wings via the brachial arteries, flight muscles via the pectoral arteries, and the head via the carotid arteries.
Carotid arteries run along a groove at the base of the cervical vertebrae close to the axis of rotation. This prevents the arteries from being occluded when the neck is tuned.
Pelvic limbs are supplied by the external iliac artery and the ischiatic artery which meets the femoral artery at the stifle to form the popliteal artery. This arteriovenous network forms the rete mirabile in many cold water birds.
The cranial vena cavae are paired and receive blood from the head and neck from the jugulars, the wings and breast from the subclavian veins.
At the angle of the jaw, there is a transverse anastomosis between the two jugulars, sloping caudally to the right which allows the blood to bypass from one side to the other incase of an occlusion of the vein.
Blood from the hindgut, pelvic limbs, and lower body enters the renal portal system which then joins the the caudal vena cava.
Most of the gastrointestinal tract, pancreas, and spleen drains into the hepatic portal vein and the lover.
Unique to birds is the caudal mesenteric or coccygeal mesenteric vein which drains the hindgut mesentary and connects to the hepatic portal vein to the renal portal vein. Because blood can flow either way, blood flow can be switched between the kidneys and liver.
The renal portal system is controlled by portal valves at the junction of the common iliac and renal veins, containing innervated smooth muscle.
In times of stress, the renal portal valves open and allow bloodflow to bypass the kidney, take a shortcut via the coccygeal mesenteric vein to the liver, and directly into the caudal vena cava.
Avian hearts are 50-100% larger than mammals of the same size due to high oxygen demands due to flight.
Necessity for an increased cardiac output is achieved by a high stroke volume, fast heart beats, and slightly lower peripheral resistance. Birds also have stiffer arteries to improve blood flow and maintain high blood pressure ranging from 108-250 mmHg (Human average is 150 mmHg)
Turkeys have the highest pressure of all vertebrates at 350 mmHg.
The consequences of such high pressure can result in aortic rupture, heart failure, and hemorrhage during bouts of stress.
Major avian arteries appear white due to the collagen fibers on the tunica adventitia. These fibers produce the stiff arteries that combat high blood pressure. The downside to this is that birds are susceptible to athersclerosis of the aorta and braciocephalic trunk. (Common in aged amazon parrots)
Because avian ventricles are completely divided, they cannot shunt blood away from the lungs while diving the way reptiles can. Therefore diving birds use selective vasoconstriction, bradycardia, and a drop in cardiac output.
Avian coagulation occurs faster in birds than mammals
Avian erythrocytes have a shorter life span compared to that in mammals.
Avian thrombocytes are analogous to mammalian platelets. They originate from stem cells instead of megakaryocytes.
Thrombocytes are involved in the clotting process although they have little thromboplastin and do not appear to trigger the intrinsic clotting pathway.
It is the extrinsic thromboplastin shed by damaged tissue that plays a more major role in the clotting process. This reliance on extrinsic clotting pathways may exist to prevent exsanguination in animals with such high blood pressure.
Birds have primary and secondary lymphoid organs.
Primary: Bursa of Fabricius and the thymus.
Secondary: spleen, intestinal lymphoid tissue, and bone marrow.
The thymus consists of 3-8 flattened, pale pink lobes lying along the neck, close to the jugular vein.
T-cells of the thymus act in cell mediated immunity such as delayed hypersensitivity reactions.
B lymphocytes are produced by the Bursa of Fabricius.
Monocytes appear very rarely in peripheral blood smears.
Heterophils are similar to mammalian neutrophils, having a polymorphic nucleus. (Stain acidophilic)
Eosinophils account for ~2% of total leucocytes. Presumed to be associated with tissue damage or parasitic infection.
The bursa of fabricius is a dorsal diverticulum in the proctodeum, which contains folds of lymphoid tissue. It reaches maximum size at sexual maturity and begins to involute thereafter.
