The digestive system of pigeon includes alimentary canal and digestive glands. This system helps in nutrition. In comparison to the size of the bird, the length of the alimentary canal is short and its different parts are modified in such a way that assimilation of simplified food occurs in a short time. The digestive system is highly modified in pigeon due to loss of teeth and also due to an adaptation for aerial life.
(i) Alimentary canal:
The alimentary canal (Fig. 1.113A) consists of the following parts:
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Mouth:
Mouth is present at the tip of the head and is bounded by horny upper and lower beak. Teeth are totally absent.
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Mouth cavity and pharynx:
Mouth leads into mouth cavity and pharynx.
These regions contain:
(a) Internal nostril:
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It is a single opening on the roof of the pharynx through which both the nares open internally.
(b) Tongue:
It is a prominent structure with free pointed anterior end. Some taste buds and numerous mucous glands are present on the surface of the tongue.
(c) Glottis:
It is an aperture which leads into trachea and is placed near the base of the tongue.
(d) Gullet:
It is the last part of the pharynx which continues as an opening into the next part of the alimentary canal.
Oesophagus:
It begins from the gullet and runs through the ventral part of the neck as a straight tube. Near the junction of neck and trunk it comes between skin and muscle and enlarges to form a sac, called crop. The crop plays a dual role in the life of pigeon and other birds. It acts as a large temporary reservoir of crude food grains.
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The presence of crop enables the bird to ingest a considerable quantity of food grains very quickly. Food may be regurgitated from the crop to feed the young. Besides its normal function of storage, the crop glands present in the epithelial lining of the crop produce a proteinaceous, white slimy secretion called pigeons milk.
This is produced in both the sexes during breeding season. The young squabs are fed by the parents by regurgitation of the fluid. The secretory activity of the crop-glands is controlled by the hormone called prolactin of anterior pituitary.
The composition of pigeons milk is given:
Water … 65-81%
Protein … 13.3-18.8%
Fat … 6.9-12.7%
Ash … 1.5%
It has a great nutritive value, even more efficient than mammalian milk. From the crop, the oesophagus enters the cavity of trunk and passes dorsal to the heart to open into the next part of the alimentary canal called stomach.
Stomach:
It is divisible into two distinct parts:
(a) Proventriculus:
It is the first part of the stomach to receive the oesophagus (Fig. 1.113A). It is tubular and internally lined by numerous gastric glands. A red-coloured small spleen remains morphologically attached to the outer side of proventriculus.
(b) Gizzard:
It is the second part of stomach and is semi-circular in outline. The walls of the gizzard are highly muscular. It contains very little inner space. Internally the wall contains numerous ridges and its epithelial lining becomes very thick and tough. It has innumerable small tubular glands.
A few stones, which are swallowed by the bird, remain within the lumen and are responsible for crushing the food (Fig. 1.113B). The function of gizzard is purely mechanical and acts mainly as the grinding machine to crush the food. The gizzard compensates the role of teeth in pigeon.
Intestine:
Intestine begins from the gizzard near the opening of proventriculus and is divisible into following parts:
(a) Duodenum
(b) Ileum
(c) Rectum.
(a) Duodenum:
This is the first part of the intestine and is distinctly ‘U’-shaped in appearance. Internally, the duodenum contains villi, crypts of Lieberkuhn and goblet cells.
(b) Ileum:
It is the second part of the intestine having numerous villi inside it. The length of the ileum is shorter than that of other vertebrates. The first and the last parts of the ileum are loop-like and the middle part is spirally coiled.
(c) Rectum:
It is the last part of the intestine and is of same thickness as that of ileum. Near the junction of ileum and rectum, a pair of small lateral blind leaf-like caeca originates.
Cloaca:
Rectum opens into a chamber called cloaca. The cloaca is spacious, muscular and is elaborated into three chambers: coprodaeum, urodaeum and proctodaeum (Fig. 1.113C). The coprodaeum receives the intestine. The urinogenital ducts open within the urodaeum and the proctodaeum opens externally by the vent.
In the nestlings, a special thick-walled glandular chamber, bursa fabricii, remains in close association with cloaca and communicates with the proctodaeum. It degenerates in the adult bird. The bursa fabricii may have some local protective function in young ones.
