The Gastrointestinal system is also called the digestive or

alimentary tract. This system forms a tube like tract from the

mouth to the anus. The primary functions include: the prehension, transport, and

breakdown of food. It also carries waste materials to be eliminated

Food is chewed in the mouth, swallowed by way of the pharynx and

esophagus, passes through the neck and thorax into the stomach

The food is partially digested before being passed to the small intestine for further digestion and absorption. Here the remaining ingesta moves to the large intestine, where it is retained until it is excreted through the anus.

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In animals that remasticate and regurgitate food, the altered

process allows fermentation of the food by bacteria and protozoa.

These digest cellulose and produce energy for the body.

These animals are called ruminants

The movement of food through the digestive tract is called peristalsis, an involuntary

wavelike motion. Animals are classified according

to their natural dietsCarnivores: eat meat

Omnivores: eat plants and meatHerbivores: eat plants

The Mouthlips

The lips (labia) form the entrance to the mouth. Mucous membranes on the inside of the lips extend to cover

the surfaces of the oral cavity. The lips of sheep, goats and horses

are soft and flexible to aid in picking up food.

Lips of cattle and pigs are stiff and do little more than close the

mouth

Members of the camel family have deeply split upper lips that allow them to

graze close to the ground without disturbing the roots of plants.

Conical papillae (cone shaped projections) found in the inner lips of ruminants prevent food from escaping from the mouth during chewing

Oral CavityThe oral cavity is formed by the arch of the upper and lower jaws and is bounded by the lips and

cheeks. It contains gums (gingivae), teeth (dento, donto),

and the tongue.

The roof of the mouth is called the palate. This is divided into the hard and soft palates. The hard palate is a rigid bony structure covered with a mucous membrane. The soft palate is the partition between the mouth

and nasopharynx. It is formed by a muscular tissue and covered by a

mucous membrane

The oral cavity serves as a receptacle for food and is where food particle size is broken down

Cheeks (bucco)The cheeks are formed by the

buccinator muscles and a subcutaneous fat pad called the

buccal pad. The muscle keep the food between the teeth for

chewing.

TongueThis is composed of skeletal

muscle with fibers pointing in three direction and covered by a mucus membrane. It keeps food between the teeth during chewing

and aids in swallowing by exerting pressure against the hard

palate.

The elevations on the sides and upper surface of the tongue are called papillae. There are three

types categorized by their appearance.

Filiform: threadlikeFungiform: mushroomlike

Vallate: rim shapedThe fungiform and vallate

contain taste buds.

The tongue is also used for grooming and licking and is used as a ladle function to

drink liquids. It is thought that animals determine through

taste whether food is poisonous.

Filiform papillae of the feline tongue

Gingivae (gums)The gums consist of mucous membranes with supporting fibrous tissue. This tissue is

richly vascular but poorly innervated. The gums form a

collar around each tooth.

Teethteeth perform a variety of

functions including cutting and grinding food and as a defense mechanism. Deciduous teeth (baby teeth) are found in most species. These fall out and are replaced by permanent teeth.

Carnivores have brachydont teeth (short crowned) with a

structure similar to human teeth. Each tooth has a crown, neck

and root. The crown is encased in enamel, the hardest substance in the body. The root is encased in cementum. Dentin underlies both enamel and cementum and makes up the bulk of the tooth

Front teeth are called incisors, these teeth are used for shearing cutting and defense. In the dental formula they are designated by the letter I In ruminants, upper

incisors are absent and have been replaced by the dental pad

Canines are also called fangs, eyeteeth or tusks. They are used for tearing and defense. They are

designated by the letter C

The cheek teeth are called premolars, designated by the letter P

and the molars, designated by the letter M. These are used to grind food into pieces easy to swallow.

Dental formulas are written to indicate the number of teeth found

on one side of the mouth. The canine dental formula:I 3/3, C 1/1, P 4/4, M 2/3

                                                                                                                                                                              

The surfaces of the teeth:lingual: next to the tonguebuccal: next to the cheek

labial: next to the lipsocclusal: the chewing or biting

suface

Salivary GlandsIn animals these are composed of 3 pairs of glands and a variety of

saliva secreting glands. The primary salivary glands are the:

parotid, mandibular and sublingual. The dog also has a salivary gland by the eye called

the zygomatic gland

The fluid secreted is called saliva. The function of saliva is to disolve or lubricate food and

in some species, to initiate carbohydrate digestion. The smell, site, or thought of food

can initiate saliva secretion. In some species (ruminants), salivary flow is continuous

The pharynx:This acts as a common passageway for air and food. It closes off the airway while swallowing and opens into the

nasal and mouth cavities.