In ratites the bursa and proctodeum form a single large cavity which has often been mistaken for a urinary bladder.
The avian lymphatic system has less numerous vessels than in mammals.
Most species have paired thoracic ducts on either side of the spine that drain the lymph from the hind limb and abdominal vessels and deliver it to the jugular veins at the base of the neck.
Anseriformes are the only birds with lymph nodes. They possess 2 primitive pairs: a cervicothoracic pair near the the thyroid gland and a lumbar pair near the kidneys.
The spleen lies to the right of the coelom, between the proventriculus and the ventriculus and does not form a significant blood reservoir.
Birds have no diaphragm and instead only have a horizontal septum that seperates the lung from the viscera.
Airsacs connected to the lungs act as bellows but do not aid in gas exchange.
Avian lungs undergo little change in volume during respiration.
The nasal cavity is compressed laterally and divided medially by a very thin septum.
Many species have an operculum at the center of each nare to prevent the inhalation of foreign bodies.
Nasal conchae are highly vascular, epithelial folds in the nasal cavity that increase the surface area over which air flows.
Nasal conchae are divided into rostral, middle, and caudal parts.
Caudal conchae picks up scent particles. Ciliated epithelium filters out foreign particles and mucus secreted by the goblet cells flushes these through the choana into the oropharynx.
A rete mirabile controls the water and heat loss by warming or cooling air entering the nasal cavity.
The paranasal sinus is particularly well developed in psittacines and becomes superficial ventromedialy to the orbit.
The paranasal has numerous diverticula which extend into the premaxilla, around the ear and rostral orbit and into the lower beak.
The paranasal sinus communicates dorsally with the caudal and middle conchae and also with the cervicocephalic airsac and its caudal extent.
In psittaciformes and anseriformes the right and left sinuses communicate.
Nasal/salt glands lie dorsal to the orbit and open into the nasal cavity at the level of the rostral concha from where the hypertonic solution is then sneezed from the nostrils.
The laryngeal opening (rima glottis) is slitlike and unlike mammals, is not covered by an epiglotis. Passage of air is regulated by a dilator/constrictor muscle.
Analagous to the mammalian larynx.
Rudimentary in species like ostriches and vultures.
Can be classified as tracheobronchial, tracheal, or bronchial depending on location. (Tracheobronchial is the most common.)
Located at the bifurcation of the trachea and has a median cartilage called the pessulus.
Psitticines lack a median pessulus.
The syrinx consists of a series of modified tracheobronchial cartilages, two vibrating tympaniform membranes, and muscles which vary the membrane tension. These membranes line the medial and lateral bronchi and sound is produced during expiration by the vibration of air through the syrinx.
The surrounding interclavicular airsac gives the voice resonance by pushing against those membranes.
As the trachea is narrowed at the site of the syrinx, it is a common site of obstruction by fungal granulomas.
Each primary bronchus runs through the whole length of the lungs (where they’re called mesobronchi) and terminates in the caudal airsacs.
Each bronchus gives rise to 4 secondary bronchi named according to the area of the lung to which they supply air. (mediodorsal, medioventral, laterodorsal, lateroventral)
These terminate in tertiary bronchi called parabronchi where blood gas exchange takes place.
All bronchi are lined by smooth muscle causing dilaiton and contraction.
Parabronchi make up the bulk of the lung tissue. They have invaginations called atria which lead to a labrynth of microscopic air capillaries extending out to perform gas exchange.
Most parabronchi are a parallel series of hundreds of tubes called paleopulmonic brinchi.
In most birds there are also irregular branched parabronchi (neopulmonic bronchi) which never compose more than 25% of the para bronchi.
Airflow through paleopulmonic bronchi is always unidirectional while neopulmonic bronchi airflow is bidirectional.
No airway valves have been found so it is thought that shape and alignment of the secondary bronchi creates a pressure differential between the cranial and caudal airsacs influencing air flow through the parabronchi.
