The digestive system includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body. Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated. The digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver, gallbladder, and pancreas are major accessory organs that have a role in digestion. These organs secrete fluids into the digestive tract.
Food undergoes three types of processes in the body: o o o
Digestion
Absorption
Elimination
Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are utilized by the body cells in metabolism.
The digestive system prepares nutrients for utilization by body cells through six activities, or functions.
Ingestion
The first activity of the digestive system is to take in food through the mouth. This process, called ingestion, has to take place before anything else can happen.
Mechanical Digestion
The large pieces of food that are ingested have to be broken into smaller particles that can be acted upon by various enzymes. This is mechanical digestion, which begins in the mouth with chewing or mastication and continues with churning and mixing actions in the stomach.
Chemical Digestion
The complex molecules of carbohydrates, proteins, and fats are transformed by chemical digestion into smaller molecules that can be absorbed and utilized by the cells. Chemical digestion, through a process called hydrolysis, uses water and digestive enzymes to break down the complex molecules. Digestive enzymes speed up the hydrolysis process, which is otherwise very slow.
Movements
After ingestion and mastication, the food particles move from the mouth into the pharynx, then into the esophagus. This movement is deglutition, or swallowing. Mixing movements occur in the stomach as a result of smooth muscle contraction. These repetitive contractions usually occur in small segments of the digestive tract and mix the food particles with enzymes and other fluids. The movements that propel the food
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particles through the digestive tract are called peristalsis. These are rhythmic waves of contractions that move the food particles through the various regions in which mechanical and chemical digestion takes place.
Absorption
The simple molecules that result from chemical digestion pass through cell membranes of the lining in the small intestine into the blood or lymph capillaries. This process is called absorption.
Elimination
The food molecules that cannot be digested or absorbed need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of feces, is defecation or elimination.
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Concerned with prehension, mastication, Digestion, absorption and expulsion of undigested portion.
Organs making digestive system
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Oral cavity.
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Pharynx.
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Oesophagus.
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Stomach / forestomach.
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Intestine – small & large.
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Accessory glands.
Oral cavity
• includes lips, teeth, tongue, cheek and cheek muscles and major salivary glands ( empties to buccal cavity)
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Lips for prehension.
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Tongue – prehension, mastication, taste, ingestion (swallowing), during chewing, it moves
Dentition of farm animals.
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Teeth- types incisors cutting canine tearing flesh premolar grinding molar grinding
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Herbivores have a more specialized digestive system than that of a carnivore because it is more difficult to digest vegetation than meat.
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The teeth are flat so that grass and plant material can be ground down, rather than the sharp teeth of carnivores designed to tear flesh.
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Glands – salivary gland
parotid (in front of ears) :serous
- Mandibular (below lower jaw): mucous
- Sublingual (below tongue): mixed.
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Tongue- taste, prehension.
Pharynx.
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The common passage for the food and air.
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Swallowing & breathing mechanism
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Esophagus.
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Is a muscular tube Continuation of pharynx joins stomach or forestomach.
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Separated from stomach by the esophageal sphincter.
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At neck it is seen towards the left side of trachea.
Stomach
Non-ruminant or Monogastric stomach
Are single-stomach animals. They have one stomach (mono=one, gastric=stomach).
Muscular gland lined sac that receives ingesta from the esophagus and conducts both physical and chemical digestion.
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Stomach lies behind the left side of the diaphragm.
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Is pear shaped.
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Divided into four regions- oesophagal, cardiac, fundic and pyloric.
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The thick wall of stomach has gastric glands (cardiac, pyloric and which aids in digestion.
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Presence of folding that receives ingesta from the oesophagus and conducts both physical and chemical Gastric glands- enzymes (pepsin), HCL and mucus.
Functional and non-functional caecum
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In the stomach, food is mixed with pepsin and HCL to help to break down solid particles.
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Horses is not ruminant as the oesophagus leading to stomach has the sphincter valve which is very strong, it is almost impossible for horse to vomit.
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The caecum in horse has got the similar function like that of ruminant stomach, as the caecum do contain the micro-organisms which helps in fermentation.
