DIGESTIVE TRACT
1. Mouth, beak and tongue
The salivary glands are present which secretes saliva (12 ml in 24 hours).
o Lips, soft palate, cheeks and teeth are absent. Instead there is upper and lower horny mandible to enclose the mouth. The two mandibles are referred to as the beak. Teeth are compensated by the beak and the presence of a gizzard. The beak is used for apprehension of food and a potent weapon of defense.
o Taste buds are distributed as isolated follicles on the base of the tongue and floor of the pharynx (fewer than mammals). Touch cells are present at the edge of the beak and on the anterior portions of the tongue. These are not concentrated on specialized papillae as in mammals. Poultry accepts or rejects feed from impulses receive from the tactile cells concerned.
o Cleft palate is a long narrow slit in the center of cleft palate which is open to the nasal passages.
o Oropharyngeal glands are small and number at least 100. There is no epiglottis nor is there a soft palate in fowl.
2. True pharynx is absent.
3. Oesophagus is present.
4. Crop is a diverticulum of oesophagus; absent in water fowl and purely insect eating and fruit eating birds e. g., the green woodpecker. Crop stores, softens and mixes food. Little or no digestion takes place here. Pigeon crop milk is produced by female or male during the brooding season 10-16 days. Crop has only one opening (entrance and exit).
5. Proventriculus is glandular and called as a true stomach.
6. Gizzard is a muscular grinding organ with horny lining secreted by epithelium. Food is masticated here. Empty gizzard remains inactive. Once food enters, muscular contractions start.
At the pyloric opening there is a valve formed by a fold of the mucous membrane which prevents grit and large particles of food from passing out of the gizzard.
7. Yolk stalk is a remnant at midpoint of gut also called Meckel’s diverticulum.
8. Small intestine is about 1.5 m long in an average adult chicken. Duodenum receives food from gizzard. Jejunum and ileum cannot be differentiated. Intestinal juice contains large number of enzymes; protease, maltase, sucrase, lactase etc. many ducts empty near the posterior end of the duodenum. The two most caudal wards are bile ducts. The remaining are the pancreatic ducts.
9. Large intestine is about 10 cm long in an average adult bird but of twice the diameter of small intestine.
10. Caeca are two blind pouches (6 inches) located between small and large intestines (only one caecum in domestic mammals). Exact function of caecum is unknown. Caecum is absent in parrots, many pigeons, all woodpeckers and in European hoope.
11. External openings cloaca is a bulbous area at the end of the alimentary tract. It is common junction for digestive, reproductive and urinary systems. Cloaca has three parts; coprodeum receives from rectum, urodeum is an opening from ureters and the male or female reproductive ducts and proctodeum which is duct from the cloacal bursa called bursa of Fabricius. Vent is thee externalopening of the cloaca. Size and shape vary with status of egg production in a laying bird.
12. Accessory organs
o Liver: bilobed, relationship of body weight to liver weight is 37:1. Liver produces 0.58 mL bile/g of liver weight at the rate of 100 ml per 24 hours.
o Gall bladder: 2.9 cm long and can store upto 2.5 ml bile. Bile is weakly acidic. Gall bladderi isabsent in Humming birds and in some genera of pigeons, parrots, cuckoos and
woodpeckers. Two bile ducts in chicken; hepatic duct comes directory from the liver and cystic duct comes from the gall bladder.
o Pancreas: pink or pale yellow in color, embedded in duodenal loop. Rich in Langerhansi islands Pancreatic juice contains amylase, lipase and trypsin. Pancreatic digestion together with emulsification of fats by the bile, takes place in the upper end of the small intestine.
