• Digestive system consists of digestive tract or alimentary canal along with the associated digestive glands.
• Alimentary canal:
• Alimentary canal of frog is complete.
• It is long and coiled tube. The tubes have varying diameter.
• It extends from mouth to cloaca.
• It consists of:
  • Buccal cavity
  • Pharynx
  • Oesophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Cloaca

1. Mouth:

• It is the beginning to the alimentary canal.
• Mouth is a very wide gap. It extends from one side of the snout to the other.
• Two bony jaws bound the mouth, and the jaws are covered by immovable lips.
• The upper jaw is fixed.
• The lower jaw is flexible i.e. it can move up and down to close or open the mouth.

2. Buccal cavity of frog:

• Mouth opens into buccal cavity.
• Buccal cavity is large, wide and shallow.
• It has ciliated columnar epithelial lining that contains mucous glands.
• These mucous glands secrete mucus that helps in lubricating the food.
• Frog lacks salivary glands.
• Teeth:
  • The lower jaw lacks teeth.
  • However, teeth occur in a row of either side on the premaxillae and maxillae bones of the upper jaw. The teeth are backwardly pointed.
  • Vomers (two small bones in the roof of the mouth) also consists of two groups of vomerine teeth.
  • The function of teeth is to simply hold the prey and prevent it from slipping out.
  • Teeth are not meant for chewing.
  • The nature of teeth is homodont (similar), acrodont (not set in a socket).
  • But teeth are attached to the jaw bone by a broad base made of a bone-like substance.
  • The crown is the free part of tooth.
  • It is made up of dentine (a hard ivory-like substance), which is traversed by numerous fine canals or canaliculi.
  • Enamel covers the tip of the crown.
  • Enamel is a very hard, resistant and glistening substance.
  • Tooth contains a central pulp cavity open at the side.
  • It is filled with a soft nourishing pulp, containing connective tissues, blood vessels, nerve and odontoblast cells that produces new material for the growth of tooth.
  • Frog are polyphyodont in nature, i.e. teeth is replaced several times in life.
• Tongue:
  • In frogs, tongue is large, muscular, sticky and protrusible.
  • It lies on the floor of mouth cavity.
  • The anterior end of tongue is attached to the inner border of lower jaw.
  • The posterior end is free and bifid.
  • This free end can be flicked out and retracted immediately after catching the prey.
  • The slimy surface of tongue facilitates in capturing the prey.
  • The change of pressure in large sublingual lymph sac causes the protrusion of tongue.
• Internal nostrils:
  • Just in front of vomerine teeth, the roof of buccal cavity contains anteriorly, a pair of small openings of internal nares.
  • By these internal nares, the nasal cavities open into buccal cavity.
  • These serves in respiration.
• Bulging of orbits:
  • The roof of buccal cavity shows two large oval and somewhat pale areas, behind the vomerine teeth. These areas are the bulging of eye balls.
  • In course of swallowing the food, frog depresses the eyes.
  • This causes the orbits to bulge inwards which in response pushes the food towards the pharynx.

3. Pharynx:

• Posteriorly, the buccal cavity reaches short pharynx without any clear demarcation.
• So, sometimes these are termed as single bucco-pharyngeal cavity.
• Several apertures open into pharynx.
• A median elevation on the floor carries the glottis.
• Glottis is a longitudinal slit like aperture.
• The glottis leads to the laryngo-tracheal chamber.
• A wide eustachian aperture is present on either lateral side in the roof.
• This aperture opens into the middle ear.
• In male frogs, on the floor of pharynx, the small opening of a vocal sac is present on either side near the angle of two jaws.
• Now, the pharynx tapers behind to lead to esophagus through the gullet.
• Gullet is the wide opening that leads to Oesophagus.

4. Oesophagus:

• Oesophagus is a short, wide, muscular and highly distensible tube.
• Its mucous epithelial lining is folded longitudinally and contains some mucous glands.
• During the passage of food, its expansion is allowed by longitudinal foldings.
• An alkaline digestive juice is secreted by the glandular lining of oesophagus.
• Oesophagus enlarges to join with stomach in the peritoneal cavity.