Prolonged dorsal recumbency should be avoided during general anesthetics due to the mass of the viscera compressing caudal airsacs.
On lateral radiographs the tertiary parabronchi are seen end on and this gives a honeycomb appearance to the lung.
Cross current exchange allows more efficient absorption of oxygen without incurring high levels of carbon dioxide in the blood.
Airsacs are 2 layer thick squamous epithelium.
Cranial airsacs (cervical, clavicular, cranial) connect to the ventral bronchi.
Caudal Airsacs (caudal, thoracic, abdominal) connect to the primary bronchi.
Cervical Airsac
Between lungs. Dorsal to the esophagus. Vertebral diverticula from C3-T5. Communicates with the skull via the infraorbital sinus.
Clavicular Airsac
Present in the thoracic inlet and has 2 parts. The intrathoracic diverticulum around the heart and sternum. The extrathoracic spreads between the bones of the pectoral girdle and into the proximal humerus. Extends into the syrinx and is used in sound production.
Cranial and Caudal Thoracic Airsac
Lie in sequence to the dorsolateral thoracic cage. Caudal airsacs are the only ones which do not communicate with pneumatic bone.
Inspiration: 6 muscles. Primary muscles are the external intercostals and the costosternalis.
Expiration: 9 muscles. Primary muscles are the internal intercostals and the abdominal muscles.
Gastrointestinal System
Oropharynx is lined by keratinized stratified squamous epithelium.
Avian pharyngotympanic tubes are not covered by folds, in order to minimize buildup of pressure.
Lymphatic tissue called pharyngeal tonsils lines the choana of the infundibular cleft.
Psittacines with vitamin a deficiency tend to display blunted choanal papillae due to squamous metaplasia. These block salivary ducts and result in in secondary bacterial infections.
Stomach is divided into the glandular proventriculus and the muscular ventriculus.
Oxynticopeptic cells produce HCl and pepsinogen.
The ventriculus possesses 4 bands of smooth muscle in various directions, allowing them to
crush food.
Some psittaciformes, columbiformes, and struthioniformes lack a gallbladder.
Most birds lack bilirubin reductase, therefore biliverdin cannot be converted to bilirubin.
Hepatic bile acids are emptied into the duodenum and then returned to the liver via enterohepatic circulation. Hepatic malfunction can be diagnosed by elevated bile acids.
The pancreas lies within the antimesenteric border of the duodenal loop.
The pancreas is composed of three lobes which contain 1-3 ducts.
Pancreatic enzymes are produced by stimulation of the hormone secretin and vagal stimulation.
The duodenum, jejunum, and ileum demonstrate very little histological variability
The axial loop carries the Mechel diverticulum and marks the boundary between the ileum and jejunum.
The supraduodenal loop is the most distal part of the ileum.
The bile and pancreatic ducts open into the distal part of the ascending duodenum, opposite the cranial gizzard.
the jejunum and ileum are indicated by the yolk sac remnant, the meckel diverticulum.
Intestinal vili in birds do not contain lacteals (lymphatic capillary that absorbs dietary fats in the villi). Lipids are instead absorbed through a well-developed capillary network.
3 epithelial cells: goblet, chief, and endocrine cells.
Chief cells have a brush border to absorb food.
Goblet cells produce mucin protein.
Endocrine cells produce somatostatin, gastrin, and secretin.
Birds don’t possess true mesenteric lymph nodes and instead have lymph nodules (Peyer patches) in the lamina propria to provide lymphatic drainage.
Water and electrolytes are absorbed into the colon by antiperistaltic movements.
Ceca are paired and arise at the junction of the ileum with the rectum and are retrograde from the ileocecal junction.
The cecal mucosa had vili and contains scattered lymph follicles in the wall.
The main function of the ceca is to help the digestion of cellulose. (More prominent in ground dwelling birds.)