Relation of stomach with oesophagus
Ruminant (or multi-stomached) system
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Rumination - the regurgitation, rechewing and reswallowing of ingested feed
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Cud - mass of regurgitated ingesta; bolus
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Process of rumination
– regurgitate bolus from rumen
– rechew and reinsalivate
– reswallow
– repeat with another bolus
• multiple stomach forestomach,
• cattle, sheep, goat, yak, buffalo
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Four compartments are
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–
–
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Rumen
Reticulum
Omasum largest
Abomasum stomach
Rumen
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Large muscular sac, subdivided into
Smaller sacs by muscular pillars
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Large fermentation vat; also called the "paunch" Extends from the diaphragm to the pelvis filling the left side of the abdominal cavity.
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Ruminants evolved to consume and subsist on roughage - grasses and shrubs built predominantly of cellulose.
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The rumen is a fermentation vat par excellance, providing an anaerobic environment, constant temperature ( Temperature = 39oC (103oF)) and pH, and good mixing.
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Its purposes is to store large quantities of feed, keep the feed mixing by strong contractions, and to provide a suitable environment for micro-organisms.
• The “finger-like” projections lining (papillae) the bottom and sides of rumen wall absorbs the fatty acids produced by micro-organisms during fermentation.
Functions of Microorganisms
• digest roughages to make V olatile F atty A cids (waste product, but are the primarily source of energy for animals).
• make protein (through kerb’s cycle)
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Make vitamins K and B complex
(Very similar to cecum of hind gut fermentors)
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The function of the rumen is to house microorganisms.
Rumen
• saturated with gasses
• constant motion.
Reticulum
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It is the most cranial compartment.
• It is also called as the “honey-comb”
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It is located immediately behind the diaphragm places it almost in apposition to the heart.
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It aids to help bring bolus of feed back up to the mouth for rechewing.
• houses microorganisms
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It also serves as a receptacle for heavy foreign objects that animal takes.
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“Hardware Disease”
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When metal object such as wire or a nail is swallowed and punctures the reticulum wall. This condition may prove lethal for two reasons
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The bacteria and protozoa can contaminate the body cavity resulting in peritonitis.
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The heart and diaphragm may be punctured by the object causing failure of these tissues.
Omasum
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Once the feed has been reduced in size by chewing and digestion by bacteria and protozoa, it passes to spherical-shaped organ the third compartment called the
Omasum.
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It is located to the right of the rumen and reticulum just caudal to the liver.
• It is also called as “many-piles”, “bookstomach”.
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Inside of Omasum is thrown into broad longitudinal folds or leaves. Got full of folded tissue therefore the name.
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The leaves got small blunt papillae on them which absorbs FA, water and electrolytes like K and Na beside it assist in grinding roughage before it enters the next compartment
T/S of ruminant stomach
• A = Abomasum
B = Rumen &
Reticulum
C = Omasum
D = Liver
Abomasum
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It is the true stomach in ruminant, located ventral to the omasum and extends caudal on the right side of rumen.
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Got at the terminal part of abomasum sphincter (thickening of circular smooth muscle fibers) at the junction of the stomach and small intestine.
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Similar to the stomach of mono-gastric animals.
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The abomasum contains many folds to increase its surface area.
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The pH coming to abomasum is around 6.0 but it is quickly lowered to about 2.5 by the acid creating the suitable environmental condition for the enzymes.
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The chief enzymes secreted are pepsin.
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It has mucus and HCL that aids in digestion.
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Here actual digestion takes place.
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Summary for ruminant stomach.
Chamber Function a) Rumen
(part of forestomach) b) Reticulum
(part of forestomach) c) Omasum
(part of forestomach) d) Abomasum mechanical and chemical breakdown of food; breakdown of food by microbes; production of volatile fatty acids; absorption of volatile fatty acids, lactic acid, ammonia, inorganic ions and water
=enzymatic digestion
Young Ruminants-
Essentially they are non-ruminants.
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Rumen and reticulum are non-functional abomasum (secretes rennin for milk to coagulate) is largest part of stomach.
