Body Cavities and the Digestive System

The primary duties of the digestive tract are:

The basic units of the digestive tract responsible for these functions are: Other organs associated with this process (primarily salivary glands, liver, and pancreas) assist in food breakdown by secreting digestive enzymes

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)

Body cavity and mesenteries (pp. 187-189)

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

The primary mesenteries are: The transverse septum develops in fishes, amphibians, and most reptiles and separates the pericardial cavity from the pleuroperitoneal cavity (Fig. 5.35, p. 188)

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:

Generally all these mesenteric derivatives come from the dorsal mesentery. The ventral mesentery usually disappears.

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

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:

Teeth can vary in their permanence, their attachment, and their structural differentiation. Attachment (Fig. 13.10, p. 480):
  • 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
  • Structural differentiation: Examples of heterodont teeth may be seen in the four tooth types of mammals: The numbers and types of teeth by a dental formula I, c, p, m that describes only one side of the mouth: Other terms associated with teeth include (Fig. 13.15, p. 482): Pharynx
    The general structure of the oral cavity is dependent in part on the primary mode of feeding that an animal uses
    An animal may be a filter feeder, a suction feeder, a carnivore or herbivore, and they all will develop specific modifications of their oral cavity

    The oral cavity is bounded:

    The hard palate of some species possesses palatine rugae that act to help hold food in the mouth
    The soft palate ends in the uvula, a fleshy flap which apparently serves no purpose

    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:

    Seven primary openings are present to the pharynx:

    Morphology of the gut wall

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

    The cross-sectional anatomy of the gut wall consists of four main layers (Fig. 13.25, p. 491): Esophagus
    The esophagus is generally short and unmodified, and is controlled by involuntary muscle movements (peristalsis), with the exception of cows and birds that are able to regurgitate food for feeding young (birds) or further digestion (cows)

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

    The stomach serves three functions The stomach assumes different shapes depending on the vertebrate class (Fig. 13.13, p. 495): The movement of food into the stomach is controlled by the action of the cardiac sphincter and the movement of food out of the stomach is controlled by the pyloric sphincter

    Glands associated with the stomach include the:

    Specialized modifications of the stomach found in birds and ruminant animals

    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

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

    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)

    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:

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


    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