Cloaca consists of the copradeum, urodeum, and proctodeum.
The bursa of fabricius is located in the dorsal wall.
In males, the cloaca lies in the midline. In females, the cloaca is pushed to the right by the enlarged left oviduct.
The coprodeum is the most cranial compartment where the rectum empties. In some species, it is lined by vili.
The coprodeum is separated from the urodeum by the coprourodeal fold. If the rectum is filled with feces, the fold can bulge out of the vent, dispelling feces without mixing them with urates.
The coprourodeal fold also clode the coprodeum during egg laying to prevent feces from being expelled at the same time.
The urodeum is separated from the other compartments of the by the circular mucosal folds. Ureters and genital ducts empty into its dorsal wall.
The left oviduct opens into a small mound, which is covered by a small membrane in anseriformes until sexual maturity.
In male passerines, the terminal vas deferens dilates during the breeding season to form a conical projection in the cloaca, called the seminal glottus/cloacal promontory.
The proctodeum is separated from the urodeum by the uroproctodeal fold.
The proctodeal Bursa of Fabricius is the site of B-lymphocyte production and differentiation, necessary for humoral immunity.
Ureters terminate in the cloacal urodeum.
Kidneys are retroperitoneal in the ventral (renal) fossa of the synsacrum.
Kidneys are intimately associated with the lumar and sacral plexi and blood vessels.
Kidneys are large and extend from the caudal synsacrum cranially as far as the lungs.
Kidneys are divided into cranial, middle, and caudal parts by the externial iliac and ischiadic branches of the abdominal aorta. (The middle portion appears to be lacking in passerines.)
Herons and penguins have a fused caudal kidney in the midline.
The spinal nerves of the lumbar and sacral plexi run through the kidney parenchyma. This explains why many renal problems present as lameness.
The avian kidney does not have a distinct demarcation between the cortex and the medulla and has no renal pelvis.
In the avian kidney, the ureter runs along the ventral side of the kidney and branches into collecting ducts that each drain a lobule, consisting of a large area of cortical tissue and a small cone of medullary tissue.
Cortical tissue contains both types of nephron.
The medullary cone contains only the loop of henle, collecting ducts, and vasa recta capillary network.
Avian cortical nephrons (90%) resemble those found found in reptiles while medullary nephrons resemble mammalian nephrons.
Cortical nephrons excrete uric acid by secreting it into the proximal convoluted tubule. Cortical nephrons have no loop of henle.
Dual afferent blood supply.
The high pressure cranial, middle, and caudal renal arteries subdivide into the afferent glomerular arteries which provide glomerular filtrate. This is influenced by the state of hydration and arginine vasotocin produced from the posterior pituitary gland.
The external iliac vein drains the pelvic limb, branches to form the the common iliac vein and caudal renal portal vein.
The renal portal vein provides 2/3 of the renal blood flow to the kidney and supplies the the proximal convoluted tubules that are responsible for the secretion of urates.
Glomerular filtration does not clear urates so it is thought that the renal portal vein plays a significant role in the elimination of them.
The renal portal system has a unique smooth muscle valve lying at the junction of the common iliac and renal veins. This has a rich nerve supply and controls the amount of venous blood entering the kidneys.
Normally the valve remains closed, allowing venous blood from the hind limbs and pelvic region to enter the kidney and take part in tubular secretion and resorption.
In emergencies, high sympathetic activity stimulates the release of adrenaline which opens the valve to divert blood away from the kidney to the heart and brain.
Diverted blood flow can take three routes
Via the open valve directly into the caudal vena cava.
2) Via the cranial portal vein to the internal vertebral venous plexus.
3) Via the coccygeal mesenteric vein to the hepatic portal vein and liver.
Ureters have a branched network throughout the kidney parenchyma, terminating in the collecting ducts of each lobule.
Ureters are lined with pseudostratified columnar epithelium which secretes urates to help pass urates.
