Digestive Organs
· Digestive system consists of the alimentary canal or digestive tract (from mouth to anus) and accessory organs of digestion (teeth, tongue, salivary gland, pancreas, liver).
· The alimentary tract:
· It is a long, coiled tube of varying diameters that begins from the mouth and terminates into the anus.
1. MOUTH:
· It is small transverse, slit-like subterminal aperture at the snout.
· It is bounded by upper and lower soft fleshy and movable lips and leads into the vestibule.
· Its upper lip is cleaved, exposing incisor teeth for gnawing effectively whereas the lower lip is not divided.
2. VESTIBULE:
· It is a small, narrow space that is externally bounded by lips and cheeks and internally by the gums of jaws.
3. BUCCAL CAVITY:
· It is surrounded by jaws and cheeks on sides, palate on the roof and tongue on the floor.
· It is separated from the nasal chamber by Palate.
· Buccal cavity contains the following structures:
a. Palate:
· It lies on the roof of the buccal cavity.
· It is divisible into the anterior hard palate which is formed by premaxillae, maxilla, palatine bones.
· It consists of palatine rugae (transverse ridges).
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| Rabbit Alimentary canal with associated glands |
· Each canal lodges a sensory Jacobson's organ which helps to recognize the different types of food.
· Posterior Soft palate is composed of connective & muscles and the posterior end of the soft palate is modified into a conical structure called Uvula that is lined by mucus membrane.
· Posterior Soft palate is composed of connective & muscles and the posterior end of the soft palate is modified into a conical structure called Uvula that is lined by mucus membrane.
· Uvula separates the buccal cavity from the pharynx.
b. Tongue:
· It is a fleshy large, monofid, flexible and slightly protrusible structure that occupies the floor of the buccal cavity.
· Its dorsal surface had a median groove and contains many taste buds.
· Anterior tip free. Posterior part attached to the front of the hyoid bone.
· Upper surface of the tongue bears 4 types of papillae or taste buds.
· Fungiform: Mushroom-shaped. Lie on the margin of the anterior tongue.
· Filiform: Small filamentous. Distributed on the whole of the dorsum of the tongue.
· Foliate: Margin of the posterior tongue.
· Circumvallate: Dorsum of posterior region. Largest papillae.
· Functions:
· It receives the taste of food, manipulate the food and mixes the food with saliva.
· Its dorsal surface had a median groove and contains many taste buds.
· Anterior tip free. Posterior part attached to the front of the hyoid bone.
· Upper surface of the tongue bears 4 types of papillae or taste buds.
· Fungiform: Mushroom-shaped. Lie on the margin of the anterior tongue.
· Filiform: Small filamentous. Distributed on the whole of the dorsum of the tongue.
· Foliate: Margin of the posterior tongue.
· Circumvallate: Dorsum of posterior region. Largest papillae.
· Functions:
· It receives the taste of food, manipulate the food and mixes the food with saliva.
c. Teeth
· They are present in both jaws.
· They are present in both jaws.
· They are developed from the maxillae and premaxillae of the upper jaw and dentary of the lower jaw.
· Teeth are heterodont, thecodont and diphyodont.
· Dental formula: 2033/1023
· Teeth are heterodont, thecodont and diphyodont.
· Dental formula: 2033/1023
· Total teeth in rabbits is 28.
· In rabbits, canine teeth are absent
· Space behind incisor and premolar teeth is called Diastema
· In rabbit incisors, teeth are chisel-shaped and well developed throughout the life for the gnawing and cutting process.
· In rabbits, canine teeth are absent
· Space behind incisor and premolar teeth is called Diastema
· In rabbit incisors, teeth are chisel-shaped and well developed throughout the life for the gnawing and cutting process.
4. PHARYNX:
· It is a short but wide cavity behind the soft palate.
· It opens to the nasal cavity through internal nares and buccal cavity through the buccopharyngeal opening anteriorly and to the oesophagus and larynx posteriorly.
· Entrance to the larynx is Glottis which lies on the floor of the pharynx ventral to oesophageal opening or Gullet.
· It is a short but wide cavity behind the soft palate.
· It opens to the nasal cavity through internal nares and buccal cavity through the buccopharyngeal opening anteriorly and to the oesophagus and larynx posteriorly.
