Pharynx and tongue
The pharynx is continuous with, and 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 believe that there are taste buds located on the tongue, but this belief is not universally held. In any case, the sense of taste appears to be very weak if at all present.
The mouth has two major functions:
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- To pick up the food particles.
- To direct the food into the oesophagus – as part of the bird’s eating behaviour.
Oesophagus, crop and proventriculus
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The oesophagus is wide and is capable of being significantly stretched. It connects the mouth region to the crop in close association with the trachea. The crop is a large dilation of the oesophagus located just prior to where the oesophagus enters the thoracic cavity. The crop provides the capacity to hold food for some time before further digestion commences. This capacity enables the bird to take its food as “meals” at time intervals but permits continuous digestion. Inside the thoracic cavity, the oesophagus enters or becomes the proventriculus which is a very glandular part of the digestive tract (often called the glandular stomach).
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 and the mucous it produces is a lubricant that aids the passage of the food along the alimentary canal. The structure below the crop is similar to that above except there is less lymphoid tissue below the crop. 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 which is used to feed the baby pigeons in the nest.
Proventriculus
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The glandular stomach, or proventriculus, is relatively small and tubular. The wall is very thick and is composed of five layers:
- Outer serous membrane
- Muscle layer composed of three separate layers:
- Two thin longitudinal layers
- Thick circular layer
- Layer of areolar tissue containing blood and lymph vessels
- Thick layer composed mainly of glandular tissue
- Mucous membrane
The glands form the greater part of the thickness of the organ. Simple single glands group to form lobules each of which converges into a common cavity near the surface. The cavities converge to form a common duct that leads to the surface through the apex of a small papilla (see figure below). These glands produce a number of juices or enzymes that are used in the digestion or breaking down of food into its constituent nutrients. The mucous membrane is raised into folds and between these folds are numerous simple tubular glands that produce hydrochloric acid as well as lymphoid tissue.
Source: https://t-tees.com
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