• dry feed stimulates reticulorumen
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Esophygeal groove allows milk consumed to go through rumen and reticulum to the intestines.
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Ratio of Rumen/Reticulum to Omasum/Abomasum according to age
Age R/R O/A
Birth 1
6 months 4
1 year 8-10
Rumen Size
Species
1000 lb cow
1000 lb ewe
Maximum
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1
1
~55-60 gallons
~5-10 gallons
Normal Content
25-30 gallons
3-5 gallons
Small intestine
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Once the feed has enter disheveled by the enzymes and by the high acid content of the stomach it is moved along and stopped in the small intestine, which is tiny and very long.
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The feed is semi-solid.
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As the feed enters the small intestine, it mixes with secretions from pancreas and liver which elevate the pH of the
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Digesta from 2.5 to between 7-8.
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Enzymes digestion and maximum absorption takes place.
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It is made up of 3 parts viz duodenum, jejunum and ileum.
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It contains finger-like projection called villi (villus) that aid in absorption of digested food.
Function s of small intestine.
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Digestion of proteins.
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Digestion of carbohydrates.
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Digestion of fats.
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Absorption of the end products of digestion.
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There are three digestive juices are pancreatic juice, bile, and intestinal juice.
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Three parts of small intestine.
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Duodenum
– 1st part attached to pylorus, liver (which secretes bile salt and pigment) and pancreas (which secretes enzymes for the digestion) open. most digestion occurs here.
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Jejunum – 2nd part, some digestion and some absorption occur.
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Ileum
– 3rd part, joins colon. mostly absorption.
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Digestive tract of the mono-gastric mammals.
Digestive enzymes in intestine.
Enzyme Function Source
Trypsin
Chymotrypsin carboxypeptides pancreatic amylase lipases disaccharides digest proteins secreted from pancreas
Digest CHO
Digest lipids. secreted from pancreas digests carbohydrates secreted from small intestine dipeptidases digest peptides secreted from small intestine
Bile juice.
• made in liver
• stored in gall bladder
• active in the small intestine
• emulsifies fat to aid in digestion which simply means that bile helps pancreatic lipase I the breakdown of fats in feed.
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So no enzymes.
Large intestine
• the terminal part of ileum joins the caecum (horse), colon (dog) or caecum and colon (in pig and ruminant).
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The caecum, colon and rectum makes up the rest digestive system.
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It is shorter, but larger I diameter than the small intestine.
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Its main function is absorption of water and reservoir of waste materials.
Caecum
• Blind sac” junction of small & large intestine.
• It is also referred as “blind gut”.
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I case of horse ad rabbits, caecum is very essential in the digestion of fibrous feeds.
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Got microbes that breaks down feed that was not digested I the small intestine.
Colon
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Food may reach here in a little ad stay here for appro.60 hours.
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Microbial digestion continues, and most of the nutrients made through microbial digestion are absorbed here.
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In addition to the vitamins and FA, water is also absorbed resulting to fecal formation, which goes to rectum.
Intestine of cattle
Rectum
Caecum
Descending colon
Ascending colon
Transverse colon
Small Intestine
Stomach
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Organ
Lips
Teeth
Tongue
Function
Ingestion and fragmentation of food
Fragmentation of food
Fragmentation and swallowing
Salivary Glands
Esophagus
Stomach
Fragmentation and moistening of food; swallowing
Passage of food from oral cavity to the stomach
Completion of fragmentation and beginning of digestion
Small Intestine - duodenum Digestion; emulsificaton of fats by enzymes from the pancreas and bile from the liver
Small Intestine - jejunum & ileum Completion of digestion and absorption
Large Intestine- cecum
Large Intestine - colon
Absorption of water from liquid residue.
Absorption of water from liquid residue
Large Intestine - rectum Storage of feces prior to defecation
Anus Route for defecation of feces outside the body
The central fact of being bird is that everyone wants to eat. Almost every bird is on predators menu. Therefore they need maximum energy for survival.
Consequently their anatomy reflects adaptations for evasion and escape.
