The primary duties of the digestive tract are:

• to ingest food and undergo chewing and primary processing of food
• swallowing (deglutition)
• further digestion and breakdown of the food items where nutrients and water obtained from the food are then absorbed and unprocessed food is eliminated by egestion

• the buccal cavity - teeth, tongue, and palate
• the pharynx and esophagus
• stomach, small intestine, large intestine and external opening - cloaca (vent) or anal opening

The formation of the digestive system begins early in development with the formation of the archenteron, from which most of the digestive system is derived (Fig. 13.2, p. 473)

• the archenteron is continuous with the yolk sac, which will feed many vertebrates during development, or will become a more minor part of the body and instead the developing embryo will receive nutrients through the umbilical cord
• the archenteron is composed of endoderm, and lines the digestive tract and organs
• the remainder of the digestive system is composed of either endoderm derivatives or derivatives of the lateral plate mesoderm
• differentiation of the archenteron leads to the development of three different regions of the gut:

• the foregut - extends toward the head and merges with an ectodermal pocket (stomodeum) to form the buccal cavity
• the midgut - remains connected to the yolk sac during development
• the hindgut - extends toward the tail and merges with an ectodermal pocket (proctodeum) to form the adult cloaca or digestive outlet

In higher forms of animals, most of the body organs are not embedded in solid tissue

• coelomic cavities are formed to contain the visceral organs
• the coelom is lined by epithelium (serosa) and the organs within the body cavity are anchored by mesenteries, or thin membranes that connect the organs to each other and to the body wall

• the dorsal and ventral mesenteries which suspend the gut within the coelom
• the falciform ligament holds the liver to the ventral body wall
• the lesser omentum connects the stomach and intestine to the liver
• greater omentum (also called the lace apron) is a folded membrane that acts as a storage organ for fat deposits
• the mesogaster extends from the stomach to the ventral body wall
• the reproductive organs are connected by either a mesorchium (for the testes) or the mesovarium (for the ovaries)

A coelomic fold or pleuroperitoneal membrane grows ventrally and fuses with the transverse septum to form the diaphragm in mammals, which divides the coelom into the thoracic cavity and the abdominal cavity - the thoracic cavity is further divided into:

• the pleural cavity surrounds the lungs, with the lungs themselves separated by the mediastinum
• the pericardial cavity surrounds the heart, and the membrane surrounding it the pericardial sac

The Mouth and Oral Cavity

The primary responsibility of the oral cavity is acquisition of food and initial processing

The oral cavity is formed by an invagination at the anterior part of the embryo, called the stomodeum that differentiates and eventually becomes connected to the archenteron or gut tube

The Tongue
The tongue contributes to feeding behavior by either conducting water through the mouth (as is the case in fishes) or by actively manipulating the prey within the mouth

In fishes the tongue is a primary tongue that is not muscular and is used mainly for channeling water. For some species, such as lampreys and other parasitic fishes, the tongue has bony plates that act like teeth for rasping and obtaining a blood meal.

In tetrapods the tongue is used for manipulation of food in the absence of water. It is derived from the lingual swellings and is anchored by the hyoid and the mandibular arches. The muscular elements contributing to tongue movement include the glossal muscles.

Specialized adaptations of the vertebrate tongue include a sticky end to assist in prey capture in amphibians and an extremely long tongue and elaborate hyoid apparatus in woodpeckers that feed in holes drilled into wood.

The tongue also has gustatory or taste receptors in mammals, or it can be associated with heat receptors in the mouth that are used for detecting prey

Oral glands
Oral glands are generally absent in most fishes, primarily because the aquatic environment assists in moving food through the pharynx and into the esophagus

Tetrapods that are terrestrial feeders require oral glands that secrete enzyme-containing saliva that lubricate food and facilitate swallowing

Seven primary glands present in most tetrapods are named according to position (Fig. 13.37, p. 500). They include the following glands: labial, lingual, palatine, nasal, maxillary, parotid (which are largest in herbivores) and mandibular (that are largest in carnivores)

In some species that secrete hemolytic or neurotoxic poisons, specialized poison glands develop, and are closely associated with fangs for delivery of the toxins

Teeth
Development of teeth is similar to the development of scales in that they evolve from epidermal eruptions in the skin of the jaws
The basic structure of a tooth consists of three main regions:

• enamel - the surface layer of the tooth that is hard and protective
• dentine - makes up the bulk of the tooth
• pulp cavity - contains the blood vessels and nerves that feed and innervate the tooth

• Polyphyodont - continuous succession of teeth throughout life (shark)
• Diphyodont - replacement of milk or deciduous teeth by permanent teeth (mammals)
• Monophyodont - single set of teeth retained throughout life (whales, marsupials)

