Digestive is a process by which the complex insoluble food particles are converted into soluble from by action of enzymes. The digestive system consists of the alimentary canal and digestive gland.
The alimentary canal is a long, coiled path starts from mouth and ends in cloaca.It consists of organs such like mouth or buccal cavity, pharynx, esophagus,stomach, small intestine, large intestine and cloaca.
Mouth:The digestive system of a frog starts with the mouth. Mouth is a wide opening lies in between upper and lower jaw. Mouth helps in consumption of food. This process is known as ingestion.
Buccal cavity: Mouth leads into the buccal cavity and it is a wide space between upper and lower halves of the head.
A) Structure present on the upper jaw: The upper jaws of frog are fixed or immovable. The following structure found on the upper jaws:
a) Maxillary teeth: maxillary teeth are the situated on the either side of upper jaw. These are sharply pointed and not fixed in the sockets. These are directly attached with the maxilla. Such type of teeth is known as acrodont teeth. All teeth are of the same size and are called homodont type. These teeth breaks occasionally and is replace by new ones.this is known as polyphyodont condition. At the tip of the mouth, the maxillary teeth are more pointed called the premaxillary teeth. It helps in closing the tip of mouth.
b) Vomarine teeth: these are the two small patches of teeth bending backwards and are found one or either side of the median line of the roof of the buccal cavity. They are called the vomarine teeth as they are born in vomer.
Structure of teeth: the teeth of frog acrodont, homodont and polyphyodont.
The tooth is conical in its shape and consists of two regions: a base and crown. The wide lower part of teeth is called root or base. It is formed of a bone like material, whereas the upper part is called the crown. It is composed of hard substance called dentine. The crown is covered with hard and shining white substance called enamel.
The inner side of teeth is hollow and is called pulp cavity.it nourishing the tooth during the growth.
Function of teeth:
The teeth of frogs not help in cutting, grinding and mastication of food. They are simply serving to preserve the captured smooth prey from the slipping out.
c) Fossae: there are three pits are situated at tip of upper jaw just near the premaxillary teeth called fossae. The middle one is larger is called median sub rostral fossa while other two are smaller and is called lateral sub rostral fossae.
d) Internal nares: internal nares are two in no.lying closed outside the patches of vomarine teeth.
e) Bulging of eyeballs: there are two large and oval area on the roof of upper jaws and called the bulging of eye ball. These occupy the maximum portion of upper jaws and form a socket for eyes.
f) Eustachian tube: there are two small spindle shaped opening one on either side situated laterally near the jaw angles called Eustachian tube. It connects the cavity of the pharynx and middle ear to open into internal ear.
B) Structure resent on the lower jaw:
The lower jaw of frog is moveable and it can up and down in vertical plane. It bears no teeth. The following structures are found on lower jaw:
a) Tongue: the tongue of frog is long, large fleshy thick, fleshy, bifid and producible. The tongue is attached in front inside the tip of lower jaw and free behind. The tongue is bilobed at its free end.The upper surface of tongue bears the taste buds in the form of small papillae and mucous glands. The secretion of which keeps tongue moist. Neither taste buds nor mucous glands produce any digestive enzymes.
The tongue can be thrownout and retracted suddenly to capture and engulf prey. This action is accompanied by protractor and retracter muscle.
The throwing out of tongue and its retraction due to the muscular action is assisted by the sudden flow of lymph from the sublingual lymph sinus situated in tongue.
b) Glottis: Glottis is a longitudinal median slit like aperture in the pharynx behind the tongue that guards the entrance to the lungs. It leads into laryngo-trachel chamber. It is always opened during the breathing but become closed during the swallowing of food.
c) Vocal sac: in male frog, on the either side of the tongue on the angle of lower jaw there are two pores called vocal sacs. These acts as resonators at the time of cracking.
d) Gullet: Just above the glottis, there is horizontal opening called gullet. The food enters from the gullet to esophagus to stomach.
e) Prelingual elevations: there are three small prelingual elevations at the front end of lower jaw. They fit in the fossae of the upper jaw and helps in the closing of mouth.
Pharynx:The buccal cavity leads into the narrow pharynx. The buccal cavity and pharynx is collectively known as bucco-pheryngeeal cavity.
Oesophagus: Pharynx leads into a small but wider tube called oesophagus or gullet. The oesophagus opens into the stomach.
Stomach: The oesophagus opens into a large and thick sac like structure known as stomach.The anterior end of stomach is called Cardiac end while the posterior end is called pyloric end. The walls of stomach are muscular and glandular. The muscles of walls of stomach contract and relax, by which food is broken down into tiny pieces. The secretions of stomach have different enzymes which help in chemical digestion. In stomach, digestion of protein of food is started.
Small intestine (Duodenum and Ileum): After stomach, first part of intestine begins which is called duodenum. The ducts from liver and pancreas open into the duodenum. These ducts bring juices from these glands.The second part of intestine is Ilium. The secretion of pancreas is called Pancreatic Juice. Pancreatic juice enters the bile duct by small duct. This juice digests the food and brings in such form which can be absorbed by the blood through intestine. In illum the internal lining forms many longitudinal folds. Digested food is absorbed by illium.
Large intestine (rectum): It is wide tube measures about 4cm. its inner lining forms numerous folds. In rectum undigested food material is stored which is ready to expel from cloaca.
Cloaca: It is a small sac like structure used to remove undigested food material.