To be able to “eat and run” or “eat and fly” so that for them to “eat now, digest later”, after they get away from danger.
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Oral cavity. includes beak, tongue, accessory glands.
The chicken can discern shape, size and colour and can feel material with its tongue and mouth.
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Mouth/Beak –
 The presence of hard beak is most adaptive to birds.
 Made of keratin.
 Gather and break down feed, along with the takes stones.
Tongue
 The tongue is heavily keratinized.
 They have little sense of taste. Their tongue has a bone running the full length.
 Their distal end of this bone is replaced by with hyaline cartilage .
Teeth
 As stated earlier Birds require maximum energy.
 There is no tooth.
 No teeth (hence the expression “scarce as hen’s teeth”, this means that they have to swallow whatever food they take in whole. thus fowl cannot consume particles of feed that are too large enter their oesophagus.
• With the feed they also take tiny rocks which aids in digestion.
At the proximal end of buccal cavity got accessory glands.
Oesophagus
 Tube that connects the oral cavity and the stomach.
 The muscular walls produce wave-like contractions (peristalsis) that help propel food from the oral cavity in the stomach.
 The oesophagus is larger in them.
 At the end of oesophagus there is out-pocketing or a diverticulum called as “Crop”.
 Crop- is a temporary storage of food. And for it is nice example of specialization of the avian digestive system.
 Crop can store food as much as half of their body.
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It is divided into two parts. a) Proventriculus - also called the glandular stomach which is pear-shaped; receives food from the crop and secretes mucus, HCL and pepsinogen.
They also secretes enzymes but by one cell type (as you can see in other animals chief and zymogenic cells). b) Ventriculus – also called as “gizzard”. As noted earlier, birds don’t have teeth, but they still have to masticate food before it can pass into the small intestine. It is connected to Proventriculus.
The avian equivalent of teeth
Very muscular.
-Used primarily to grind & break-up food (such as seeds)
- Chicken picks up gravel and small stones in their food, called grit (to help grind the food).
-Lined with a tough, abrasive keratin which assisted by grit and stones deliberately ingested.
Small intestine
 Chief organ of digestion ad absorption
 Receives bile from liver through bile duct and pancreatic juice from the pancreas via duct.
Large intestine
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 Everything looks similar but, little variations.
 Has out-pocketing called caeca
 Aid in digestion of plant material (bacteria in the caeca help enzymatic digest the material).
Cloaca
 Receives waste from the large intestine and material from the urinary and reproductive systems.
 Divided into three sections- a) Coprodaeum-receives waste from the large intestine. b) Urodaeum- receives urine from the kidneys (via the uterus) and sperm and egg from the gonads. c) Protodaeum- stores (temporary) and ejects material; closed posteriorly by the muscular anus.
Why bird faces are white?
Clinical Importance
1.
Mouth
Site for the examination for lesion in viral disease like foot-and-mouth disease.
Lesion normally seen in the form of vesicle at lips, gum, dental pad and tongue.
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2.
Oesophagus
At the base of the mouth oesophagus is located dorsal to trachea and at middle third of the neck oesophagus courses on the left side of the trachea. This location is important from the surgical point of view in choke (oesophageal obstruction) management in animals.
3.
Rumen
Located on the left side of animal body, at paralumber fossa. Any surgical procedures associated with rumen like Trocarisation, rumenotomy, has to be performed on the left side of the animal and not on the right side.
4.
Reticulum
Due to its close location with the heart it may result in traumatic reticulopericarditis due to lodging of sharp objects like nail that may be present in commercial feed.
5.
Abomasum
In pregnant animals abomasums may get displaced either to left or right side or may result in torsion. In any of these cases feed intake and production will be hampered.
6.
Intestine
This is one of the sites where internal parasites like round worms get localized and cause harm to the animal. Apart from this, organ like liver also will be infested by parasite like live fluke
Notes for Diploma (by: Dr. Penjor and Nidup.K).
2) http://training.seer.cancer.gov/module_anatomy/unit10_1_dige_functions.html
3) Test book of Vet. Anatomy (SACK, DYCE, WENSING)
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