Digestive system

Chapter 9 Digestive system

The maintenance of life depends on a constant supply of energy. Food provides energy and the digestive system has evolved to extract it from the nutrients taken into the body and then to excrete the indigestible remains. The digestive process occurs in several stages:

The digestive system (Figs 9.1, 9.2) consists of the following parts:

There are also several accessory glands, without which the digestive process cannot be completed:

The oral cavity

The oral cavity (Fig. 9.3) is also known as the mouth or buccal cavity and contains the tongue, teeth and salivary glands. The function of the oral cavity is as follows:

The oral cavity is formed by the following bones of the skull:

The mandibles articulate with the temporal bones of the skull forming the temporomandibular joint. In carnivores the action of the joint is scissor-like to shear flesh off the bones of their prey.

The upper and lower jaws are linked by skin, forming the cheeks, under which lie the muscles of mastication (see Ch. 4). These muscles lie over the temporomandibular joint and give strength to the biting action. The entrance to the mouth is closed by the lips, composed of muscle covered in skin. The upper lip is split vertically by a division known as the philtrum (see Ch. 8, Fig. 8.1).

The entire oral cavity is lined by a layer of mucous membrane. It is reflected on to the jawbones, forming the gums. The mucous membrane covers the hard palate and extends over a flap of soft tissue at the back of the oral cavity – this is the soft palate, which extends caudally between the oral and nasal cavities and divides the pharynx into the oropharynx and nasopharynx (Fig. 9.3) (see also Ch. 8).

The tongue

The functions of the tongue are:

The tongue lies on the floor of the oral cavity and is made of striated muscle fibres running in all directions. This enables the tongue to make delicate movements. The muscles are attached at the root of the tongue to the hyoid bone (see Ch. 8) and to the sides of the mandibles. The tip of the tongue is unattached and very mobile.

The tongue is covered in mucous membrane. The dorsal surface is thicker and arranged in rough papillae, which assist in control of the food bolus and in grooming. Some papillae are adapted to form taste buds, mainly found at the back of the tongue. Taste buds are well supplied with nerve fibres, which carry information about taste to the forebrain (see Ch. 5). Running along the underside of the tongue are the paired sublingual veins and arteries.

The teeth

The teeth are hard structures embedded in the upper and lower jaw. Each jaw forms a dental arch or dentary – there are four dental arches in total. The teeth pierce the gums to sit in sockets or alveoli. The membrane covering the gums is known as the gingival membrane or periodontal membrane.


All teeth have a basic structure (Fig. 9.4). In the centre of each tooth is a pulp cavity. This contains blood capillaries and nerves, which supply the growing tooth. In young animals the cavity is relatively large but, once the tooth is fully developed, it shrivels and contains only a small blood and nerve supply. After a tooth has stopped growing the only changes occurring will be due to wear.


The teeth of a carnivore are adapted to shearing and tearing the flesh off the bones of their prey. There are four types of tooth, which are classified by their shape and position in the jaw (Fig. 9.5). This is summarised in Table 9.1.

Table 9.1 Tooth types and functions

Type Position and shape Function
Incisor (I) Lie in the incisive bone of the upper jaw and in the mandible of the lower jaw; small, pointed with a single root Fine nibbling and cutting meat; often used for delicate grooming
Canines (C) – ‘eye teeth’ One on each corner of the upper and lower jaws; pointed, with a simple curved shape; single root deeply embedded in the bone Holding prey firmly in the mouth
Premolars (PM) – ‘cheek teeth’ Flatter surface with several points known as cusps or tubercles; usually have two or three roots arranged in a triangular position to give stability in the jawbone Shearing flesh off the bone using a scissor-like action; flattened surface helps to grind up the flesh to facilitate swallowing and digestion
Molars (M) – ‘cheek teeth’ Similar shape to premolars; usually larger with at least three roots Shearing and grinding meat – NB There are no molars in the deciduous dentition
Carnassials Largest teeth in the jaw; similar shape to other cheek teeth; these are the first lower molar and the last upper premolar on each side Very powerful teeth sited close to the angle of the lips; this type is only found in carnivores

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Digestive system

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