Anatomy of the Equine Intestinal Tract

Anatomy of the Equine Intestinal Tract

In a very broad sense the equine intestinal tract can be divided into large sections based on its overall function.

Photo: Photos.com

In a very broad sense the equine intestinal tract can be divided into large sections based on its overall function.

These sections are analogous to the same segments that exist in most mammals. They include the stomach, small intestine, the large intestine, and the small colon. The stomach is a large sac that liquefies the feed that is ingested by the horse. Only a small amount of digestion occurs in the stomach. No nutrients are absorbed through the stomach. Acid from the stomach helps to break down some feed particles, and an enzyme known as pepsin begins protein digestion.

True digestion only begins in the small intestine that receives this liquefied feed material from the stomach. With assistance from the enzymes secreted by the pancreas into the small intestine, the small intestine is the primary site for digestion and absorption of sugar and starch (a complex sugar in plants), protein (that has been initially digested in the stomach), and fat. The small intestine is also the site for absorption of fat-soluble vitamins (A, D, E, and K), calcium, and phosphorous. The next segment, the large intestine, begins with the cecum and ends with the descending colon.

The large intestine in the horse works like a large fermentation vat in which tremendous numbers of bacteria and protozoa live to facilitate further digestion of plant fiber by their production of enzymes that are capable of breaking down this component of the equine diet (the horse itself does not have these enzymes). This fiber breakdown produces substances called “volatile fatty acids” that can then be absorbed and used by the horse for energy.

A second important function of the large intestine is water absorption. This function occurs very efficiently such that by the final step in the small colon, the waste material not used by the horse is formed into fecal balls. These are subsequently passed into the rectum for evacuation through the anus.

As an herbivore the horse is “designed” to graze and, therefore, must be equipped with a capacity to extract nutrition from grass and other forage. This process requires an area of the intestinal tract where the forage can ferment to release absorbable and usable forms of energy (the volatile fatty acids).

The horse, a “hindgut” fermenter, differs from “foregut” fermenters such as ruminants (cattle, sheep, goats). In horses fermentation occurs primarily in specialized areas of the lower intestinal tract: the cecum and large intestine. Ruminants perform this fermentation in the rumen, the largest compartment of a four-chambered stomach “system.”

The horse’s intestinal tract begins with the mouth and esophagus. The esophagus of the adult horse is approximately 1 1/4 to 1 1/2 meters in length. As in humans, it serves to pass food and water to the stomach through muscular contractions of the esophageal wall. The esophagus opens into the stomach at the esophageal sphincter. The size of the equine stomach, a “j-shaped” organ, varies but generally holds between 8 and 15 liters, depending partly on the size of the horse.

 Two distinctly different types of mucosa (intestinal lining) exist within the stomach. One is continuous with the esophageal sphincter. This mucosa is termed the “non-glandular”and “squamous”portion because it does not have any stomach glands that secrete acid,mucus, or digestive enzymes. This “non-glandular” portion of the stomach extends about halfway into the stomach where it meets the second type of stomach lining called the “glandular” portion of the stomach.

When someone is looking in the stomach with an endoscope, this junction, called the margo plicatus, is seen as a distinct line midway into the stomach. From this junction the glandular portion of the stomach extends to the pylorus. The “glandular” harbors the glands that produce and release stomach acid and the protein-digesting enzyme pepsin. The pylorus of the stomach is the very farthest portion of the stomach from the mouth.

It terminates in a muscular sphincter, the pyloric sphincter, which leads into the first portion of the small intestine. The small intestine is generally about 22 meters (60-65 feet) long. It is composed of three distinct sections. The duodenum is the first segment and is only about a meter long. At about 12 to 15 centimeters from the pyloric sphincter, the pancreatic duct and bile duct empty into the duodenum (just beyond the location of the pyloric sphincter).

The next and largest segment of the small intestine is the jejunum. It is highly mobile and exists in several coils primarily within the top portion (toward the spine) of the left half of the abdomen. The last part of the small intestine is about a meter long and is called the ileum. The small intestine does not hold any significant volume since the feedstuff travels through relatively quickly. If it is “holding” feed, it is probably abnormal. The mesentery (ligamentous attachment of the intestine to the body wall) is connected to the top of the abdomen (toward the spine) near the first and second lumbar vertebrae at the site known as the “root of the mesentery.”

