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 endLarge 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.