Urates reach the cloaca are are refluxed by retroperistalsis into the rectum and colon where there is mixing with feces and reabsorption of water.
The cloaca therefore plays a role in avian thermoregulation.
All birds have superocular nasal/salt glands ≠ hadrian/lacrimal glands.
Nasal glands are similar to renal tissue, using a system of countercurrent blood flow to remove salt from the bloodstream. The salt is then passed down the nasal passage and sneezed out of the nostrils.
Some birds can produce metabolic water and conserve it by producing minimal urine.
Many birds can preserve water by cooling warm air as it passes through the nares, using a rete mirabilis.
Prior to egg laying, PTH demineralizes medullary bone to allow calcium to be deposited in the egg yolk and shell. Excess phosphate is excreted via the kidneys causing diuresis. Many hens become polyuric prior to egg laying.
Birds and reptiles are uricotelic. They excrete 60% of their nitrogenous waste in the form of chalky white urates. Urea is formed as only as a by product of detoxification in the kidney, and to a lesser extent, the liver.
Urates help to conserve water and produce insoluble waste products in yolk, leaving the water fraction to be reabsorbed.
Uric acid is synthesized in the liver and excreted in the kidney by glomerular filtration (10%) and tubular secretion (90%).
Urates are also produced independently of urine flow, being secreted by the reptilian style cortical nephrons and so will still be produced even by very dehydrated birds.
Gout is caused by hyperuricemia or kidney damage. If the proximal renal tubule are damaged, uric acid cannot be excreted, so urate levels rise.
Rate of excretion of urates is relatively independent of glomerular filtration, high levels or urates can be produced, even in dehydrated birds. If they cannot be evacuated by the kidney, they will precipitate and cause renal gout.
High blood uric acid levels will also cause gout in joints and organs, such as the kidneys, spleen, and pericardium.
Dehydration can lead to irreversible gout. Urea levels can be monitored to estimate dehydration, but 70% of the kidney needs to be damaged before developing elevated blood uric acid levels. (Fast carnivorous birds for 24 hours in order to avoid false positives.)
GnRH spikes when hypothalamic photoreceptors become active.
Birds have 2 types of GnRH which trigger the release of FSH and Lutropin luteinizing hormone.
After breeding season is completed, the pineal gland and prolactin cause an increase in fat and increases food intake.
The avian tunica albuginea is much thinner than that of mammals.
The pampiniform venous plexus is not present in birds.
In domestic fowl, the epididymis has an appendix which is attached by connective tissue into the ventral aspect of the adrenal gland. Surgical castration is not always effective because the tissue of the appendix can produce androgenic nodes.
Passerines have the highest body temperatures, so the vas deferens elongates distally to form a cloacal promontory called the seminal glomus, to act as a site of sperm storage. The seminal glomus keeps sperm ~4º C lower than the core temperature.
The left ovary lies caudal to the adrenal gland and near to the cranial tip of the kidney. It consists of a vascular medulla, with nerve fibers and smooth muscle and a peripheral cortex. It is suspended by the mesovarium and receives its blood supply from the cranial renal artery.
The oviduct occupies the left dorsocaudal side of the celomic cavity. It’s a coiled tube, suspended by the mesosalpinx.
The oviduct wall consists of ciliated epithelial lining, glands, and smooth muscle.
Five parts
Infundibulum: funnel that catches the egg. Location where fertilization occurs.
Magnum: coiled around numerous tubular glands. Location of albumin production. Site of calcium, sodium, and magnesium addition to the yolk.
Isthmus: Divides the magnum from the uterus.
Uterus: Holds egg. Very vascular to aid in calcium deposition.
Vagina: Sperm host glands are located in the uterovaginal junction.
Endocrine Control
FSH is mainly responsible for follicular growth. Developing follicles produce estrogen from the theca and interstitial cells, and progesterone from the granulosa.
