Digestive system

                             

      The digestive system of Frog consists of alimentary canal and associated digestive glands.

Alimentary canal: -

      It is a long, coiled tubular structure of varying diameter extending from mouth to cloaca. It is complete. It consists of mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, cloaca and cloacal aperture.

·         Mouth: -  It is anterior opening of alimentary canal and is a wide gape at the top of snout. It remains bounded by immovable upper jaw and movable lower jaw. Lower jaw moves up and down to close and open the mouth.

·         Buccal cavity: - Mouth opens into a wide cavity called as buccal cavity. It remains lined by ciliated epithelium which contains mucus glands which produce mucus to lubricate food.  It contains different parts on upper jaw and lower jaw.

Parts on upper jaw

Fossae: -  These are three small pits at the top of upper jaw.

Indulging’s of eye ball: - Two large oval areas each on either side of median line on the roof of buccal cavity are indulging’s of eye. Frog depresses the eyes causing these to bulge inwards and push the food back during swallowing of food.

Internal nares: - On the roof, in front of vomer bones, a pair of small openings called as internal nares are present. These help in respiration.

Teeth: - The maxillae and premaxillae bones of upper jaw contains teeth while lower jaw is without teeth. Two small bones on the anterior roof of buccal cavity called as vomer bones also contains teeth. These teeth are called as vomerine teeth.

The teeth are non-masticatory in nature and they simply hold the prey(food) inside buccal cavity.

All teeth are similar i.e. Homodont or Isodont. Teeth simply remain attached on jaw bone. they are not set in cavity i.e. acrodont. They are replaced many times in life time i.e. polyphyodont.

Each tooth consists of broad base and free part or crown. The free part is made up of dentine which remains covered at top by enamel. It contains central pulp cavity which contains pulp, blood vessels, nerves and odontoblasts.

Parts on upper jaw

Tongue: - A large, muscular and sticky tongue is present on the floor of buccal cavity. It remains attached with the lower jaw in front while is free and bifid at back. It helps to capture food.

Prelingual tubercles: - These are three small elevations at the top of lower jaw. They fit into fossae to completely close mouth.

·         Pharynx: - Buccal cavity opens into another short cavity called as pharynx. There is no proper demarcation between buccal cavity and pharynx. Thus, both are called as bucco-pharyngeal cavity. Pharynx consists of a number or openings.

On the roof of pharynx each on either side opening of Eustachian tube is present which connects pharynx with middle ear.

A median slit like aperture on the floor of pharynx is called as glottis which opens into laryngo-tracheal chamber.

In male frog, on the floor, each on either side opening of vocal sac is present.

Posteriorly, pharynx tapers to form a wide opening called as gullet which leads into oesophagus.

·    Oesophagus: – It is short, wide and muscular structure which carries food from pharynx to stomach. Its internal epithelial lining remains folded and contains mucus glands.

·         Stomach: – Oesophagus opens into a wide sac located at left side of body cavity  called as stomach. It remains attached with body wall by mesentry called as mesogaster.  Its large broader anterior region is called as cardiac stomach while narrower posterior region is called as pyloric stomach. Internally it remains folded and glandular. It contains multicellular gastric gland and unicellular oxyntic cells. The gastric gland produces pepsinogen while oxyntic gland produce HCl.

The pyloric end of stomach is constricted at its opening to small intestine and is called as pyloric constriction which contains pyloric valves.

Stomach stores food and digestion takes place in it.

• Small intestine: -  It is long, coiled tubular structure and remains attached with body wall by mesenteries. It consists of duodenum and ileum.

Duodenum: It is short, tubular structure which runs ahead forming U with stomach. It receives a common hepato-pancreatic duct. Internally, it remains folded.

Ileum: It is longest part of alimentary canal and is highly coiled. Internally, it remains folded. However, true villi are absent. Intestinal gland is present internally. Digestion of food and absorption of digested food takes place in it.

• Large intestine or Rectum: It is short, wide tube which opens behind into cloaca by anus guarded by anal sphincter. Internally, it contains folds Absorption of water takes place here.
• Cloaca: It is small sac like structure in which large intestine and urino-genital ducts open. Cloaca opens outside by cloacal aperture.