· Entrance to the larynx is Glottis which lies on the floor of the pharynx ventral to oesophageal opening or Gullet.
· Glottis is guarded by a flap of cartilage called epiglottis.
· Soft palate divides the pharynx into three parts.
· Nasopharynx: It lies dorsal to the soft palate where the internal nare is opened.
· Soft palate divides the pharynx into three parts.
· Nasopharynx: It lies dorsal to the soft palate where the internal nare is opened.
· Laterally it has a pair of eustachian tubes.
· Oropharynx: it lies below to soft palate.
· Laryngopharynx: it contains a median vertical slit called glottis that opens into the larynx.
· Laryngopharynx: it contains a median vertical slit called glottis that opens into the larynx.
· It posteriorly opens into the oesophagus through the gullet.
5. OESOPHAGUS or FOOD PIPE:
· It is a long narrow elastic muscular tube leading to the stomach.
· It runs parallel to the trachea at the dorsal side and pierces the diaphragm to opens into the stomach.
· Digestive glands are absent.
· Mucous glands are present.
· It has inner longitudinal folding which increases the diameter of the oesophagus.
· It conducts the bolus towards the stomach.
· It has inner longitudinal folding which increases the diameter of the oesophagus.
· It conducts the bolus towards the stomach.
6. STOMACH:
· It is a large, J –shaped sac.
· It is the broadest part of the alimentary tract which lies transversely in the abdomen behind the diaphragm.
· It has lesser and greater curvature.
· It is differentiated into 3 parts: cardiac region, pyloric region and fundic region.
· Presence of cardiac sphincter at the opening of the oesophagus into the cardiac part.
· Opens into the duodenum through the pyloric sphincter.
· Functions:
· Stomach performs these functions- temporary storage of food, mechanical churning and partial digestion of food.
7. SMALL INTESTINE (SI):
· Small intestine is divisible into duodenum, jejunum, ileum.
a) Duodenum:
· It forms a U-shape structure with the stomach.
· It is about 20 cm long. It is the shortest part of the small intestine.
· Middle curvature of duodenum bears pancreas.
· It receives an opening of the bile duct in the proximal limb and pancreatic duct.
· Bile duct is guarded by the sphincter of Oddi but the absence of ampulla of Vater.
· Its inner wall contains Brunner's gland and Crypts of Lieberkuhn.
· It is the main site of digestion.
· Middle curvature of duodenum bears pancreas.
· It receives an opening of the bile duct in the proximal limb and pancreatic duct.
· Bile duct is guarded by the sphincter of Oddi but the absence of ampulla of Vater.
· Its inner wall contains Brunner's gland and Crypts of Lieberkuhn.
· It is the main site of digestion.
b) Jejunum:
· it is the middle part of the small intestine. intestinal glands are larger in the jejunum than the ileum.
c) Ileum:
· It is the longest terminal part of SI.
· It is about 2 – 2.5 m in length
· Which consists of a series of complicated coils.
· It is expanded at its distal end to form sacculus rotundus which opens into the caecum, in between them, the ileocaecal valve is present.
· Its inner wall bears lymphoid tissue or oval Patches called Payer’s patches which
· produce lymphocytes.
· It has a number of folds known as valvulae conniventes or plica circulars.
· Its main function is the absorption of digested food.
· It is about 2 – 2.5 m in length
· Which consists of a series of complicated coils.
· It is expanded at its distal end to form sacculus rotundus which opens into the caecum, in between them, the ileocaecal valve is present.
· Its inner wall bears lymphoid tissue or oval Patches called Payer’s patches which
· produce lymphocytes.
· It has a number of folds known as valvulae conniventes or plica circulars.
· Its main function is the absorption of digested food.
d. Caecum:
It is about 50 cm in Length and thin-walled tube.
· It contains spiral valves in it.
· Distally it is terminated into a small 25 cm long narrow thick-walled blind tube called the vermiform appendix.
· Function: cellulose of plants is digested by the bacterial action in caecum in herbivores.
· Distally it is terminated into a small 25 cm long narrow thick-walled blind tube called the vermiform appendix.
· Function: cellulose of plants is digested by the bacterial action in caecum in herbivores.
8. LARGE INTESTINE:
· It is more than a one-meter long tube which is divisible into 2 parts- anterior sacculated colon (45 cm long) and posterior beaded rectum (75cm long) due to deposition of faecal pellets.