Increasing estrogen stimulates a LH surge and, under its influence, the follicle splits to release the primary oocyte.
Estrogen also mobilizes calcium from the bone, increasing plasma calcium concentration for egg production.
Continued secretion of progesterone further inhibits ovulation.
Prolactin stimulates the production of crop milk.
Oviposition in birds in controlled by prostaglandins and arganine vasotocin/oxytocin.
Pituitary gland is located caudal to the optic chiasma.
Pineal gland is located between the cerebral hemispheres and the cerebellum.
Thyroid glands are paired and lie cranial to the thoracic inlet, lateral to the trachea, and medial to the jugular veins.
Avian thyroids do not have C-cells that produce calcitonin. This is produced instead by the ultimobranchial bodies.
Avian thyroglobulin has a higher percentage of iodine than mammals which is why many birds easily develop iodine deficiency.
T3/T4 regulate molting by stimulating the production of new feathers. They also control metabolism, regulation of heat and growth, the reproductive organs, and increasing egg production.
2 pairs of parathyroid glands lie caudal to the thyroid. Often fused.
Parathyroid glands secrete PTH which controls calcium and phosphorous metabolism. Serum calcium is raised by both increasing Ca reabsorption from the kidney tubule and releasing it from bone via osteoclast activity. Phosphorous levels are decreased by decreasing tubular reabsorption.
Ultimobranchial bodies are flattened glands lying caudal to the parathyroid glands and consist of C cells which secrete calcitonin. The role of calcitonin in birds is unknown as it does not appear to lower serum calcium. It may play a role in limiting extensive bone reabsorption.
Andrenal glands are small and ovoid and lie cranial to the kidneys and gonads on either side of the aorta and vena cava.
The avian adrenal cortex is not well differentiated from the medulla.
The medullary part of the adrenal gland secretes adrenaline and noradrenaline (norepinephrine).
The cortical part of the adrenal gland secretes corticosterone and aldosterone.
Avian corticosterone has both glucocorticoid and mineralocorticoid activity and therefore plays a bigger role than aldosterone in electrolyte balance.
The pancreas lies in the mesentery of the duodenum and is often divided into three lobes.
The endocrine portion of the pancreas has 3 types of islets: alpha, beta, and delta.
Alpha cells secrete glucagon which regulate carbohydrate metabolism, increasing serum glucose levels by gluconeogenesis, lipolysis, and glycogenolysis.
Beta cells produce insulin, which lowers the serum level of glucose by stimulating tissue uptake and storage.
Delta cells produce somatostatin which regulates the levels of glucagon and insulin.
Birds have much higher glucagon levels and lower insulin levels than mammals.
F (PP) cells, situated in the exocrine tissue of the pancreas secrete avian pancreatic polypeptide which inhibits gastrointestinal motility and gallbladder and pancreatic secretion. It also induces a sense of satiety via CNS.
Endocrine cells are scattered along intestinal epithelium with the majority of cells situated within the pylorus.
Hormones secreted by the gastrointestinal tract include somatostatin, secretin, and avian pancreatic polypeptide.
Cerebral hemispheres are composed of the corpora striata, indicating that birds use less learning and memory and more instinct and stereotypical behavior.
Well developed cerebellum for motion and large optic lobes for vision.
The spinal cord becomes enlarged at the brachial and lumbosacral plexi.
The glycogen body is a pea shaped, glycogen rich cleft which lies on the dorsal surface of the lumbosacral plexus and has unknown function.
The roots of the lumbosacral plexus are in contact with the dorsal surface of the cranial kidney. The sacral plexus is totally embedded within its middle division.
Some of the roots of the pudendal plexus are embedded in the caudal division.
The optic nerve is the most well developed of the avian cranial nerves.
No consensual light reflex in birds because there is complete decussation of the optic nerve fibers at the optic chiasma.
External adnexa: birds have upper and lower eyelids and a nictitating membrane.
