The Equine Lower Gastrointestinal System - Its Anatomy and Vulnerability

By Stephen Champion, BSc (Hons) BVM&S MRCVS,
Veterinary Surgeon, CHAMPIONVET.com
Written for Equi-Ads Magazine, January 2002

Should your horse be unfortunate enough to suffer from a dreaded bout of colic, the chances are that the problem will lie in the lower gastrointestinal system. For the purposes of this article, this is the part of the digestive system is from the stomach to the anus. To understand from where the pain of colic is originating, it is important to understand and reflect upon the anatomy and topography (where it lies in a space) of the gastrointestinal (GI) system. Due to its enormous length, the anatomy is complicated – but I shall endeavour to make it as simple as possible!

Except in advanced pregnancy (when the uterus dominates the potential space), the equine abdomen is filled mostly by the large intestine. This is where the roughage in the horse’s diet, (the plant cell walls from the forage), is broken down by microbes into useful energy. We shall describe this later, but will firstly tackle the upper parts of the GI tract.

Food reaches the stomach via the oesophagus, or food pipe, from the mouth. It enters the stomach through an opening called the cardia, which is well developed and bendy and responsible for the horse’s reputation not to be able to vomit. Not true – horses can vomit, although it is indeed rare.

The equine stomach is remarkable due to its small size in relation to the size of the horse, with a capacity of only 5 to 15 litres, depending on the size of the animal. It lies mainly in the left half of the abdomen. The left side of the stomach comprises the fundus and body, which run into the pyloris towards the right side, which in turn joins the intestines. The former two parts are mainly responsible for the grinding of food, whereas the latter is the glandular part and secretes acid to help digest food. The stomach is entirely protected by the rib cage and so it is almost impossible to see if the stomach is distended. Extreme distension can be seen on the left side of the horse and is associated with very serious consequences. Your vet may pass a stomach tube to see if the stomach is distended; to relieve the distension; and to see what type of matter or fluid is contained within the stomach. It is not uncommon for the stomach to rupture when grossly distended, which is entirely incompatible with life and the horse will usually be immediately euthanased.

Food leaves the stomach into the small intestines, which measure 25 metres in length (about the length of a standard public swimming bath!). The first part of its length is called the duodenum, which runs into the jejunum, which in turn joins the ileum. The small intestines have a smaller calibre than the other intestines and are responsible for a large part of the digestion of the food. The gall bladder and pancreas discharge their products into the duodenum via the bile duct and pancreatic duct respectively, which facilitate the digestive process. The walls of the intestines are fed with blood for their function from blood vessels contained within connective tissue called mesentary. Colics associated with bacterial diarrhoea and anterior enteritis originate from this part of the GI system.

The ileum, the last part of the small intestine, joins the large intestine at the bottom of the caecum. This is a massive organ, shaped like an enormous comma and is situated in the right side of the abdomen. It has a volume of up to 30 litres and may measure up to a metre between extremities! It acts as an enormous fermentation drum, rather like a cow’s rumen or the fermentation bin of a home brew beer kit! Large amounts of gas can be produced, which your vet may listen for through the right flank. Caecal dilatation (enlargement through gas accumulation) or torsion (twisting) can be potentially life-threatening.

The large intestine generally is characterised not just by its enormous size, but also by its divisions into a sacculated form, called haustra. These are constantly modified, propelling and churning food in a process called haustral flow.

The base, or top part, of the caecum, emits the start of the colon. The colon consists of ascending and transverse parts, together called the large colon, and the descending part, also called the small colon. The ascending colon consists of four parallel limbs, separated by three flexures, or bends. Remembering the sequence of these structures has been the blight of many veterinary students preparing for their exams! However, their endeavours are not in vain, as these bends are susceptible to impaction by food material, which can be the cause of mild to moderate bouts of colic, often lasting several days. Your vet may palpate the colon of your horse through the wall of his rectum, by performing a rectal examination. Most commonly the pelvic flexure will be impacted, and liquid paraffin or other lubricants may be prescribed to help the impaction pass through.

The ascending colon, in its various parts, joins the transverse colon, which in turn joins the small or descending colon. This is several metres long, although thinner than the other parts, and “floats” within a conventional mesentery. It lies mainly within the left part of the abdomen, forms the typical faecal balls into their characteristic shape, and ends in the rectum, which is located within the bony structure of the pelvis. The indigestible portion of the horse’s food material is reunited with the world via the anus.

The lower digestive tract of the horse is vast, diverse in form and function and has the task of digesting a food source very high in fibre. Because of this, it is prone to disease and mishap. An understanding of the anatomy of your horse’s lower digestive tract is essential when formulating a ration, and when dealing with cases of colic and other gastrointestinal disease.

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