· Cloaca absent.
· It is more than a one-meter long tube which is divisible into 2 parts- anterior sacculated colon (45 cm long) and posterior beaded rectum (75cm long) due to deposition of faecal pellets.
· Cloaca absent.
· The rectum opens directly to the outside by anus having an anal sphincter.
Histology of Alimentary Tract:
· There are 4 basic histological layers.· From inner to outwards, they are: Mucosa, Submucosa, Muscularis, Serosa or Adventitia.
1. MUCOSA:
· It consists of epithelium, lamina propria and muscularis mucosa.
· Epithelium: Stratified squamous epithelium is found in the oesophagus.
· Whereas Simple columnar epithelium is found in other parts alimentary canal.
· Inner wall of the Oesophagus is thrown out into numerous longitudinal folds where digestive glands are absent but abundant mucus glands are present.
· In the Stomach, Numerous gastric pits are present at the bases of which gastric glands are opened.
· Inner wall of the Oesophagus is thrown out into numerous longitudinal folds where digestive glands are absent but abundant mucus glands are present.
· In the Stomach, Numerous gastric pits are present at the bases of which gastric glands are opened.
· Gastric glands consist of the following types of cells:
· Mucus gland or goblet cell: it secretes mucus and is present in all parts of the alimentary canal.
· G-cells: it secretes Gastrin and is present in the pylorus.
· Parietal / Oxyntic cells: it secretes HCl for acidifying the food, killing microorganisms, dissolves iron and copper and activates the pepsinogen into pepsin.
· Peptic / Zymogenic cells secrete Pepsinogen enzyme and present in the Fundus region of the stomach
· Argentaffin cells secrete Serotonin as a vasoconstrictor.
· Small intestine is thrown into numerous finger-like projections called villi which increase the surface area for absorption.
· Between the villi, crypts of Leiberkuhn that consists of Paneth cells, Argentaffin cells and undifferentiated columnar epithelial cells.
· Mucus gland or goblet cell: it secretes mucus and is present in all parts of the alimentary canal.
· G-cells: it secretes Gastrin and is present in the pylorus.
· Parietal / Oxyntic cells: it secretes HCl for acidifying the food, killing microorganisms, dissolves iron and copper and activates the pepsinogen into pepsin.
· Peptic / Zymogenic cells secrete Pepsinogen enzyme and present in the Fundus region of the stomach
· Argentaffin cells secrete Serotonin as a vasoconstrictor.
· Small intestine is thrown into numerous finger-like projections called villi which increase the surface area for absorption.
· Between the villi, crypts of Leiberkuhn that consists of Paneth cells, Argentaffin cells and undifferentiated columnar epithelial cells.
2. SUBMUCOSA:
· It consists of connective tissue, blood vessels, lymphatics and nerve fibres.
· In Duodenum, Submucosal glands / Brunner’s glands are present in this layer.
· It consists of connective tissue, blood vessels, lymphatics and nerve fibres.
· In Duodenum, Submucosal glands / Brunner’s glands are present in this layer.
3. MUSCULARIS MUCOSA:
· It has outer longitudinal and inner circular muscle layers.
· In the Stomach, an additional oblique muscle layer (inner to circular muscles) is also present.
4. SEROSA / ADVENTITIA:
· It is the outermost layer of the alimentary canal which is formed by the visceral peritoneum.
· It consists of squamous epithelial cells.
SALIVARY GLANDS
· 4 pairs of salivary glands are found in the buccal cavity.
· 4 pairs of salivary glands are found in the buccal cavity.
· They are of the following types:
a) Parotid Glands:
· These lie below and anterior to the ear opening or base of the external ear pinnae.
· Their ducts open behind upper incisors.
· Their ducts are called Stenson ducts
b) Infra-Orbital Glands:
· It lies below the eyeball or orbit.
· Their ducts open near the upper molars.
c) Submaxillary or Submandibular Glands:
· They lie inside the posterior end of the lower jaw or jaw angle of the mandible.
· Ducts of these glands are Wharton ducts that open just behind the lower incisor.
d) Sublingual Glands:
· These are present beneath the tongue on the inner sides of the mandibles.