Modified feathers (filioplumes) act like mammalian cilia.
The harderian gland lies craniomedial within the orbit behind the nictitating membrane and produces a mucoid secretion that moistens the cornea. The lacrimal gland lies at the caudolateral margins. Both glands empty via dorsal and ventral punctae into the nasolacrimal duct. The salt gland lies dorsomedial to the orbit and empties separately into the nasal cavity.
The eyeball consists of a small unprotected anterior portion covered by the cornea, and the posterior part that is protected by the two orbits separated by a thin bony septum
The shape of the eye is formed by 10-18 scleral ossicles, visible by radiography. These are a ring of overlapping bones, which strengthen the eye and provide an attachment for the ciliary muscles, permitting greater accommodation.
Because the eye ball fills the orbit, the extra-ocular muscles are less developed than in mammals. The single occipital neck joint and long flexible neck compensate instead by allowing the bird to rotate its neck.
The iris has striated muscles, allowing them some voluntary control, so atropine cannot be used to cause dilation.
Dilation of the avian iris is effective with mydriatics such as the muscle relaxant vecuronium.
There are three methods of lens accommodation
Diurnal birds use the posterior sclerocorneal muscles to compress the lens.
Nocturnal birds use the anterior sclerocorneal muscles.
Diving birds use the sclerocorneal muscles and iris sphincter muscles.
The avian retina lacks blood vessels and tapetum lucidum to prevent shadows and scattering of light.
The retina is vascularized by a black, vascular comb-like structure called the pecten. This structure extends from the optic disk into the vitreous body towards the lens and is unique to birds. It appears to aid nourish the relatively avascular retina, aid in pH balance, and facilitate fluid movement within the eye.
Many diurnal birds lack rods and many nocturnal birds lack cones.
The optic disk is mainly obscured by the pecten.
Ground dwelling birds have no foveae while falconiformes have 2.
The avian external ear has no pinna.
Skin surrounding the ear is loose and can be drawn forward by the dermo-osseus muscle to reduce the opening to a vertical slit.
In some coraciformes, a vertical skin flap called the operculum lies rostral to the external ear opening.
The middle ear is the air filled cavity between the tympanic membrane and inner ear.
There is only one bony ossicle called the columella (equivalent to the mammalian stapes).
The pharyngotympanic tubes communicate with the middle ear and pharynx cia the common infundibular cleft.
Unlike mammals, the cochlea is short and not coiled and the semicircular canals are larger and more thick walled.
The eighth cranial nerve in birds receives both vestibular and auditory sensory fibers.
The nares and the caudal conchae are lined by olfactory epithelium and are connected to the olfactory bulbs of the brain.
There is no avian vomeronasal organ.
Avian tastebuds are confined to glandular non-cornified epithelium at the base of the tongue and in the roof and floor of the oropharynx. In parrots, they lie at either side of the choana. and at the rostral end of the laryngeal mound.
Birds have widely distributed dermal mechanoreceptors called Herbst corpuscles. Some also have these corpuscles in the beak tip.
Only 3 glands: uropygial, aural, and vent
Epidermis consists of the superficial stratum corneum and deep stratum germinativum.
Avian epidermis acts like a holocrine sebacious gland, secreting a thin lipid film that helps in the maintenance of plumage.
The dermis is composed of connective tissue and contains feather follicles.
The uropygial gland is a bilobed holocrine gland drained by a papilla dorsocaudally and is covered by down feathers. It secrets oil and acts as a bacteriostat. The gland is not present in all birds.
Aural sebaceous glands around the external ear secrete a waxy substance.
Vent glands secrete mucus but their function is unknown.
Podotheca (non feathered area of the legs) are keratinized epidermal plates.
Patagia are skin folds where wings, neck, and legs join the body. (Useful sites for subcutaneous injections) The main patagia of the wing are the propatagium (wing web)between the shoulder and carpus and the metapatagium between the thorax and wing.
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