Aims and Objectives
Aim
•Explain the anatomy of the horse’s digestive system and the process of digestion.
Objectives
•Visualise the digestive system within the horse’s body
•LABEL the parts of the digestive system from mouth to anus
•Know where and how the food is digested
THE ANATOMY OF THE HORSE’S DIGESTIVE SYSTEM
The process of digestion in the horse
The digestive system is responsible for the
intake (selection, intake and grinding),
and digestion (processing and absorption),
of food. The process is complete when the
nutrients have been absorbed and the
waste is eliminated.
The horse’s digestive system starts at the
mouth and ends at the anus. It is one
continuous length and is extremely long
so it must loop and coil to be able to fit
inside the small amount of space left
over by the large lungs and diaphragm.
Nostril --The horse has an acute sense of smell and will be selective when choosing food
Lips --The digestive process starts at the lips. He used his lips and the whiskers around the muzzle to
select and gather up his food
Incisors -- the incisor teeth grasp and bite off the selected portion
Tongue--The tongue has taste buds which provide the horse with essential information about the
food. The tongue and cheek muscles manipulate the food to the back teeth. The tongue rolls the
bolus to the back of the mouth
Grinding teeth--to the grinding teeth (premolars and molars) where the food is ground up and mixed
with saliva. This chewing action is very important as it is this action that stimulates the production of
saliva
Pharynx – passes through into correct tube see next slide
Oesophagus -- This tube is about 1.5 metres in length. It is a passageway from the mouth to the
stomach and no digestion occurs here
Cardiac sphincter-- is a strong band of muscle called a sphincter muscle. This acts as a one way valve into
the stomach
Stomach -- The stomach is relatively small, about the size of a rugby ball
Pyloric sphincter—the exit from the stomach to the small intestine
Small intestine -- The small intestine can be divided into 3 areas. The duodenum about 1 meter long, the
jejunum about 20-24 metres long and the ileum about 1 – 1.5 meters long , in all 20m – 27 metres long and
has a capacity of 55-70 litres
Caecum--The caecum is a large vat which has a blind end
Large colon-- about 3 – 4 meters long and has a capacity of 90-110 litres. It has to fold into 4 to fit into the
remaining space in the horse’s abdomen
Small colon-- The small colon is roughly 4 meters in length and has a capacity of 16 litres
Rectum--The rectum is a short tube used as a storage area that holds waste
Anus--The anus is another sphincter muscle that empties the rectum when there is a build-up of waste
The Pharynx
The pharynx is the muscular passage between the mouth
and the oesophagus.
In this area air and food pass down different tubes to the
lungs and stomach.
The trachea takes air to the lungs and the oesophagus
takes the bolus of food to the stomach.
These tubes lie next to each other and there are 2 safety
mechanisms to ensure food does not enter the lungs.
The soft palate and the epiglottis provide a moveable
passageway for the food and air to move down the correct
tube. When breathing the soft palate is depressed forming
a constant air way and the epiglottis is open.
When eating and swallowing food the soft palate becomes
raised and opens the passage to the oesophagus. The
epiglottis folds back to cover the opening to the trachea,
preventing food from entering the trachea which could
prove fatal.
The second mechanism is the cough reflex, which can
propel any stray food violently from the trachea.
The Stomach
Once the food has passed through the pharynx it enters the oesophagus. This tube is about 1.5 metres
in length. It is a passageway from the mouth to the stomach and no digestion occurs here.
At the end of the oesophagus there is a strong band of muscle called a sphincter muscle. This acts as a
one way valve into the stomach. The horse cannot vomit and this sphincter muscle prevents food or
gases escaping back into the oesophagus.
The entry to the stomach is via the cardiac sphincter. The stomach is a j shaped sac. It is here that
chemical digestion of the food begins.
The stomach is relatively small, about the size of a rugby ball and can expand to hold roughly 9-18
litres. The horse is a trickle feeder and the stomach is constantly being topped up. Over feeding the
horse can result in food being forced into the small intestine before it has been broken down properly
by the gastric juices, with a risk of colic or other digestive upset. As the horse eats little and often the
stomach is never empty.
The stomach has four distinct regions:
The Stomach
The fact the stomach is never empty is important, as hydrochloric acid is constantly produced, (It is not
triggered by food). If the stomach were empty the acid would attack the lining of the stomach causing
ulcers.
Food can stay in the stomach for between 30 minutes and 3 hours depending on the type of food. The
majority of feed spends about 45 minutes to 1 hour in the stomach. It leaves the stomach as a soupy
liquid called chyme which contains all of the ingested food and acidic gastric juices.
The chyme passes out of the stomach via another sphincter muscle called the pyloric sphincter into the
small intestine. The chime is passed along the small intestine by wave like muscular contractions called
peristalsis.
The Small Intestine
The small intestine can be divided into 3 areas. The duodenum about 1 meter long, the jejunum about
20-24 metres long and the ileum about 1 – 1.5 meters long , in all 20m – 27 metres long and has a
capacity of 55-70 litres.
The duodenum forms an s shaped curve from the stomach which contains ducts from the pancreas and
the liver. These are situated close to the stomach exit and produce pancreatic fluid and bile. The
chyme is turned alkaline by these fluids.
The jejunum and the ileum lie to the left of the horse’s abdomen and are looped and coiled to fit the
space. The small intestine can move quite freely in this space as they are only attached at the stomach
and the caecum.
The small intestine is where the non fibrous food stuff is broken down and absorbed. (The concentrate
portion of the diet.) The inside lining of the small intestine has finger like protuberances which
increase the surface area. Peristalsis forces the chyme to the walls so that breakdown and absorption
are more effective. There are glands which are responsible for producing digestive enzymes.
The Hind Gut
The horse’s natural diet of grass contains large amounts of complex insoluble carbohydrates or fibre
and this is not broken down until the chyme reaches the hind gut.
This consists of the caecum, the large colon and the small colon, the rectum and anus
The caecum is a large vat which has a blind end. The entrance to the caecum lies near the horses right
hip and runs forward and down 0.6-0.9 metres to finish midway along the horses belly at the base of
the abdomen. It contains vast numbers of bacteria which can break down the fibre by a fermentation
process. Energy is released from this fermentation process in the form of volatile fatty acids.
The resulting waste is passed into the large colon. This is about 3 – 4 meters long and has a capacity of
90-110 litres. It has to fold into 4 to fit into the remaining space in the horse’s abdomen. At the pelvic
flexture the large colon narrows and turns back on itself and this can cause problems as it is susceptible
to blockage at this point. There is further absorption of nutrients and water in the large colon. The
food takes 36 to 48 hours to travel the length of the large colon.
The small colon is roughly 4 meters in length and has a capacity of 16 litres.
By the time the food reaches here all of the digestion and absorption of the nutrients has been carried
out and the job of the small intestine is to absorb water. The waste is non-digestible food matter and as
it passes along the small colon it is formed into loose balls.
The rectum is a short tube used as a storage area that holds waste until it is ready to be passed out via
the anus.
The anus is another sphincter muscle that empties the rectum when there is a build-up of waste.