· they have short ducts Multiple open near the termination of Wharton ducts or below the free part of the anterior end of the tongue.
· Functions:
· All these glands secrete the saliva which is alkaline
· watery liquid. Saliva lubricates the food for easy
· swallowing of food and contains ptyalin enzyme to digest
· the starch into maltose.
LIVER:
· It is the largest gland in the animal body and is attached to the diaphragm with the help of the falciparum ligament.
· It is divided into 5 lobes:
· Right central lobe
· Left central lobe
· Left lateral lobe
· Caudate lobe
· Spigelian lobe
· It is divided into 5 lobes:
· Right central lobe
· Left central lobe
· Left lateral lobe
· Caudate lobe
· Spigelian lobe
· Gall's bladder is the elongated dark green thin-walled structure present in a groove of the posterior face of the right central lobe which stores bile temporarily.
· A hepatic duct is developed from each lobe and unite with the cystic duct of the gall bladder to form a common bile duct.
· A common bile duct is opened into the duodenum through the sphincter.
· Each lobe of the liver consists of many small lobules which are separated by connective tissue called Glisson's capsule.
· Presence of phagocytic Kupffer's cell.
· Hepatocytes secret bile into bile canaliculi which combine to form hepatic ducts which join cystic ducts to form a common bile duct that opens into the proximal limb of the duodenum.
PANCREAS:
· It is a broad leaf-like, diffuse pinkish gland lying between two limbs of the U-shaped duodenum.
· It is a mixed or heterocrine gland because it consists of both endocrine & exocrine parts.
(a) Exocrine part (it covers 97-89%) which is pancreatic acini or lobules for the secretion of pancreatic juice. This juice contains amylase, trypsinogen and lipase enzymes.
(b) Endocrine part is Islets of Langerhans It covers (1-3%) of the pancreas which secretes pancreatic hormones with these cells.
Types of cells in Islets of Langerhans:
i) É‘ or alpha cells (15-25%): secrete glucagon hormone which increases blood sugar level.
ii) êžµ or beta cells (70-80%): secrete insulin which reduces blood sugar level-so insulin and glucagon are antagonistic hormones
iii) Delta (ẟ) cell: secretes somatostatin.
iv) F-Cell: secretes pancreatic polypeptides(pp hormone) which control the secretion of digestive enzymes.
· Pancreatic duct opens into the distal limb of the duodenum.
(b) Endocrine part is Islets of Langerhans It covers (1-3%) of the pancreas which secretes pancreatic hormones with these cells.
Types of cells in Islets of Langerhans:
i) É‘ or alpha cells (15-25%): secrete glucagon hormone which increases blood sugar level.
ii) êžµ or beta cells (70-80%): secrete insulin which reduces blood sugar level-so insulin and glucagon are antagonistic hormones
iii) Delta (ẟ) cell: secretes somatostatin.
iv) F-Cell: secretes pancreatic polypeptides(pp hormone) which control the secretion of digestive enzymes.
· Pancreatic duct opens into the distal limb of the duodenum.
GASTRIC GLAND
· They are found in the mucous membrane of the stomach
· as simple branched alveolar glands in numerous numbers.
· They secrete gastric juice which contains HCL, mucus and pepsinogen or rennin enzyme.
Physiology of Digestion
· Food and Ingestion:· Food: The rabbit is a herbivorous animal.
· It consists mainly of green leaves, vegetables, cereals, roots and barks, etc.
· Gnawing: Its food is cut into smaller pieces by means of its sharp incisor teeth
· In gnawing the upper and lower incisors work against one another like chisels.
· Food pieces are taken into the buccal cavity by the movable lips through the mouth.
· Mastication:
· Inside the buccal cavity, the premolars and molars grind or chew the fragments of food. Their flat surfaces traversed by transverse enamel ridges are well-adapted for mastication.
· Swallowing:
· The masticated food is manipulated by the tongue.
· The masticated food is manipulated by the tongue.
· Mucus and saliva are mixed to moisten and lubricate food which changes into a small ball or bolus.
· Bolus is pushed down to the stomach by peristaltic movement.
· Bolus is pushed down to the stomach by peristaltic movement.
· During swallowing, the epiglottis closes the glottis and the soft palate pushed upwards to prevent the entry of food into the larynx and nasal passage, respectively.
