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:

Fig. 9.3 Lateral view of the dog’s head to show the oral cavity/digestive system.

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 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.

Structure

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.

Fig. 9.4 A Shape and features of incisor and canine teeth. B Shape and features of premolar, molar and carnassial teeth.

Function

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.

Fig. 9.5 Skull of an adult dog showing the permanent dentition.

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

Infection of the roots of the upper carnassial teeth is known as a malar abscess. The abscess bursts through the bone and the skin to discharge pus just below the eye.

Dentition

Dogs and cats have two sets of teeth in their lifetime:

• Deciduous dentition – also called the milk or temporary teeth. These are present in the jaw at birth and erupt (push through the gums) as the animal grows. They are usually smaller and whiter than the permanent teeth and fall out as the adult teeth begin to erupt.

• Permanent dentition – also called the adult teeth. These are larger than milk teeth and as the animal ages they show signs of wear. They last a lifetime.

Eruption times vary with the species, the type of tooth and the species of animal and are summarised in Table 9.2.

Table 9.2 Eruption times

Tooth type	Dog	Cat
Deciduous dentition
Incisors	3–4 weeks	Entire dentition starts to erupt at 2 weeks and is complete by 4 weeks
Canines	5 weeks
Premolars	4–8 weeks
Molars	Absent
Permanent dentition
Incisors	3.5–4 months	12 weeks
Canines	5–6 months
Premolars	1st premolars 4–5 months; remainder 5–7 months	Variable; full dentition present by 6 months
Molars	5–7 months

Dental formulae

Each species has a characteristic dental formula and this enables the veterinary surgeon to monitor the numbers of teeth lost as a result of disease or ageing. Each type of tooth is referred to by its initial letter: I is for incisor, etc. The following numbers show how many teeth of each type there are in the upper and lower jaws on one side of the head. The numbers are then multiplied by two to give the total number of teeth in the mouth.

Dog dental formula

Permanent: I3/3, C1/1, PM4/4, M2/3×2 = 42 teeth

Deciduous: I3/3, C1/1, PM3/3×2 = 28 teeth

Cat dental formula

Permanent: I3/3, C1/1, PM3/2, M1/1×2 = 30 teeth

Deciduous: I3/3, C1/1, PM3/2×2 = 26 teeth

Salivary glands

These are paired glands lying around the area of the oral cavity. Their secretions, known collectively as saliva, pour into the cavity via ducts. Saliva contains 99% water and 1% mucus – there are no enzymes in the saliva of the dog and the cat (Fig. 9.6).

Fig. 9.6 Location of the salivary glands in the dog.

The positions of the glands are:

• Zygomatic – close to the eyeball within the orbit

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