Acrodont - simplest teeth that have no roots and may break off easily from jaw (fish and amphibians)

Pleurodont - teeth attached by one side to the inner surface of the jaw bone (lizards)

Thecodont - teeth set into sockets and relatively immobile

• Homodont - teeth essentially all alike
• Heterodont - teeth differentiated into a variety of uses

• Incisors - most anterior teeth, adapted for securing food, grooming, or nibbling
• Canines - next posterior, spikelike teeth that are used for holding, piercing and ripping
• Premolars - behind the canines, used for grinding
• Molars - most posterior, used for crushing, with a greater surface area than premolars

• Human 2/2, 1/1, 2/2, 3/3 = 16 x 2 sides = 32 teeth (omnivore)
• Cat 3/3, 1/1, 3/2, 1/1 = 15 x 2 sides = 30 teeth (carnivore)
• Deer 0/3, 0/1, 3/3, 3/3 = 16 x 2 sides = 32 teeth (herbivore)

• Diastema - space that occurs between incisors and premolars
• Hypsodont - teeth with high crowns
• Brachyodont - teeth with low crowns
• Bunodont - grinding surface slightly raised into separate rounded tubercles and entirely covered in enamel
• Lophodont - cusps of the teeth drawn out into ridges
• Tusks - excessively developed teeth, either incisors (elephants) or canines (walrus)
• Fangs - associated with poison glands, excessively developed canines
• Egg tooth - largely dentine, found in birds and reptiles and used as a means of hatching from a cleidoic egg - not a true tooth

The oral cavity is bounded:

• laterally by the lips and cheeks
• cranially by the hard palate (composed of the palatine bones) and soft palate
• caudally by the jaw and associated musculature

The pharynx or throat follows from the oral cavity, and is continuous to the esophagus

In fishes, both food and air enters the mouth and empty into the pharynx and the nasal pits are not continuous with the mouth.

In amphibians, crossing of food and air occurs at the pharyngeal chiasma, such that amphibians need not open their mouth to obtain air

In other tetrapods the pharynx opens into separate pouches or regions composed of:

• the nasopharynx - part of pharynx used for respiratory function into which the Eustachian tube from the ear empties
• the oropharynx - the part of the pharynx used in digestion
• the laryngopharynx - the part of the pharynx used in speech

• Choanae (2) - also called posterior nares, that are continuous with external nares
• Eustachian tubes (2) - extend from the auditory canal
• Isthmus fauces (1) - used in conducting digestive enzymes
• Glottis (1) - opening to the trachea and lungs. Can be covered by cartilagenous epiglottis during swallowing
• Esophagus (1) - opening to the rest of the digestive tract

Morphology of the gut wall

The postpharyngeal digestive tract is lined by epithelium derived from endoderm
This epithelium will be of two types:

• squamous epithelium lines the oral cavity, pharynx, esophagus, and cloaca
• columnar epithelium lines the stomach and intestine

• mucosa - contains columnar epithelium, lymphocyte nodules (act in immune function), and villi that increase the surface area available for absorption and protect the crypts of Lieberkuhn and the goblet cells that secrete mucus that lubricates the lining of the intestine and prevents from self-digestion.
• submucosa - vascular tissue that helps to transport nutrients into the primary circulatory system
• muscularis externa - circular and longitudinal sheets of smooth muscles that work to move food along the gut tube to the next part of the digestive process
• adventitia - a surface layer of fibrous connective tissue that envelops the serosa when combined with a mesentary

In birds the crop develops as a blind outgrowth of the esophagus with several functions:

• the site of food storage
• in some birds it may contain digestive enzymes for processing foods high in cellulose or for secretion of a milky food-like substance called "crop milk" in pigeons (stimulated by the secretion of hormones similar to those used in mammalian milk secretion)

• acts to store food prior to digestion
• processes food mechanically by strong muscular movements
• chemically modifies food through secretion of digestive enzymes.

• in the fishes, the stomach is spindle-shaped and elongate
• in higher vertebrates the stomach becomes more sac-like and assumes a more transverse position in the body cavity

Glands associated with the stomach include the:

• gastric glands that secrete hydrochloric acid and pepsinogen (that forms pepsin)
• other mammals secrete rennin that breaks down milk protein, lipase, that breaks down fats, and chitinase, that breaks down chitin (found in the exoskeletons of insects and crustaceans).

