(I've always been fascinated by all things equine, including their anatomy and how their various body systems work. As a project to learn more about these amazing animals, I've decided to do a series of posts on the equine digestive system. As these come along, they'll be added to the sidebar. Please keep in mind that I am neither a vet nor an expert on these topics - if you have corrections and additions or resources to direct us to, please put that in the comments. Also, you may find it helpful to visit this article, which has excellent pictures of the digestive system and the location of the various organs.)
After the food exits the stomach, it enters the small intestine through the pyloric valve. The best way to think about the horse's digestive system as a whole is that the foregut - mouth, esophagus, stomach and small intestine - is "rapid transit", and the hindgut - caecum, large colon, small colon and rectum - is "slow boat". The small intestine is very long - up to 70 feet with a capacity of about 48 quarts - and the food moves through it rapidly - in 30 minutes to an hour. The small intestine has three parts - the duodenum, jejunum and ileum. Due to its length, it is intricately folded within the abdomen - see in particular figures 7 and 8 in the article linked in the previous paragraph.
It is worth noting that in horses the foregut constitutes only about 40% of the digestive capacity of the total digestive system by volume, whereas in ruminants, the foregut, including the rumen, constitutes 70% of the digestive capacity. Due to the rapid transit and small capacity of the equine foregut, it is easy for its capacity to be overwhelmed by infrequent large meals or by meals with too high a content of non-structural carbohydrates - undigested food and easily-digested carbohydrates are then passed into the hindgut with often undesirable consequences.
The small intestine is a major organ of digestion. In the first part of the small intestine, the duodenum, bile and salts from the liver (an organ that has over 50 functions including the production of bile) together with pancreatic enzymes are added to the food and major digestion of carbohydrates and proteins as well as emulsification of fats occurs. Unlike humans, horses do not have a gall bladder for the storage of bile, rather bile enters the small intestine directly and continuously from the liver. Bile also has an important function of buffering the highly acid contents of the stomach. (It is amazing, although it really shouldn't be surprising, how many features of the equine digestive system work much more effectively with a diet of small, frequent to continuous meals that are relatively low in non-structural carbohydrates - e.g., forage - and how easily this system can be disrupted by large, infrequent meals or meals that have too many easily-digested carbohydrates. For more on the topic of equine nutrition in light of this, please visit safergrass.org.)
Although they aren't digestive organs per se, both the liver and pancreas are essential to digestion. The pancreas is an amazing organ and produces insulin as well as digestive enzymes - insulin resistance in horses is a topic beyond the scope of these posts but is often induced/related to diet, much as it is in humans.
The small intestine is the place in the digestive system where most of the soluble (non-structural) carbohydrates, together with virtually all amino acids from protein breakdown, are absorbed. It is also a conveyor passing along the less-easily digested structural carbohydrates, including cellulose, to the hind gut.
Due to its relatively small diameter, relative lack of expandability and great length, and the way it lies folded within and is attached to the abdominal walls, the small intestine, particularly the second section, the jejunum, is subject to a variety of problems, including twisting, displacement or entrapment.
Next up is that truly amazing equine organ, the cecum.