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.

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.

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:

Production of saliva is continuous but may be increased by such factors as the sight and smell of food (particularly in dogs), fear, pain and irritant gases or other chemicals such as organophosphates. Salivation also often occurs just prior to vomiting and may be a warning sign to the owner!

The function of saliva is:

Pharynx

The pharynx forms a crossover point between the respiratory and digestive systems. It is a muscular tube lined with mucous membrane, connecting the back of the nasal and oral cavities with the oesophagus and the larynx and trachea. The soft palate extends caudally towards the epiglottis of the larynx and divides the pharynx into the nasopharynx and oropharynx (Fig. 9.3; see also Ch.8). The walls of the pharynx contain diffuse areas of lymphoid tissue known as the tonsils. Their function is to protect the animal against disease (see Ch. 7). The most obvious are the palatine tonsils lying one on each side of the pharynx in a shallow recess. The Eustachian or auditory tube connects the pharynx to the middle ear. It enables the air pressure on either side of the tympanic membrane to equalise and thus maintains the flexibility and function of the ear drum (see Ch. 5).

The pharynx conveys food from the mouth into the oesophagus by means of a process known as deglutition or swallowing (Fig. 9.7). This occurs in the following stages:

Fig. 9.7 Function of the pharynx. A Positions of structures during swallowing of food. B Position of structures during nose breathing.

Oesophagus

This is a simple tube that carries food from the pharynx to the stomach (Figs 9.1, 9.2). In the neck, the oesophagus lies dorsal to the trachea and slightly to the left of it. It passes through the thoracic cavity, running within the mediastinum, dorsal to the heart base and between the two lungs. The oesophagus enters the abdominal cavity via the oesophageal hiatus of the diaphragm, which separates the thorax and abdomen.

The walls of the oesophagus are lined with stratified squamous epithelium arranged in longitudinal folds. This protects against damage by food and allows for widthways expansion as the boluses pass down. Within the walls are circular and longitudinal bands of smooth muscle fibres. Contraction of these muscles brings about a series of peristaltic waves that force the food along the tube (Fig. 9.8). Food can pass in the reverse direction – antiperistalsis – seen during vomiting. The average time taken for food to pass down the oesophagus is 15–30 seconds but this depends on the type of food: liquids take a shorter time than dry foods.

Fig. 9.8 Representation of peristalsis. Muscular contraction within the tube forces the food bolus along.

Stomach

The stomach of the dog and cat is described as being simple and digestion is said to be monogastric. The functions of the stomach are:

The stomach is a C-shaped, sac-like organ lying on the left side of the cranial abdomen. Food enters from the oesophagus via the cardiac sphincter and leaves from the pyloric sphincter (Fig. 9.2). The inner curve of the sac is called the lesser curvature and the outer curve the greater curvature. The entire organ is covered in a layer of visceral peritoneum or mesentery, as are all the organs in the abdominal cavity. The mesentery attached to the inner curvature is called the lesser omentum, while that attached to the greater curvature is called the greater omentum. The spleen lies within the layers of the greater omentum. The walls of the stomach are thick and easily distended by food. When empty, the stomach lies under the ribs but when full the stomach may occupy a third of the abdomen.

Structure of the stomach wall

The stomach can be divided into three regions: the cardia, fundus and pylorus. Most of the gastric glands lie within the fundic region of the stomach. The walls of the stomach are lined with mucous membrane, the gastric mucosa. This is thrown into deep longitudinal folds known as rugae which flatten out when the stomach fills with food. Within the mucosa are the gastric pits (Fig. 9.10), consisting of three types of cell responsible for the secretion of gastric juices:

• Goblet cells – found in all parts of the stomach. They secrete mucus to lubricate the food and to protect the stomach wall from damage by digestive enzymes (autodigestion)

• Chief cells – found within the fundus. They secrete pepsinogen, the precursor to the active enzyme pepsin; pepsin breaks down proteins to peptides

• Parietal cells – found within the fundus. They secrete hydrochloric acid (HCl); this creates an acid pH which enables pepsin to work effectively.

Fig. 9.10 A Structure of the stomach. B Enlargement to show a gastric pit.

Food enters the stomach via the cardiac sphincter. Distension of the stomach stimulates the secretion of the hormone gastrin from the stomach walls, which initiates the production of the gastric juices. Two types of muscular movement of the stomach break up and mix the food with the juices:

• Peristalsis – pushes the food through the stomach (Fig. 9.8) and contributes to the rumbling of an empty stomach

• Rhythmic segmentation – breaks up and mixes the food boluses (Fig. 9.11).

Fig. 9.11 Rhythmic segmentation within the gastrointestinal tract serves to break up and mix food boluses.

Small intestine

The small intestine is the major site of enzymic digestion and absorption. It is a long, relatively narrow tube and may be up to 3.5 times the body length. Food passes along the small intestine and is mixed with digestive juices by peristalsis and by rhythmic segmentation. It is divided into three parts, each of which has a similar structure but shows functional adaptations (Figs 9.2, 9.9):

Structure of the intestinal wall

Each part of the small intestine has a similar structure (Fig. 9.12). The epithelial layer is folded into millions of tiny, leaf-shaped folds called villi (sing. villus). Their function is to increase the surface area of the epithelium to maximise the efficiency of the digestive and absorptive processes. Extending from each epithelial cell is a border of tiny microvilli, which form a ‘brush border’ to further increase the surface area. (see Ch. 2, Fig. 2.2)

Fig. 9.12 A Cross-section through an area of intestine to show structure. B Enlargement to show the villi and absorptive epithelium.