5. Stomach:

• Stomach is present on the left side in the body cavity.
• It is attached to the dorsal bodywall by a mesentery termed as mesogaster.
• It is around 4 cm long, broad and slightly curved bag or tube with thick muscular walls.
• The anterior part is large, and broad. It is called as cardiac stomach.
• The posterior part is short and narrow. It is called the pyloric stomach.
• Several prominent longitudinal folds are present in the inner surface of the stomach.
• It allows the distension of stomach when food is received.
• Its mucous epithelium has multicellular gastric glands.
• These glands secrete the enzyme pepsinogen and unicellular oxyntic glands, secreting hydrochloric acids.
• The pyloric end of stomach is slightly constricted.
• Pyloric valve guards its opening into small intestine. 
• Pyloric valve is a circular ring like sphincter muscle.
• Stomach serves for storage as well as digestion of food.

6. Small intestine:

• Small intestine is a long, coiled and narrow tube.
• It is about 30cm long, and is attached mid-dorsally to bodywall by mesenteries.
• It comprises of two parts:
  •  A small anterior duodenum
  • A much longer posterior ileum
• Besides, intestinal glands, the mucosal lining of the small intestine consists of two types of cells.
• They are:
  • Goblet cells:
  • Large cells containing oval vacuoles and granular substances which produces mucus.
  • Near the base of the cell, nucleus is present.
    • Absorbing cells:
    • Small cells with nuclei near the base.
• Duodenum:
  • Duodenum runs ahead being parallel to stomach and forms a shape like U.
  • It receives a common hepatopancreatic duct.
  • Liver and pancreas bring bile and pancreatic juice respectively.
  • Low transverse folds are formed by the internal mucous lining.
• Ileum:
  • Ileum is the longest part of alimentary canal.
  • Before enlarging posteriorly to join rectum, it makes several loops.
  • The internal mucus lining forms many longitudinal folds.
  • However, as in case of higher vertebrates, there are no true villi and definite glands and crypts.
  • In the small intestine, digestion of food and absorption of digested food takes place.

7. Large intestine or rectum:

• Large intestine is short, wide tube about 4cm long.
• It runs straight behind to open into cloaca by anus.
• The opening is guarded by an anal sphincter.
• The inner lining of large intestine forms numerous low longitudinal folds.
• Itserves for the re-absorption of water and the preparation and storage of faeces.

8. Cloaca:

• It is the small terminal sac-like part.
• The anus and the urinogenital apertures open into cloaca.
• Cloaca opens to outside by the vent or cloacal aperture, lying at the hind end of body.

Digestive glands of frog:

• Keeping aside gastric glands and intestinal glands, two large glands that are linked with the alimentary canal of frog are the liver and the pancreas.
• Liver:
  • The largest gland in the body of vertebrate is the liver.
  • It is reddish-brown in colour.
  • It is multi-lobed gland and lies close to the heart and lungs.
  • 3 lobes are present in the liver of frog i.e. right, left and median.
  • Liver consists of innumerable polygonal cells that secretes bile.
  • Bile is a greenish alkaline fluid.
  • Bile is stored in the thin-walled sac called as gall bladder.
  • Gall bladder is large, spherical, and greenish in color.
  • A common bile duct is formed when cystic ducts from gall bladder and hepatic ducts from liver lobes combines.
  • It runs through pancreas and joins the pancreatic duct to form a hepatopancreatic duct.
  • Now, it ultimately opens into duodenum.
  • Bile lacks any digestive ferments and only emulsifies fats.
  • Thus, liver is not a true digestive gland.
• Pancreas:
  • Pancreas of frog is much branched, irregular flattened and is yellow in color.
  • It lies in the mesentery between stomach and duodenum.
  • It carries out both exocrine and endocrine function.
  • The endocrine part is formed by scattered islets of Langerhans. It produces insulin hormone which is related to sugar metabolism.
  • The exocrine part secretes pancreatic juice. This juice contains of several digestive enzymes.
  • Since pancreas lacks independent duct, the pancreatic juice reaches the duodenum through the hepatopancreatic duct.

