Digestive structures in birds
In birds, alimentary canal is a long tube like organ starting from the beak at the head and ending with the vent or cloaca in the abdominal region.
Special features of the avian digestive system includes presence of beak, Absence of teeth, Proventriculus, Gizzard, Paired caeca and Vent.
Digestive structures in birds are Salivary glands, Pharynx, tongue, Oesophagus, crop, Proventriculus, Gizzard of birds
Structures of mouth in birds
In the fowl ,the lips and cheeks are replaced by the beak – an area of dense and horny skin lying over the mandible.
Absence of teeth, but so called egg tooth found on the end of the beak of newly hatched chickens to aid their escape from the egg at the time of hatching which disappears after a day or two.
The hard palate, forming the roof of the mouth. It has five transverse rows of backwardly pointing, hard, conical papillae.
Numerous ducts of the salivary glands pierce the hard palate to release their secretions into the mouth cavity.
Salivary glands of birds
A thick layer of stratified squamous epithelium covers the free surface. The salivary glands run the whole length of the hard palate, the groups of glands merging to form one mass of glandular tissue under the epithelium. Lymphoid tissue is found in most glands.
- The salivary glands are:
- Maxillary – in the roof of the mouth
- Palatine – on either side of the nasal opening in the roof of the mouth
- Apheno-pteryoid glands – in the roof of the pharynx on each side of the common opening for the eustachian tubes
- Anterior sub-mandible glands
- Posterior sub-mandibular glands
- Lingual glands – in the tongue
- Crico-arytenoid glands – around the glottis
- Small gland in the angle of the mouth.
Pharynx and tongue of birds
The pharynx is continuous with or follows the mouth.
The combined cavity of the mouth and the pharynx is often referred to as the oropharynx.
The common opening for the two eustachian tubes is located in the middle of its dorsal wall (roof).
The tongue is long and pointed and conforms to the shape of the beak in which it operates.
The epithelium of the tongue is thick and horny, especially towards the tip.
A transverse row of simple, large and horny papillae with their tips directed towards the rear of the mouth cavity are located on the posterior end.
The hyoid bone provides the framework to support the tongue. The entoglossal bone extends longitudinally in the median plane.
Small patches of lymphatic tissue are located throughout the corium. Mucous glands are located in the tongue with short ducts directed towards the rear.
Some but not others believe that there are taste buds located on the tongue.
In any case the sense of taste appears to be very weak if at all present.
The mouth has two major functions- To pick up the food particles and direct the food into the oesophagus.
Oesophagus and crop of birds
Oesophagus is wide which connects the mouth region to the crop.
The wall of the oesophagus is composed of four layers of tissue, the innermost being mucous membrane.
The mucous membrane is an important barrier to the entry of microbes.
It secretes mucous to lubricate and to aid the passage of the food along the alimentary canal.
The crop is a large dilation of the oesophagus located just prior to the oesophagus entering the thoracic cavity. In some birds there are two crops and carnivorous birds donot have crop.
The crop provides the capacity to hold food for some time before the commencement of digestion. This capacity enables the bird to take its food as “meals” at time intervals and permits continuous digestion.
The crop structure is similar to that of the oesophagus except there are no glands present in fowls. Ducks and geese have glands in the crop mucous membranes. In pigeons, the surface cells of the crop slough off during brooding to form pigeon’s milk – used to feed the baby pigeons in the nest.
Inside the thoracic cavity, the oesophagus becomes the proventriculus very glandular part of the digestive tract often called as glandular stomach.
Proventriculus in birds
The glandular stomach or proventriculus is relatively small and tubular.
- The wall is very thick composed of five layers-
- Outer serous membrane
- Muscle layer composed of three separate layers
- Two thin longitudinal layers.
- One thick circular layer.
- Layer of areolar tissue containing blood and lymph vessels
- Thick layer composed mainly of glandular tissue
- Inner mucous membrane.
Gizzard of birds
The muscular stomach or gizzard is located immediately succeeding the proventriculus.
It is placed partly between the lobes and partly behind the left lobe of the liver.
Under the outer layer very powerful masses of red muscle are located.
The inner surface is lined with a creamy colored, thick, horny tissue raised in ridges.
The gizzard almost always contains quantities of hard objects such as gravel or other grit that aids in the disintegration of food – the primary function of the gizzard.
The gizzard consists of number of layers of tissues in some of which straight tubular glands are located.
The innermost layer is a strong, flexible skin able to withstand the potentially damaging effects of the muscular action of grinding the food often in the presence of stones or other insoluble material.
The glands of the gizzard produce a liquid, keratinized material that passes to the surface of the horny lining where it hardens to replace tissue worn away by the grinding action of the organ.