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 diets Carnivores: eat meat Omnivores: eat plants and meat Herbivores: eat plants

The Mouth lips 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 Cavity The 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.

Tongue This 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: threadlike Fungiform: mushroomlike Vallate: rim shaped The 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.

Teeth teeth 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 tongue buccal: next to the cheek labial: next to the lips occlusal: the chewing or biting suface

Salivary Glands In 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 opening body : 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

Duodenum This attaches to the pyloric end of the stomach. It receives the pancreatic and common bile ducts. Digestion and absorption takes place here.

Jejunum This is the middle section held in place by the mesentery. Vigorous peristaltic waves rapidly move fluid contents into the ileum.

Ileum This 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 Intestine This is divided into the cecum, colon, and rectum.

The Cecum This 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 Colon The colon is divided into three parts: the ascending, transverse and descending.

The Rectum the 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 Pancreas This 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.