THE PERITONEAL CAVITY AND VISCERAL
The birds diaphragm is different to that of mammals as it does not form a complete wall between the thoracic and abdominal cavities. Thus, all the main organs are located within a unique peritoneal cavity which can be divided into several celomic compartments; the peritoneal sacs.
Starts at the beak, which has a bony base and is formed of the nasal bone, maxilla, premaxilla and the mandible. All the bones are covered by a hard keratinised sheath called the rhamphothecae. The beak’s form depends on the diet of the bird and substitutes the lips, gums and teeth of mammals. Birds such as parrots use the beak to differentiate between foods.
In captive birds an overgrowth of the beak may cause problems in the normal prehension of food. A vet is usually required to remove the overgrown beak. The oral and pharyngeal cavities join into the oropharynx, which includes a long hard palate with keratinised papillae, the tongue and the laryngeal mound.
Neither soft palate nor nasopharynx are present, this means that both the choana and the auditory tubes open directly into the oropharynx through their correspondent openings.
The tongue is shaped according to the beak’s shape. In web-footed birds the tongue’s borders are covered by lingual bristles which together with the keratinised lamellae of the bill filter and strain the food. In parrots the tongue is hard, fleshy (formed of its own muscles) and very mobile. This allows them to make sounds and words. Salivary glands are commonly absent, except in some insectivorous birds.
The oropharynx is followed by the oesophagus. In dehydrated or injured birds the entrance to the oesophagus must be approached to re-hydrate the animal. The opening of the glottis, which lies ventrally to the oesophagus, must be avoided in this procedure.
The first portion of the oesophagus is located between the trachea and the cervical muscles. However, it then moves to the right side and keeps this position further down the neck. In ducks the caudal part of the oesophagus contains an aggregation of lymphoid tissue known as the oesophageal tonsil. In almost all species a dilatation, forming the crop, is located in the mid portion of the oesophagus. The crop accumulates food but no digestion takes place. In columbiformes (pigeons), it produces a protein rich juice, the crop milk, which is regurgitated by the adults to feed to their chicks. The shape of the crop differs between species, from a simple dilatation (in aquatic birds), to a sac (birds of prey and grain eaters), to a double sac (pigeon) or an “S” shape (psittacine species). Both the oesophagus and the crop are found subcutaneously and are easily palpable and accessible surgically.
Once past the heart and the lungs the oesophagus
opens into the stomach. The stomach has two parts: the proventriculus and
The ventral aspect of the proventriculus or glandular stomach is in contact with the left lobe of the liver. The mucosa of the proventriculus produces mucous, enzymes (pepsin) and hydrochloric acid. In carnivorous birds (birds of prey) hydrochloric acid is essential for digestion of meat and bones, which form part of the diet.
The gizzard or muscular stomach, is further caudal and also linked to the liver, although its main contact is the sternum and the ventral part of the left abdominal wall.
Birds eat sand and grit in order to break up the coarse material on which they feed, functioning in a similar way to teeth. The muscular wall is more powerful in grain eating birds than in carnivorous birds. The mucosa secretes a carbohydrate-protein complex forming the cuticle which protects against any damage caused by the ingested grit.
In birds of prey this part of the stomach
tends to retain hairs, feathers and bones that are regurgitated in the form
of “pellets”. The study of these pellets in an ecosystem allows
an insight into the dietary preferences of the species living there.
The intestine remains in the ventral peritoneal
sac, in contact with the gizzard and the reproductive organs, occupying the
caudal part of the peritoneal cavity. It has duodenum, jejunum, ileum, two
caeca and rectum
The length and development of the digestive tract depends on the diet. It is longer in grain eating and herbivorous birds than in carnivorous and fruit eating birds.
The vitelline diverticulum of the jejunum is a remnant of the yolk duct, which during the first few days of life provides nutrients to the recently hatched chick. The caeca, absent in parrots open into the transitional zone between the small and large intestine. The length of the caeca also depends on the diet, being short in grain eating birds and long in herbivorous birds.
The caeca allow the digestion of cellulose, absorption of water and in certain birds, such as pigeons, also act as an immune organ due to their rich lymphoid tissue. The rectum opens into the cloaca, where the genital and urinary tracts also join.
cloaca is divided into three compartments:
a) Coprodeum: most cranial compartment. The end of the digestive system.
b) Urodeum: middle compartment into which the urogenital ducts open
c) Proctodeum: caudal compartment, which opens via the cloacal orifice. Dorsally there is the Bursa of Fabricius, a small lymphoreticular organ. Bird’s excrements are frequent but small.
The liver is surrounded by four peritoneal sacs (two ventral sacs and two dorsal sacs). It has two principal lobes, right and left. The hepatopancreatic duct emerges from the left lobe, and drains bile directly to the duodenum.
Two hepatocystic ducts emerge from the right lobe and carry bile to the gall bladder (absent in some parrots, pigeon and ostrich). Then the bile drains to the duodenum by the cysticoenteric duct. The pancreas consists of three lobes (dorsal, ventral and splenic), each one of them with its own efferent pancreatic duct which drains into the duodenum.
The shape of the spleen varies depending
on the species. It is located between the gizzard, proventriculus and bile
duct and does not act as a reservoir for blood.
In general, due to their high rate of metabolism, birds ingest food equivalent of up to approximately 25-30% of their body weight per day. Their high energy requirements mean that small and young birds cannot survive for a long time if they are left for more than a few hours without food .