Digestive system

Chapter 9 Digestive system

• The digestive system of the carnivore is described as monogastric. The tract is relatively short, as meat is easy to digest, and the stomach is simple.

• The teeth of the carnivore are sharp, pointed and powerful and the jaw has a scissor-like action. This enables meat to be cut and torn off the bones of prey.

• The majority of the tract is a long tube of varying diameter. Each part has a similar structure but shows functional adaptations.

• Food passes down the tract and is mixed with digestive juices by coordinated muscular movements known as peristalsis and rhythmic segmentation.

• Digestion occurs by the action of enzymes, each of which is specific to a particular food type.

• Digestive juices containing enzymes are secreted by intrinsic exocrine glands in the stomach and small intestine and by extrinsic glands such as the pancreas and gall bladder.

• Digestion results in the production of soluble molecules that can pass through the epithelium of the small intestine into the blood capillaries (amino acids and monosaccharides) or lacteals (fatty acids and glycerol).

• The products of digestion are carried around the body where they are metabolised and used by the tissues.

• Any unwanted or indigestible food remains are excreted in the faeces.

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:

• Ingestion – the process of taking food into the body; this takes place in the mouth

• Digestion – the process of breaking down the food into small chemical units; this occurs in the stomach and small intestine

• Absorption – the process whereby the chemical units pass into the blood and are carried to the liver; this occurs in the small intestine

• Metabolism – the process in which the chemical units are converted into energy for use by all the organs of the body; this takes place mainly in the liver

• Excretion – the removal of any remaining indigestible material.

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

• Oral cavity or mouth – lips, tongue, teeth

• Small intestine – duodenum, jejunum, ileum

• Large intestine – caecum, colon, rectum, anus.

Fig. 9.1 Organs of the digestive system of the dog, left lateral view.

Fig. 9.2 The digestive tract removed from the body.

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

• Salivary glands

• Gall bladder

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:

1 To pick up the food – this is known as prehension and involves the use of the lips and tongue

2 To break up the food into small boluses to aid swallowing – this is known as mastication or chewing and involves the use of the tongue, cheeks and teeth

3 Lubrication of the food with mucus and saliva making it easier to swallow

4 In omnivores and herbivores, digestion of carbohydrates begins in the mouth with the secretion of salivary enzymes. This does not occur in carnivores, e.g. dog and cat, as food is held in the mouth for a very short time before it is swallowed.

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 incisive bone and the maxilla form the upper jaw

• The palatine bone forms the roof of the mouth – the hard palate

• The mandible forms the lower jaw; the paired mandibles join in the midline at the mandibular symphysis.

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:

1 To aid the ingestion of food

2 To carry the receptors (taste buds) for the sensation of taste or gustation

3 To help in the formation of a food bolus ready for swallowing

4 To groom the fur, particularly in cats

5 To assist thermoregulation – the tongue is used to apply saliva to the fur, which smoothes it down into a thinner and cooler layer; in dogs, panting, during which saliva evaporates from the tongue, helps to cool the body

6 To produce vocalisation – production of sound involves complicated movements of the tongue and lips.

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.

In an anaesthetised animal, the sublingual vein, running under the tongue, may be used for intravenous injections or for collecting blood samples, while the sublingual artery may be used for monitoring the pulse rate.

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.

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	Variable; full dentition present by 6 months

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Tags: Introduction to Vetrinary Anatomy and Phisiology

Jul 18, 2016 | Posted by admin in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off

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