Digestive System of Pigeon (Columba livia): Anatomy and Physiology of Digestion

 

The Digestive System Of  Pigeon (Columba Livia): Anatomy, Digestive Glands, and Physiology of Digestion

The digestive system of a pigeon (Columba livia) exhibits notable adaptations correlated with its granivorous diet and avian flight. 

The digestive system comprises the Alimentary canal and associated digestive glands, providing all the nutrients needed for rapid and efficient digestion. 

The digestive system of pigeons is highly modified due to the loss of teeth and also as a Flight Adaptation for aerial life. 

The absence of teeth and the presence of specialised structures such as the crop and gizzard are the key features of this system.

I. 

Anatomy of the Alimentary canal 

Digestive System of Pigeon: Diagram 

The alimentary canal of Columba livia is a continuous, long, regionally specialised and tubular structure of varying diameter.

It begins at the mouth, comprising of buccal cavity, pharynx, oesophagus, stomach, small intestine and large intestine, terminating with a cloacal aperture.

A.

Buccal cavity and Pharynx 

Beak:

The mouth is the aperture to the inlet and is present at the tip of the head, bounded by a horny upper and lower beak, without teeth. The beak is perfectly shaped for picking up grains and seeds. The mouth leads into the buccal cavity and pharynx.

The buccal cavity comprises of tongue and pharynx.

The tongue is a prominent structure with a free pointed interior end containing some taste buds and numerous mucus glands on the surface that contribute to food lubrication.

Pharynx :

Bucco-pharyngeal cavity of Pigeon: Diagram 

The posterior part of the buccal cavity is termed as pharynx, which features several openings, including the 

Internal nares, opening of the nostrils into the root of the pharynx.

Eustachian tubes ( pharyngotympanic tubes ):

It is an opening behind the nares and reaches up to the cavity of the middle ear.

Glottis: 

It is an aperture near the base of the tongue, leading into the trachea.

The gullet 

It is the last part of the pharynx leading to the oesophagus. It runs on the ventral part of the neck.

It's a long, wide, distensible and thick-walled tube.

B.

Oesophagus and Crop

Oesophagus This is a distensible tube extending from the gullet to the stomach, ventrally along the neck.

Crop (Ingluvies): 

It is a thin-walled, bilobed, elastic sac, near the junction of the neck and trunk, just in front of the sternum.

Crop cut open (Pigeon): Diagram 

 The crop serves two primary functions,

• It acts as a large temporary reservoir of food grains, allowing rapid ingestion of food, which can be processed gradually.
• Pigeon milk production The lining of the crop produces a soft, cheesy, proteinaceous, white slimy nourishing secretion called 'pigeon milk ' or 'crop milk', produced in both sexes during the breeding season. This is fed by the parents by regurgitation of the fluids. This secretary activity is controlled by the prolactin hormone of the anterior pituitary. It is formed by degeneration of the epithelial cells lining the crop. The composition of pigeons milk is water: 65 - 81%, protein: 13.3 - 18.8%, fat: 6.9 to 12.7% and ash: 1.5%.

( The pigeon's milk shows great value, even more than mammalian milk. ).

The crop also contains mucus-secreting glands.

In the trunk, the oesophagus again becomes thick-walled and passes dorsal to the heart to open into the stomach.

C.

Stomach: 

The avian stomach is bipartite, comprising two distinct regions.

Sagittal section of Stomach (Pigeon): Diagram 

Proventriculus ( Glandular Stomach): 

This is the interior smaller and glandular portion. Its thick walls contain glands that secrete mucus, hydrochloric acid (HCL), and pepsinogen initiating the chemical digestion of proteins.

 It also contains a small oval reddish spleen on the right side, attached by the peritoneum to the proventriculus.

 Gizzard :

The second part of the stomach is called the gizzard which is semicircular having highly muscular walls and narrow lumen. 

 Internally it contains numerous ridges making the epithelial lining very thick and keratinised. 

It contains innumerable small tubular glands. 

Birds ingest small stones and grit called gastroliths, which reside within the gizzard. The powerful muscular contractions of the gizzard aided by gastroliths, mechanically grind and triturate the food. The gizzard effectively compensates for the absence of teeth.

 

Function : 

The gizzard functions as a grinding machine to crush the food ( it works as teeth in a pigeon ).

  

( The gizzard of grain-eating and piscivorous birds show differences in their structure. The grain-eating bird, the gizzard is highly muscular while in piscivorous, it is simple and thin-walled. )

D.

Small Intestine :

The mid-gut or the small  intestine is a narrow convoluted tube of about 75 cm long, beginning  from the gizzard near the opening of the proventriculus and is divided into the following parts,

Duodenum  : 

The initial U- shaped segment receives the chyme from the gizzard.

The duodenum leaves the gizzard such that the pyloric opening of the gizzard into the duodenum and the cardiac opening of the proventriculus into the gizzard lie side by side. 

( Please check out the diagram to understand the concept. )

In between the limbs of the U-shaped loop of the duodenum lies the pancreas which gives three ducts to the duodenum. Also, the duodenum receives two bile ducts from the liver introducing crucial digestive enzymes and bile salts.