Associated digestive glands:

            The glands associated with alimentary canal are liver and pancreas.

•   Liver: It is largest gland located ventral to the anterior region of stomach. It is reddish brown and multi-lobed. It consists of right lobe, median lobe and left lobe. The cells of liver are called as hepatic cells which produce greenish alkaline fluid, bile. It is stored in gall bladder. The bile contains bile salts and bile pigments. The bile salts are Sodium bicarbonate, Sodium glycolate and Sodium taurocholate while bile pigments are bilirubin and biliverdin. The Common bile duct runs down from gall bladder. It runs through pancreas and receives pancreatic duct to form hepatopancreatic duct which opens into duodenum. Bile emulsifies fats.

The functions of liver are:

-      It produces bile which emulsifies fat. The conversion of large particle of fat into smaller ones is called as emulsification.

-       It converts excess glucose into glycogen and stores glycogen as reserve food.

-       The process of deamination takes in it.

-       The Kupffer cells are phagocytic in nature and they engulf microorganisms.

-       The old RBC get destroyed in it.

·        Pancreas: It is flattened, leaf like yellow coloured gland lying between stomach and duodenum. It acts as both exocrine and endocrine gland. The exocrine part secretes pancreatic juice with digestive enzymes. This juice passes to duodenum by hepato-pancreatic duct. This juice contains trypsin, amylase, and lipase. The trypsin responsible for digestion of protein, amylase is responsible for digestion of carbohydrate and lipase is responsible for digestion of fat. The endocrine part is Islets of Langerhan's which produce hormones insulin and glucagon which are responsible to regulate amount of glucose in blood.

Food, Feeding and Physiology of digestion

Frog is carnivorous and feeds upon insects, Earthworms, small fishes, molluscs, even small frogs and tadpoles.

The food is captured by rapid flicking action of tongue. Food is swallowed directly without mastication. Food is lubricated by mucus produced by mucus gland present on inner lining of buccal cavity. Through oesophagus food comes into stomach. Foods move down in oesogphagus by a wave of contraction of its muscular wall, called peristalsis.

The actual digestion begins from stomach where food remains for 2 to 3 hrs. Here, food gets mixed with pepsinogen and HCl produce by gastric gland and oxyntic gland respectively. The inactive pepsinogen changes into active pepsin in presence of hydrochloric acid and pepsin acts upon protein to convert it into peptones and proteases. Hydrochloric acid provides acidic medium and kills bacteria and fungi. Hcl also makes food soft. The muscular contractions of wall of stomach help in mixing of digestive enzymes with food and converts food in semi-solid paste like form called as chyme. The chyme passes to duodenum from pyloric valves. In small intestine three substances, bile, pancreatic juice and intestinal juice mix up with food.

Bile is alkaline fluid produced by liver which neutralizes acidic food, emulsifies fat and activates pancreatic lipase.

The pancreatic juice contains different enzymes which act in alkaline medium. It contains trypsinogen, amylase and lipase. The inactive trypsinogen changes into active trypsin by intestinal enterokinase. The trypsin acts upon protein, proteoses, peptones etc. and converts them into aminoacids. The amylase converts carbohydrate into glucose. The lipase converts emulsified fat into fatty acids and glycerol.

The intestinal juice or succus entericus contains different enzymes besides enterokinase and acts upon all type of foods. Erepsin acts upon protein and converts it into aminoacids. Maltase converts maltose into glucose. Sucrase converts sucrose to glucose. Lipase converts fat into fatty acids and glycerol.

The simple form of food materials is absorbed by inner lining of intestine. Presence of folds like villi makes the process of absorption faster. Glucose and amino acids pass into blood capillaries of folds. While fatty acids and glycerol pass into lymphatic vessels or lacteal in folds.

Undigested food passes into rectum for storage and preparation of faces. The faecal matter passes into cloaca and passed out through cloacal aperture. The passing out of faecal matter through cloacal aperture is called as egestion.