• birds have the stomach divided into two sections (Fig. 13.33, p. 497):
• the proventriculus is glandular and secretes digestive enzymes
• the gizzard is muscular and processes food mechanically due to lack of mechanical processing by teeth
• ruminant mammals have a four-part stomach (Fig. 13.34, p. 498):
• the rumen  acts as a storage pouch
• the reticulum or honeycombed stomach contains colonies of anaerobic bacteria that break down cellulose
• the omasum is a highly folded part of stomach that is responsible for processing proteins
• the abomasum represents the true stomach.

Intestine
The intestine is the primary site for absorption and digestion. Assisting in the digestive process are enzymes released by the pancreas and liver.

Small intestine
Digested organic materials and water are absorbed from the intestinal lumen (interior space) and into the circulatory system of the small intestine

• is characterized by having villi which cover the intestinal lining and increase the surface area available for absorption, and is highly vascularized
• is usually of a smaller diameter than the large intestine
• is divided into three regions in birds and mammals (Fig. 13.26, p. 491):
• the duodenum is the site of much of the digestion by enzymes secreted by the liver and pancreas and is where most of the absorption occurs
• the jejunum and ilium are best delineated in mammals on the basis of histological features of the mucosal wall
• in some species of fish, the small intestine has between four to over a hundred pyloric caeca (blind sac) that evaginate from the intestine and also increase surface area of the intestine

Large intestine
Following processing in the small intestine, undigested food is passed into the large intestine

• generally lacks the mucosal villi associated with the small intestine
• serves as the site of water reabsorption and storage of undigested food prior to elimination
• the large intestine may also have an intestinal caecum in some lizards and mammals to harbor colonies of cellulose-digesting bacteria
• mammals may also have a vermiform appendix that may act in immune response of the body but most often becomes problematic if infected
• in mammals and birds the large intestine is the colon, with its dorsal part connects to the rectum through which wastes pass

Cloaca
Unlike mammals, with their wastes passing through the colon and out into the body through a separate exit, most vertebrates have a cloaca (Latin for sewer)

• opens to the body surface through the cloacal aperture
• is the common exit pathway for products of the digestive, excretory and reproductive systems.

Liver and gallbladder
The liver is the largest organ of the body and is composed of several lobes arranged around a central vein, a derivative of the hepatic vein
The functions of the liver are strongly linked to both the digestive as well as to the circulatory system:

• produces bile that emulsifies fat and facilitates its absorption
• bile is stored in the gallbladder after its secretion by the bile canaliculi, and then discharged into the small intestine through the cystic duct
• involved in detoxification of noxious substances (which is why, in people who drink too much, the liver becomes diseased due to it being overtaxed)
• the site of storage of excess food in the form of glycogen or lipid, which is released under hormonal influence into the blood as it is needed
• the site of cholesterol synthesis and iron metabolism - where the iron part of hemoglobin is salvaged

Pancreas
All vertebrates have a pancreas which functions as both an exocrine and endocrine organ

• the exocrine part of the pancreas is the acinar gland, which produces digestive enzymes into the intestine.
• the endocrine portion of the pancreas is the pancreatic islets of Langerhans, which has two types of cells which function in maintaining glucose level in the blood:
• alpha cells secreting glucagon to increase blood sugar by breaking down glycogen
• beta cells secrete insulin to decrease blood sugar by transporting it into the cells and by promoting conversion of blood sugar into glycogen

Definitions
Acrodont - simplest teeth that have no roots and may break off easily from jaw
Bunodont - grinding surface slightly raised into separate rounded tubercles and entirely covered in enamel
Diastema - space that occurs between incisors and premolars of carnivores
Diphyodont - replacement of milk or deciduous teeth by permanent teeth
Falciform ligament - holds the liver to the ventral body wall
Greater omentum - folded membrane that acts as a storage organ for fat deposits
Lesser omentum - connects the stomach and intestine to the liver
Mediastinum - area between the two pleural cavities of mammals that contains the pericardial cavity, thymus, bronchi and major blood vessels
Mesogaster - membrane that extends from the stomach to the ventral body wall
Mesorchium - membrane that connects the testes to the body wall
Mesovarium - membrane that connects the ovaries to the body wall
Monophyodont - single set of teeth retained throughout life
Pericardial cavity - cavity surrounding the heart
Pleural cavity - cavity surrounding the lungs
Pleurodont - teeth attached by one side to the inner surface of the jaw bone
Polyphyodont - continuous succession of teeth throughout life
Rugae - fleshy folds that help to hold food in the mouth
Stomodaeum - invagination at the anterior end of the embryo that forms the oral cavity and connects to the posterior portion of the digestive tract
Thecodont - teeth set into sockets and relatively immobile
Transverse septum - membrane that develops between the liver and the heart