Physiology of digestion in frog:

• Being strictly carnivorous, frog feeds on insects, worms, crustaceans, molluscs, small fish and even small frogs and tadpoles.
• The prey is caught by rapid flicking of tongue and is swallowed as a whole.
• The food is now passed to stomach.
• As salivary glands are absent in case of frogs, the food is lubricated by the mucus secreted from the lining of bucco-pharyngeal cavity and oesophagus.
• The wave of contraction of the muscular wall of oesophagus pushes food down, it is called as peristalsis.
• Gastric digestion:
  • Food remains in the stomach for upto 2-3hrs, which is sufficient time.
  • Gastric juice is secreted by the gastric glands of stomach wall.
  • The gastric juice consists of hydrochloric acid and an inactive pre-enzyme pepsinogen.
  • Pepsinogen is converted to active pepsin in presence of hydrochloric acid.
  • Now, the pepsin catalyzes the hydrolysis of proteins, breaking them into peptones and proteases.
  • Acid makes the food soft and also provides acidic medium. It kills bacteria and fungi present in the food.
  • The disintegration and mixing of digestive enzymes with food is aided by the muscular contractions of stomach wall.
  • In presence of food, stomach secretes gastrin hormone.
  • Gastrin activates cells that secrete HCl.
  • Now, the liquified semidigested acidic food is termed as chyme.
  • When the chyme reaches a proper state, the pyloric sphincter relaxes, hence chyme enters the duodenum.
• Intestinal digestion:
  • As the acidic chyme enters the duodenum, several intestinal hormones are produced which have their own respective functions.
  • Enterogastrone reaches the stomach trough blood and stops the production of gastric juice with HCl.
  • Cholecystokinin causes gall bladder to contract hence releasing bile into duodenum through hepatopancreatic duct.
  • Secretin and Pancreozymin work together to stimulate pancreas to secrete pancreatic juices into duodenum.
  • Enterocrinin activates secretion of intestinal juice, the succus entericus.
  • Thus, three important substances mix with the food in intestine for the completion of digestion.
  • They are derived from three different sources: bile, pancreatic juice and intestinal juice.
  • Bile:
    • Bile is a greenish alkaline fluid secreted by liver.
    • It lacks digestive enzymes.
    • It contains bile salts such as sodium bicarbonate, sodium glycocholate, sodium perocholate, etc.
    • Bile being alkaline in nature neutralizes the acidity of chyme, emulsifies fats, stimulates peristaltic action of intestine and activates pancreatic lipase.
  • Pancreatic juice:
    • The watery alkaline pancreatic juice contains several enzymes that acts on all 3 classes of foods.
    • Intestinal enterokinase converts inactive trypsinogen to active proteolytic enzyme trypsin. Trypsin converts proteoses, peptones and polypeptides to simple amino acids.
    • Amylase or amylopsin reduces starch (polysaccharides) to maltose (disaccharides).
    • Lipase formerly called steapsin, converts emulsified fats into fatty acids and glycerol.
  • Succus entericus:
    • Succus entericus or intestinal juice contains several enzymes, besides enterokinase.
    • These enzymes act on all classes of food stuffs.
    • Erepsin is the collective name for all proteolytic enzymes or peptidases.
    • It converts polypeptides to amino acids.
    • Maltase converts maltose to glucose.
    • Sucrase or invertase converts sucrose to glucose and fructose
    • Lactase converts lactose to glucose and galactose.
    • Lipase splits fats into fatty acids and glycerol.
• Egestion, absorption, and assimilation:
• Egestion:
  • Digestion is accomplished in the small intestine.
  • By peristalsis, the undigested part of food is slowly moved into rectum for storage and preparation of faeces.
  • At intervals, the faecal matter passes into cloaca.
  • And now it is egested through cloacal aperture.
• Absorption:
  • The final products of digestion are absorbed through the walls of small intestine.
  • The internal absorptive surface is increased by folds with villi like processes.
  • The actual mechanism of absorption is only little known.
  • Osmotic forces and other factors are seemed to play a part.
  • The epithelial lining absorbs water, mineral salts and other nutrients in the solution directly.
  • Carbohydrates are absorbed as glucose and fructose, and proteins as amino acids.
  • These pass into blood capillaries in the folds.
  •  Then it is passed into hepatic portal system and so into liver.
  • Fatty acids and glycerol pass into lymphatic capillaries or lacteals in the folds and so into the veins.
• Assimilation:
  • The absorbed food can be used for two basic purposes of nutrition:
  • Liberation of energy during respiration.
  • Assimilation as part of intimate structure of the animal.
  • Excess of glucose may be stored as glycogen in liver and skeletal muscles or converted into fats.  These are deposited in adipose tissue.
  • Amino acids may for proteins for growth and repair.
  • Or, it undergoes deamination resulting in the formation of urea to be excreted by kidneys with urine.

Digestive system of frog: Anatomy and Physiology of digestion

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