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Inside each villus is:

• A network of blood capillaries that convey small molecules from the breakdown of carbohydrates and proteins to the liver via the hepatic portal vein

• A lymphatic capillary known as a lacteal. Each lacteal carries chyle, a milky fluid resulting from fat digestion, from the small intestine to the cisterna chyli in the dorsal abdomen.

Digestion

Food is ingested in the form of proteins (polypeptides), carbohydrates (polysaccharides) and fats (lipids). For food to be a useable source of energy, it must be broken down into soluble molecules that are small enough to be absorbed through the wall of the small intestine and into the bloodstream – this is digestion. In mammals this process occurs by the use of chemicals called enzymes. Each enzyme is designed to act on a specific material or substrate, and not on any other. They are usually named after the substrate on which they act, e.g. lipase acts on lipids (fats); maltase acts on maltose.

There are four sources of digestive juices containing the enzymes: gastric juices, pancreatic juices, bile salts and intestinal juices. Digestion in the carnivore begins in the stomach.

Pancreatic juice

This secretion is produced by the exocrine part of the pancreas and occurs in response to the hormones cholecystokinin and secretin (from the wall of the duodenum) and gastrin (from the stomach wall), and stimuli from the autonomic nervous system (see Ch. 6). It contains:

• Bicarbonate – neutralises the affects of the acid in the chyme, allowing other enzymes to work

• Digestive enzymes – many are inactive precursors, which prevent autodigestion and destruction of the pancreas

• Proteases – act on proteins and include:

Trypsinogen, converted to active trypsin by another enzyme, enterokinase, present in succus entericus

Trypsin, which activates other enzyme precursors. It acts on peptides and other proteins to produce amino acids. A trypsin inhibitor within the pancreas prevents spontaneous conversion of trypsinogen to trypsin and autodigestion

• Lipases – activated by bile salts. They convert fats to fatty acids and glycerol

• Amylases – act on starches, a form of plant carbohydrate, and convert them to maltose.

Large intestine

This is a short tube of a wider diameter than the small intestine. Each part has a similar structure to that of the small intestine. However, in the lumen there are no villi and no digestive glands, but there are more goblet cells. These secrete mucus, which lubricates the faeces as it passes through. The large intestine is divided into:

• Anal sphincter – this marks the end of the digestive tract. It is a muscular ring that controls the passage of faeces out of the body. It has two parts:

Internal anal sphincter: inner ring of smooth muscle; control is involuntary

External anal sphincter: outer ring of striated muscle; control is voluntary

Fig. 9.9 Barium X-ray series illustrating the digestive tract. A The barium (radio-opaque white material) is in the oesophagus after being swallowed. B Barium enters the stomach. C Barium starts to leave the stomach and enter the duodenum. D Most of the barium is now in the small intestine (a small amount remains in the stomach and some has entered the large intestine). E Barium in the colon and rectum applied by enema.

The lumen of the sphincter is constricted and lined with deep longitudinal folds of mucous membrane that stretch to allow the passage of bulky faeces.

The liver

The liver is the largest gland in the body and lies in the cranial abdomen. The cranial aspect is convex and is in contact with the diaphragm. The caudal aspect is concave and is in contact with the stomach, duodenum and right kidney (Fig. 9.1). The liver is deep red in colour as it has a large volume of blood flowing through it. It is divided into several large lobes, in the centre of which is the falciform ligament. This is the remains of fetal blood vessels from the umbilicus and is of little significance in the adult animal (see Ch. 7). The gall bladder lies between the lobes in the centre of the caudal aspect. It stores bile, which pours into the duodenum via the common bile duct.

Histologically, the liver consists of thousands of cells known as hepatocytes (Fig. 9.13). These are responsible for all the many functions of the liver. The hepatocytes are arranged in hexagonal liver lobules surrounded by connective tissue. Running through the lobules between the hepatocytes are minute blood sinusoids carrying blood from the hepatic portal vein. Each hepatocyte is bathed in plasma, which percolates across the lobule to drain into the central vein. The central veins flow into the hepatic vein and so to the caudal vena cava. Thus the products of digestion are carried from the small intestine by the blood to every hepatocyte where they are used for metabolism. The liver tissue receives arterial blood in the hepatic artery.

Fig. 9.13 Structural organisation of the liver.

Between the hepatocytes are tiny channels known as bile canaliculi, into which bile is secreted. The canaliculi form an interconnecting network that eventually drains into the gall bladder. There is no direct connection between the sinusoids and the canaliculi.

The liver has many functions and is essential to normal health.

2 Protein metabolism – the liver has several important roles here:

Formation of plasma proteins: Albumin maintains the balance of fluids in the body; fibrinogen and prothrombin are involved in the blood clotting mechanism; globulins form part of the immune mechanism

Regulation of amino acids: there are 20 amino acids that are essential to form body protein. The liver cells use these amino acids to make new proteins and as the ‘building blocks’ of other organic compounds. Amino acids that are not essential may be converted into more useful ones by a process known as transamination

Production of urea: surplus amino acids cannot be stored in the body and are converted by the liver to ammonia and then to urea. This process is known as deamination. Urea is excreted in the urine.