The esophagus:This is a narrow tube that runs from the

pharynx to the stomach. The lumen of the tube is usually closed but dilates as needed for

the passage of food

Swallowing:There are three phases to the act of

swallowing. The first is voluntary and the others are instinctive. The voluntary phase passes food from the mouth into the pharynx. This is followed by the

reflex phase that blocks all other openings. The third phase takes place in the esophagus where food is propelled

by muscle contraction (peristalsis) through the cardiac sphincter into the

stomach

Non-ruminant stomach:The stomach is divided into

three sections:fundus: rounded section above

the esophageal openingbody: the middle section

pyloris: the lower small end

The ruminant stomach is preceded by three chambers (diverticula) where food

is soaked and digested via microorganisms. Regurgitation and

remastication (also called chewing the cud) of the food assists in fermentation.

The four chambers are: reticulum, rumen, omasum, abomasum. The first three chambers ferment the food which

supplies energy to the animal

The reticulum: (also called the honeycomb)

this is the most cranial chamber. It contains intersecting ridges

that result in a honeycomb appearance.

The rumen: (also called the paunch)This is a large muscular sack that extends from the diaphragm to the

pelvis and fills almost the entire left side of the abdominal cavity and

makes up almost 20% of the animal’s total weight. It is divided

into the ventral and dorsal sac. Both sacs have numerous papillae up to 1

cm long in length

The omasum: This is round and studded with

short blunt papillae. These papillae grind the roughage

before it enters the abomasum for further digestion.

Contractions of the omasum squeeze the fluid out of ingesta

and grind the solids.

The abomasum:This is the true stomach of the

ruminant. It is the first glandular portion of the digestive system that secretes digestive enzymes

to break down the food particles. The abomasum opens into the small intestine via the pylorus.

Ingested food is liquefied by reactions from digestive

enzymes and then passes into the duodenum. The chyme moves

through the jejunum and ileum. The ileum empties into the

colon. The contents contain both water to be absorbed by the large intestine and waste to be

eliminated

The stomach, large and small intestine are contained within the space between the

diapragm and pelvis. The abdomen is lined with a serous membrane called the

peritoneum. The mesentery (a fold of the peritoneum) connects a portion of the

intestines to the dorsal abdominal wall. The visceral peritoneum covers all or part of the organs and helps to keep them in place. The omentum (a double fold of the peritoneum) attaches to the stomach, connecting it to the

abdominal viscera

mesentery

omentum

The small intestines (entero) Occupies a large portion of the

abdominal cavity and are divided into three parts:

*duodenum*jejunum*ileum

DuodenumThis attaches to the pyloric end of the stomach. It receives the pancreatic and common bile

ducts. Digestion and absorption takes place here.

JejunumThis is the middle section held in

place by the mesentery. Vigorous peristaltic waves

rapidly move fluid contents into the ileum.

IleumThis is the longest portion of the small intestine and this is where most of food absorption takes

place

The intestinal digestive juice containing mucus and enzymes is stimulated by a

hormone called secretin. It is produced by the intestinal glands as chyme reaches the small intestine. The digestive process is completed in the small intestine and the digested food is absorbed through the

intestinal walls by villi. Villi are small thread like projections, it is these villi that are sloughed when a dog contracts parvo

virus

The Large IntestineThis is divided into the cecum,

colon, and rectum.

The CecumThis forms a pouch that joins the colon. This structure is larger in herbivores. The

primary function is to break down fibrous material. The

vermiform appendix is a narrow tubelike structure attached to the

cecum.

The ColonThe colon is divided into three parts: the ascending, transverse

and descending.

The Rectumthe section of the descending

colon located within the pelvis. It dilates to store feces until the

expulsion through the anus. The anus is composed of both

smooth and striated muscle.

The PancreasThis is an elongated gland

located near the first part of the duodenum. It it both an exocrine

and endocrine gland. The exocrine cells secrete pancreatic

juice needed for digestion. These juices are collected within the pancreas and transferred to the

small intestine

The islets of Langerhans are groups of endocrine cells that secrete insulin and glucagon. These have opposing roles in

carbohydrate digestion

The Liver (hepat)This is the largest gland in the body and

is classified as exocrine. The liver is soft and pliable and should have a reddish brown color. It’s functions include

*secreting bile*aiding in metabolizing proteins, fats and

carbohydrates*filtering and destroying foreign matter

and neutralizing toxins*storing iron, glycogen and vitamins A,

B12 and D

Gallbladder (cholecyst)The primary function is to store the concentrated bile deposited

by hepatic and cystic ducts. The stored bile is then expelled into the duodenum during digestion.