The horses digestive system (picture from www.horse-diseases.com).
I will start at the mouth and work my way along the tract! When horses are eating they chew 73-92 times a minute. They make 800 to 1200 chewing movements per kg of concentrate. The mandibular muscles in the cheek are very thick. The circular motion of the jaw shears food. Particles of hay are reduced to 1.6mm in length. The upper lip is strong to place the forage between the teeth. The tongue is not like a cow's, it moves ingested material to the check teeth for grinding. The pharynx is a muscular membranous sac, it prevents choking during swallowing. The guttural pouch has an unknown meaning but is 300-500 mm in volume.
The horses' teeth (photo from www.thehorsecity.com).
The oesophagus is the first region of the alimentary canal. It is a musculomembranous tube.
The horse’s stomach is quite small and leads into the first section of small intestine- the duodenum. The stomach is a different structure at the top than bottom. There is strong valve called the cardiac sphincter which is what stops them from being sick. 8.15 litres can fit in the stomach (I think this is an average).
The small intestine is 22-25 metres long, there are different regions: - the duodenum, jejunum and ileum. The duodenum is closely positioned to the liver and pancreas. It receives bile from the liver via the bile duct. The rate of transit for digesta going through the small intestine is very rapid. Typically 50% of food going into the stomach of a pony will have reached the distal part of the ileum after 1 hour. It has been estimated that digesta can travel through the small intestine at a rate of about 30 cm/min. Despite the rapid passage rate through the small intestine, there is still considerable absorption. Most of the fat in the diet is absorbed here– largely due to bile. However, there is still absorption of protein and carbohydrate material.
The large intestine is around 7.5-8 metres in length. The width is larger than the small intestine. It can be divided into different sections.
The caecum can be viewed as a large cul-de-sac between the ileum and the colon. Humans have a very small caecum near the appendix, we do not need ours to the extent horses do as we are not herbivores. It can hold about 25-30 litres and is around 125 cm long. It has the form of a comma and is curved. The caecum-colon border is a region of overlap which is difficult to define. There are lots of folds which increase the surface area. The entrance and exits both have muscular valves to stop back flow. The main function of the caecum is to break down complex carbohydrates. This is done in association with microbes present in the organ as horses don’t have the enzymes required. Cellulose is very difficult to digest and the microbes help with the process.
The colon is split into the large and small colon. The passage rate of digestion is quick as within 3 hours of eating, it will be in the colon. Much of the unabsorbed material spends a great deal of time in the intestine of the horse. There are 100 million microbes per mm there. The passage rate also depends on the diet, pellet diets pass faster and fresh grass is faster than hay.
The liver is the largest gland in the body. It has lobes and produces bile. The hepatic duct is 5cm long and goes through the duodenum. The horse doesn't have a gall bladder so the liver is very important in the horse.
The horses digestive system is a long and complex tract. It is understandable how horses can get colic when they have such a long intestine.