· Digestion:
· Digestion of food commences in the buccal cavity (saliva) itself, continues in the stomach (gastric juice) and is completed in the intestine (bile, pancreatic juice and intestinal juice).
· Digestion of food commences in the buccal cavity (saliva) itself, continues in the stomach (gastric juice) and is completed in the intestine (bile, pancreatic juice and intestinal juice).
· Action of Saliva:
· In the buccal cavity, the food mixes with saliva secreted by the salivary glands. It contains the enzyme ptyalin which converts starch to maltose.
· The pH of saliva is close to neutrality (approximately 6-7) which is suitable for the action of ptyalin.
· In the buccal cavity, the food mixes with saliva secreted by the salivary glands. It contains the enzyme ptyalin which converts starch to maltose.
· The pH of saliva is close to neutrality (approximately 6-7) which is suitable for the action of ptyalin.
· Action of Gastric juice:
· The food is retained in the stomach for some time.
· The food is retained in the stomach for some time.
· The muscular wall of the stomach undergoes contractions.
· As a result the food is churned, broken into smaller bits (mashed) and thoroughly mixed with the gastric juice secreted by the gastric glands.
· As a result the food is churned, broken into smaller bits (mashed) and thoroughly mixed with the gastric juice secreted by the gastric glands.
· The gastric juice contains mucus, HCl and the enzymes rennin and pepsinogen.
· Functions of Hydrochloric acid:
(i) kills bacteria of the food,
(ii) stops the action of ptyalin which cannot work in an acid medium,
(iii) activates the proenzyme pepsinogen into pepsin,
(iv) provides an acid medium (optimum pH 1.5) for pepsin to work, and
(v) stops contractions of the stomach.
· The enzyme rennin coagulates soluble milk protein caseinogen into insoluble casein.
· The enzyme pepsin partially breaks down proteins into simpler peptones.
· The semi-digested food in the stomach is called chyme.
· The semi-digested food in the stomach is called chyme.
· In the beginning of gastric digestion, the pyloric sphincter remains closed.
· As the chyme becomes more and more acidic, the pyloric sphincter opens from time to time so that little by little the chyme is forced into the duodenum by the peristaltic contractions of the stomach wall.
· Once in the duodenum, the chyme mixes with three digestive juices: bile, pancreatic juice and intestinal juice.
· As the chyme becomes more and more acidic, the pyloric sphincter opens from time to time so that little by little the chyme is forced into the duodenum by the peristaltic contractions of the stomach wall.
· Once in the duodenum, the chyme mixes with three digestive juices: bile, pancreatic juice and intestinal juice.
· Action of Bile:
· The bile, secreted by the liver, has no enzymes, but it performs the following important functions:
· The bile contains certain salts, such as sodium glycocholate and sodium taurocholate, which bring about emulsification of fat, that is, split larger fat molecules into smaller ones, for more effective digestion.
· Bile is alkaline, containing sodium bicarbonate. This neutralizes the acid of the chyme and makes it alkaline.
· This is necessary because the enzymes of pancreatic and intestinal juices can operate only in an alkaline medium.
· The colour of bile is due to the presence of bilirubin and biliverdin.
· These are excretory products derived by the disintegration of haemoglobin of the dead red blood corpuscles.
· Bile salts kill bacteria to prevent the putrefaction of chyme.
· Bile salts help in the absorption of fats and fat-soluble vitamins such as A, D, E, K.
· Bile salts kill bacteria to prevent the putrefaction of chyme.
· Bile salts help in the absorption of fats and fat-soluble vitamins such as A, D, E, K.
· Action of Pancreatic juice:
· The presence of acid chyme induces the duodenal epithelial wall to secrete the hormone secretin.
· The presence of acid chyme induces the duodenal epithelial wall to secrete the hormone secretin.
· It reaches the pancreas through the bloodstream and stimulates the secretion of the pancreatic juice.
· This juice is strongly alkaline (PH 8.8) and, like bile, neutralises the acidity of chyme.
· This juice is strongly alkaline (PH 8.8) and, like bile, neutralises the acidity of chyme.
· It contains the following enzymes:
· Steapsin: It converts fats to fatty acids and glycerol.
· Amylase: It converts starch to maltose.
· Trypnisogen: It is a proenzyme that is activated by intestinal enterokinase into the proteolytic trypsin which converts peptones into amino acids.