Within the root of the mesentery exists the large cranial mesenteric artery. The mesentery is wide and fan-shaped and carries numerous vessels and nerves to the intestines. It is attached to the small intestine along its entire length, but because of its fan shape and singular attachment, it is highly mobile and permits the small intestine to move freely in the abdomen.

Embedded within the mesentery are lymph nodes and fat. From the ileum (last part of the small intestine) arises the large structure known as the cecum, which is comma shaped and averages about 1 1/4 meters in length with a potential volume of 20 liters or more.

Ruminants and even people have a cecum (in the human it is the appendix).However, the function and size are greatly expanded in the horse. The cecum, in horses, is a large, blunt-ended structure that forms sort of a T with the small intestine (the ileum) and large intestine (large colon). The cecum is the stem of the T, and the small and large intestine connect to it but not to each other.

There are two different entrances to the cecum, one from the small intestine and one from the colon. The cecum is blind ended and extends away from the “connections” (orifaces) to the small intestine and large colon. The entrance of the ileum into the cecum is termed the ileocecal orifice (one branch of the “top” of the “T”) and about 5 centimeters from the entrance of the ileum into the cecum is the exit of the large colon from the cecum, the cecocolic orifice (the second branch of the top of the “T”).

Normally, the ileum exists on the “underside” (or belly) of the horse. The apex or tip of the cecum lies on the abdominal floor just to the right of midline and about a hand’s length back from the tip of the horse’s sternum. As its name implies, the large intestine is larger than the other parts of the intestinal tract. The large intestine, from the termination of the ileum to the anus of the horse, is about 7.5 to 8 meters in length. The terms “large intestine” and “large colon” are often used interchangeably. However, the large colon begins at the cecocolic orifice and extends about 3 to 3.7 meters to the transverse colon.

The large colon exists in various diameters. About 5 to 7.5 centimeters in diameter near the cecocolic orifice, it expands to 20 to 25 centimeters on the floor of the abdomen and reaches a diameter of approximately 8 centimeters at the pelvic flexure, or turn.

After this flexure it travels forward toward the head where it turns again at the diaphragmatic flexure and expands to nearly 50 centimeters in diameter. This segment is followed by the next section of large intestine, the transverse colon. The large intestine can really be described as the cecum, the large colon, the transverse colon, and small colon.

As already suggested, the entire intestinal system cannot exist within the abdomen in full extension. Therefore, it must be folded on itself to fit. The major abdominal turns in the horse are the sternal flexure, where the right ventral (on the floor of the abdomen) colon changes direction and turns left toward the tail. Then the left ventral colon changes direction and passes back toward the tail from the sternal flexure to turn dorsally (toward the spine) at the pelvic flexure.

From there the left dorsal colon (on top of the ventral colon nearer to the spine) courses back toward the head of the diaphragm, turns right at the diaphragmatic flexure (situated above the sternal flexure), and gives rise to the right dorsal colon (on top of the right ventral colon nearer to the spine). The right dorsal colon then courses back again toward the tail and turns left toward the middle of the abdomen to become the shorter and narrower transverse colon. The transverse colon joins the small colon just below the left kidney.

The small colon is about 3.5 meters in length and begins at the termination of the shorter transverse colon. Small colon diameter ranges from 7.5 to 10 centimeters.

The small colon is followed by the final segment, the rectum. The rectum is about 30 centimeters in length before it exits the body by the anus. The large intestine has numerous tissue bands that can often be felt on rectal examination. These tissue bands can serve as a guide in identifying what piece of intestine is being felt during rectal examination. The cecum and right and left ventral colon are segments that have four bands. The left dorsal colon has one band. The right dorsal colon has three bands, and the small colon has two bands. The small intestine does not have these soft tissue bands.

Other abdominal cavity contents are similar to those found in other mammals. These include the bladder and associated ureters, kidneys, spleen, liver, pancreas, major vessels, etc. Not all of these structures can be identified on rectal examination, but many may be visualized by ultrasound examination.

About the Author

Bradford G. Bentz, VMD, MS, Dipl. ACVIM, ABVP (equine)

Brad Bentz, VMD, Dipl. ACVIM, ABVP, ACVECC, owns Bluegrass Equine Performance and Internal Medicine in Lexington, Ky., where he specializes in advanced internal medicine and critical care focused on helping equine patients recuperate at home. He’s authored numerous books, articles, and papers about horse health and currently serves as commission veterinarian for the Kentucky State Racing Commission.

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