Cloacal aperture:
The proctodaeum of cloaca opens to the exterior through the cloacal aperture or vent. It is present on the ventral side and near the base of the tail.
Membranes and Mesenteries:
There are membranes and mesenteries which divide the body cavity into different parts and also support the viscera.
The following membranes and mesenteries are prominent:
(i) Parietal peritoneum is the membrane lining the body wall.
(ii) Ventral ligament arises from the ventral body wall to the gizzard.
(iii) Falciform ligament arises from the ventral body wall to the ventral side of the liver. It is continuous with the ventral ligament.
(iv) Oblique septum is a partition arising from the lateral body wall to the centre which separates the body cavity into an anterior and posterior compartment. This septum is continuous with the pericardial sac at the centre and encloses the posterior air-sacs.
(v) Pericardiac sac encloses the heart. It is composed of parietal pericardium and visceral pericardium.
(vi) Coronary ligament attaches the anterior side of the liver to the combined pericardial sac and oblique septum.
(vii) Gastro hepatic ligament attaches the gizzard to the liver.
(viii) Mesogastric mesentery connects the gizzard to the dorsal body wall.
(ix) Meso-duodenal ligament connects the two halves of the duodenal loop. The pancreas is also located on this ligament.
(x) Mesentery proper attaches the small intestine to the dorsal body wall.
(xi) Hepato-duodenal ligament connects the duodenum to the liver.
(ii) Digestive glands:
Following digestive glands are associated with the alimentary system:
Mucous glands:
These are present on the surface of the tongue and crop and are responsible for moistening of the food.
Salivary glands:
Paired angular and unpaired sublingual salivary glands are located in the pharyngeal region. The secretion called saliva moistens the food and also contains diastatic enzyme.
Crop glands:
The secretion called “crop milk or pigeons milk” is secreted by these glands. The secretion is used for the nourishment of the youngs.
Gastric glands:
These are present on the internal lining of proventriculus and secrete gastric juice.
Tubular glands:
These glands are present on the internal lining of gizzard. The fluid secreted by these glands is thick, yellowish green in colour.
Liver:
Paired deep brown coloured glands of immense size are present ventral to the gizzard. It produces the bile which passes through two bile ducts, one coming from each lobe of liver. The bile ducts open separately within the two limbs of the duodenum. The gall bladder is absent in pigeon. Loss of gall bladder is an adaptation to aerial life for reducing the weight of the body.
Pancreas:
It is a pinkish white gland located in between the two limbs of the duodenum. It is a combination of both exocrine and endocrine glands. The exocrine part producing pancreatic juice opens into the duodenum by two or three pancreatic ducts. The endocrine part produces insulin, which is directly poured into the blood vessel.
Crypts of Lieberkuhn and Goblet cells:
These are present in the lining of the duodenum and its secretion influences the pancreas to produce pancreatic juice.
Glands in the ileum:
These glands produce digestive juices which contain various enzymes.
Caecal glands:
Produce certain digestive juices for the digestion of vegetable fibers. They are also concerned with the absorption of water.
Physiology of digestion:
The physiology of digestion in pigeon involves the three following steps:
Ingestion:
Pigeon is a grain or seed eater. It picks up grains very rapidly which are quickly swallowed. The shape of the beak is modified in such a way that it helps in its ingestion. The swallowed food, being moistened by mucus, is stored in the crop.
Digestion:
Within the crop the food is moistened and macerated. The passage of food from crop to proventriculus is regulated. Inside the proventriculus, the gastric juices containing HCl and pepsin act over the food. Within the gizzard the food is completely churned by the action of gizzard wall and stones. The secretion of gizzard glands helps in the process.
The churned food, while travelling through the duodenum, comes in contact with the bile and pancreatic juice. The bile neutralizes the acidity. Different enzymes are present in the various digestive juices. The completely broken-down food is absorbed through the lining of small intestine. Residual part passes into the rectum where water is absorbed and the fibres are broken down by the caecal juice.
Egestion:
Residual part called faeces is temporarily stored in the cloaca where it mixes with the urine and is periodically dropped through the cloacal aperture.