· Steapsin: It converts fats to fatty acids and glycerol.
· Amylase: It converts starch to maltose.
· Trypnisogen: It is a proenzyme that is activated by intestinal enterokinase into the proteolytic trypsin which converts peptones into amino acids.
· Action of Intestinal juice:
· The intestinal juice or succus entericus, secreted by intestinal glands, is also alkaline.
· It contains several enzymes such as:
· Enterokinase which activates trypsinogen into trypsin.
· Erepsin which converts peptones to amino acids.
· Maltase which changes maltose into glucose.
· Invertase which changes cane (sucrose) to glucose and fructose.
· Lactase which changes milk (lactose) to glucose and galactose.
· Enterokinase which activates trypsinogen into trypsin.
· Erepsin which converts peptones to amino acids.
· Maltase which changes maltose into glucose.
· Invertase which changes cane (sucrose) to glucose and fructose.
· Lactase which changes milk (lactose) to glucose and galactose.
· Digestion of cellulose:
· A large food of rabbit consists of cellulose.
· Digestion of cellulose occurs in the caecum because of the presence of a vast number of symbiotic bacteria and Protozoa.
· These break up cellulose into soluble sugars and, in return, they get nitrogenous food from rabbits.
· These bacteria and rabbits show examples of symbiotic relationships.
· As a result of intestinal digestion, the chyme is converted into an alkaline watery emulsion called chyle, much of which is absorbed into the lymphatic system through lacteals.
· These bacteria and rabbits show examples of symbiotic relationships.
· As a result of intestinal digestion, the chyme is converted into an alkaline watery emulsion called chyle, much of which is absorbed into the lymphatic system through lacteals.
· Absorption of Products of Digestion:
· The end products of digestion are fatty acids, glycerol and simple hexose sugars. Their absorption occurs mainly in the ileum.
· The passage of digested food from the ileum into the blood and lymph by diffusion is termed absorption.
· The efficiency of the process is increased because of three factors:
· Slow passes of chyle through the ileum, the wall of ileum has innumerable finger-like villi which greatly increases the surface area for absorption and the wall of ileum rhythmically contract to ensure that the liquid food which is in contact with the villi is constantly changing.
· Villus contains a network of blood capillaries just below its surface.
· Hexose sugars, water, minerals and vitamins are absorbed from the gut into the capillaries of villi and taken to the liver by the hepatic portal vein.
· A lymph called lacteal is also present within each villus. Glycerol and fatty acids pass from the gut into lacteal and are carried into the blood system via the hepatic portal system.
· Slow passes of chyle through the ileum, the wall of ileum has innumerable finger-like villi which greatly increases the surface area for absorption and the wall of ileum rhythmically contract to ensure that the liquid food which is in contact with the villi is constantly changing.
· Villus contains a network of blood capillaries just below its surface.
· Hexose sugars, water, minerals and vitamins are absorbed from the gut into the capillaries of villi and taken to the liver by the hepatic portal vein.
· A lymph called lacteal is also present within each villus. Glycerol and fatty acids pass from the gut into lacteal and are carried into the blood system via the hepatic portal system.
· Egestion or Defaecation:
· After absorption in the ileum, the residual food reaches into the large intestine.
· It includes water and solid indigestible remains of food such as vegetable fibres and cellulose for the digestion of which there are no appropriate enzymes.
· Much of the water is absorbed in the colon, leaving the solid material as faeces.
· In the rectum, the faeces form small dehydrated pellets which are periodically eliminated through the anus. This is known as egestion or defaecation.
· Much of the water is absorbed in the colon, leaving the solid material as faeces.
· In the rectum, the faeces form small dehydrated pellets which are periodically eliminated through the anus. This is known as egestion or defaecation.
· Rabbit is coprophagous in nature, eating its own excreta in order to get the maximum amount of nutrients from its food.
· Faeces produced during the night alone are eaten up which are soft and moist due to incompletely digested cellulose.
· Faeces produced during the night alone are eaten up which are soft and moist due to incompletely digested cellulose.
· Thus passing through the gut once more, the faeces are subjected once again to digestion and absorption.
· Faeces produced during the day are dry and hard and not at all eaten.
· Faeces produced during the day are dry and hard and not at all eaten.
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