The digestive system is basically a tube extending from the mouth to the anus. Its function is to take in food, grind it, digest it and absorb the nutrients, and eliminate the solid waste products that result from the process. Digestion reduces the nutrients in food to compounds which are simple enough to be absorbed by the animal and used for energy and the building of tissues.


The mouth is a cavity that serves to gather food, grind it up into small pieces, and mix it with saliva and mucus to form a slippery ball, called a bolus that can be easily swallowed by the animal. The mouth is lined with a mucous membrane.

The tongue is a muscular organ covered with a mucous membrane which is a layer of specialised epithelial cells. In grazing animals, it is also covered with a layer of small stalk-like structures called papillae which help the animal to grip the blades of grass. The tongue helps in the grinding of food, the formation of the bolus, and in the swallowing of the bolus. The surface of the tongue contains glands and taste buds which play an important part in the selection of food. The taste buds are sensitive to sweet, bitter, sour and salty tastes.


An animal’s teeth play an important part in the biting, tearing and grinding of food. The general diagram of a tooth is shown below:

Figure 1: Diagram of a tooth

Looking at figure 1 and you can see that a large part of the tooth is below the gum. The enamel on the outside of the tooth above the gum gives the tooth its characteristic white colour. Teeth are classified into three groups:

Incisors:These are the sharp, cutting teeth at the front of the mouth.
Canines:The conical, pointed teeth used for ripping. There are four of these situated behind the incisors, which can easily be seen in dogs.
Molars and Premolars:These are blunt and irregular in shape. They are used to grind food into small pieces.

Figure 2: The Digestive Tract of the Horse

Soon after the birth of the animal, temporary or milk teeth grow in the mouth. These are replaced by the permanent teeth as the animal grows. In humans, the milk teeth are replaced from the age of about 8 years, and by the time the child is 13 – 14 all the permanent teeth should have grown. In animals, the age at which the milk teeth are replaced is important because this is used to judge the age of the animal, particularly with cattle, sheep and horses. The condition of the permanent teeth is also used as a guide to age; teeth that are badly worn are a sign of old age in an animal.


The lay-out of the teeth in an animal’s jaws is the same for all species of animal, and this lay-out is described according to a dental formula. To give you some idea, we will look at the teeth of cattle, sheep and pigs, and it is a good idea, when you are working with these animals to have a look at their teeth yourself, and check their age and condition.


Cattle have molars and premolars in both the top and bottom jaws. They have no canines and they have incisor teeth in the bottom jaw only. There are no incisor teeth in the top jaws of any cattle; instead they have what is called a hard pad which is a toughened area of the gum. Cattle do not bite grass in the way that horses do, they wrap their tongues around the grass and tear it off, using the papillae on the tongue to increase their grip.

The diagram below shows the bottom jaw of a mature cow or ox, and you can see that there are 8 incisor teeth at the front, then a gap followed by 3 premolars and 3 molars on each side.

Figure 3: The bottom Jaw of an ox or cow

Figure 4: One side of the top jaw and one side of the bottom jaw of an ox or cow.


Teeth are numbered from the front of the jaw, moving backwards towards the throat and occur in the following order. Incisors, canines, premolars, molars. Dental formulae are based on this order and refer to one side of the mouth only. Looking at the two diagrams above, you can see that the teeth of a mature ox or cow comprise the following:

  • Top Jaw
  • Incisors                             0
  • Canines                             0
  • Premolars                         3
  • Molars                               3
    • Bottom Jaw
  • Incisors                              4
  • Canines                             0
  • Premolars                         3
  • Molars                               3

So that the Dental Formula for Cattle is written like this:

0 0 3 3
4 0 3 3

The top line referring to the top jaw, and the bottom line, the lower jaw.


As we have already said, the age at which the permanent teeth appear, or erupt, can be used to judge the age of the animal, and in fact, is used by the abattoirs to give the age of cattle sent for slaughter. The figures given in the following table are approximate because the times of eruption vary depending on the breed of the animal and the type of feed it has been having. Better nutrition causes faster growth and earlier eruption of the permanent teeth than poor feeding.

 Age (Months) Molars Premolars Age (months) Incisors Common Term
6M 1   Milk Tooth
12 – 15M 2   Milk Tooth
22 – 24M3P 220 – 261st Pair2 Tooth
30 P 127 – 332nd Pair4 Tooth
30 – 36 P 336 – 423rd Pair6 Tooth
   42 – 484th PairFull Mouth

Figure 5: The young ox or heifer at 2 years old would have all its permanent molars, one permanent

(2 Tooth)

premolar, and the front of its mouth would look like this:

Figure 6: At 2½ years old, it would have all its permanent molars, two premolars and the front of its mouth

(4 Tooth)

Figure 7: At 3 years old, it would have all its permanent molars and premolars and the front of its mouth

(4 Tooth)

Figure 8: At 4 years old it would have all its permanent teeth and would be classed as a ‘Full Mouth’.

Full Mouth

Sheep have the same number of permanent teeth as cattle, and their dental formula is the same e.g.

 0 0 3 3

4 0 3 3

As you can see, they have no Incisors in the top jaw, but they do have the same Hard Pad as cattle, and their grazing methods are the same; they tear the grass rather than bite it. Once a sheep has become a ‘Full Mouth’, its teeth wear down quite quickly, and they also begin to fall out, so that the animal has difficulty in feeding and becomes thin and unthrifty. Once they loose their teeth they have to be culled.

The ages at which the permanent teeth erupt in sheep are given in the following table:
 Age (Months) Molars Premolars Age (months) Incisors Common Term
3 – 5M 1   Milk Tooth
9 – 10M 2   Milk Tooth
18 – 24M 3P 112 – 181st Pair2 Tooth
18 – 24 P 218 – 242nd Pair4 Tooth
18 – 24 P 324 – 273rd Pair6 Tooth
18 – 24  33 – 364th PairFull Mouth

Teeth are not so important in pigs, but to give you some idea, the two diagrams below show the upper and lower jaws of a mature pig. Baconer and porker pigs are slaughtered before they reach maturity. Pigs differ from cattle and sheep in that they have Incisors in both jaws, and well developed canines. In boars, the canines grow to form the tusks of the boar, and are used as a weapon in fighting.

Figure 9: A side view of the top and bottom jaw of a pig

The Dental Formula for a mature pig is

3 1 4 3

3 1 4 3