Ileum :

 It is the second longest part of the small intestine showing the presence of numerous, minute, finger-like processes or villi inside it. 

These villi help in increasing the surface area for secretion and nutrient absorption.

( The ileum of a pigeon is comparatively shorter than that of other animals e.g. mammals. )

The first and the last parts of the ileum are loop-like and the middle part is spirally coiled.

E.

Large Intestine :

It is a short tube. The large intestine comprises of rectum and cloaca. 

Rectum :

A relatively short tube that receives the indigestible residues from the ileum.

 A pair of small leaf-like, conical blind pouches, the rectal or colic caeca (approximately 0.33 cm in length), arise at the junction of ileum and rectum, are believed to play a role in water absorption and recent studies suggest their involvement in immunological responses and potentially the digestion of fibrous materials. 

The rectum is 4 cm long and its opening into the cloaca is controlled by an anal sphincter.

Cloaca :

 The rectum opens into a chamber called cloaca which is muscular and divisible into three chambers,  receiving the products of the digestive urinary and reproductive systems.

1.  Coprodaeum- Coprodaeum is the first part that receives the rectum.
2. Urodaeum- In this part the urinogenital ( ureter and genital ) ducts open and,
3. Proctodaeum - Proctodaeum opens to the exterior by the vent or cloacal aperture.

 

Cloaca in longitudinal section: (Pigeon)

In the nestlings, a special thick-walled glandular chamber called bursa fabricii, a lymphoid organ involved in B-cell maturation, is associated with the proctodaeum but regresses in adults. 

F.

Cloacal aperture :

 The proctodaeum of the 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.

Digestive glands :

Several glands associated with the alimentary canal contribute to the digestive process.

a.

 Mucus glands (Buccal glands)

Small glands secreting mucus are present on the surface of the tongue and crop and responsible for the moistening of food.

b.

 Salivary glands 

 Paired angular and unpaired sublingual salivary glands are located in the pharyngeal region, which produces saliva that moistens the food and also contains diastatic enzymes.

c.

 Crop glands 

 These glands produce the secretion called crop milk or pigeon milk which is used for the nourishment of young ones.

d.

 Gastric glands 

 They are present on the internal lining of proventriculus and secret gastric juice containing peptic enzymes.

e.

 Tubular glands 

Present on the internal lining of the gizzard. The fluid secreted by these glands is thick horny and yellowish-green in colour.

f.

 Liver 

 It is a deep brown coloured paired gland with immense size (right lobe larger than the left lobe) and present in the ventral to the gizzard. It produces the juice called bile, which passes from two bile ducts from each lobe of the liver. 

The bile duct opens separately within the two limbs of the duodenum. The left bile duct opens into the proximal limb and the right into the distal limb of the duodenum. The gallbladder is absent in pigeons (common pigeons), although present in some species of pigeons. 

(The loss of gall bladder is an adaptation to the aerial mode of life)

g.

Pancreas 

It is a pinkish-white gland located between the two limbs of the duodenum. It is the combination of both exocrine and endocrine glands. The exocrine part producing pancreatic juice, opens into the distal limb of the duodenum by three pancreatic ducts. The endocrine part produces insulin which is directly poured into the blood vessel.

h.

Crypts of lieberkuhn and goblet cells 

 These are present in the lining of the duodenum and secrete juice called secretin which influences the pancreas to produce pancreatic juice. 

i.

 Glands in the ileum 

These glands produce digestive juices which contain various enzymes.

j. 

Cloacal glands 

Produce certain digested juices for the digestion of vegetable fibre. They are also concerned with the absorption of water.

Physiology of digestion in Pigeon 

 The physiology of digestion in pigeons involves the three following steps :

1.

Ingestion 

A pigeon is a grain or seed eater. The shape of the beak is modified in such a way that it can easily pick up grains rapidly and quickly swallow them. The beak also helps in ingestion. The swallowed food passes through the oesophagus into the crop where it is moistened by mucus and stored in the crop.

2.

Digestion  

Digestion occurs in different organs, such as,

Crop:

In the crop, the food is moistened and undergoes some maceration. The passage of food to the proventriculus is regulated.

 

Proventriculus:

Here, the chemical digestion of proteins begins under the action of hydrochloric acid and pepsin present in the gastric juice.

 Gizzard:

The food then enters the gizzard, where it is subjected to intense mechanical churning by the muscular walls and the grinding action of the ingested gastroliths. The secretions from the gizzard glands also contribute to this process.

 Small Intestine:

The partially digested food (chyme) enters the duodenum, where it mixes with bile from the liver and various enzymes from the pancreas and the intestinal glands.

Bile emulsifies fats, while pancreatic and intestinal enzymes further break down carbohydrates, proteins, and fats into absorbable units (e.g., monosaccharides, amino acids, fatty acids, and glycerol). These nutrients are primarily absorbed across the villi lining the ileum.

Large Intestine:

The remaining indigestible residues pass into the rectum. Here, water absorption occurs, and the caeca may contribute to the breakdown of fibrous materials.

3.

Egestion 

  The residual part called faeces is temporarily stored in the coprodaeum of the cloaca where it mixes with the urine and is periodically dropped through the cloacal aperture (vent).