Equine Alimentary System - Anatomy & Physiology
Introduction
The typical horse's diet consists of 7-12% protein and 4% fat, the remainder being made up of carbohydrate. A high proportion of the carbohydrate is in the form of starch. A mature horse eats 2-2.5% of it's body weight in dry matter every day, 1.5-1.75% of this should be fibre (hay/haylage). This is to prevent a rapid drop in pH in the large intestine and also to stimulate peristalsis in the gut and prevent build up of gas.
Undigested material spends a long time in the caecum and large intestine being digested by microbial fermentation, mainly cellulose (95% after 65 hours).
In the hindgut of the horse,
- 75-85% of insoluble carbohydrates is digested.
- 15-30% of soluble carbohydrates.
- 30% of protein digestion.
Development
The ascending colon expands cranially (compared to ox, where it expands caudally) around the root of the mesentry on the left side of the mesentry.
Small Intestine
- Arrangement is similar to that of the dog's small intestine, but the position of the jejunum is influenced by the large ascending colon and caecum.
- The jejunum is restricted to the left dorsal part of the abdomen.
- Transit time of food in the small intestine is quite rapid.
- 25% of a liquid marker has reached the caecum within 1.5 hours.
- Most ingesta reaches the large intestine within 3 hours after intake.
- 70-80% of protein is digested.
- 70%+ of starch is digested by enzymes.
Caecum
- Main site of microbial fermentation, followed by the ascending then descending colons.
- On the right side of the abdomen.
- Very large, roughly 1m in length with a 30L capacity.
- Consists of a base, body and apex (blind ending).
- The base lies in the right dorsal part of the abdomen in contact with the abdominal roof.
- Apex lies on the ventral abdominal wall, and terminates at the level of the xiphoid cartilage.
- Exists at the junction with the ileum and colon.
- The caecocolic orifice is where the caecum opens into the ascending colon. This exists as a transverse slit formed by a constriction of the ascending colon.
- The ileum opens into the caecum at the ileal papilla. This is a small projection into the caecum housing the ileal sphincter and venous plexus that, together, control the ileal orifice.
- Taenia are present.
- Taenia are formed by concentration of the longitudinal muscle layer.
- Between the taenia are sacculations, or haustra.
- Haustra appear as folds on the interior surface.
- There are four taenia over the caecum: -dorsal - ventral - lateral -medial.
- The dorsal taenia provides the attachment site for the ileocaecal fold, which joins the caecum to the ileum.
- The lateral taenia provides the attachment site for the caecocolic fold, which joins the caecum to the ascending colon.
- The ventral taenia is free.
- The medial and lateral taenia are where the caecal vessels and lymph nodes are located.
- Ingesta is regularly transported from the ileum to the caecum, this movement can be heard upon auscultation of the right dorsal quadrant of the caudal abdomen.
- Ausculatation of this area is carried out in assesment of colic.
- In the horse, the caecum is responsible for the digestion of complex carbohydrates such as cellulose.
Colon
Ascending colon
- The ascending colon is very large and takes up most of the ventral abdomen.
- It is the shape of a double "U", where one "U" is on top of the other.
- There are four limbs that lie parallel to each other, and three flexures that change these direction of the limbs.
- Sequence of the limbs and flexures of the ascending colon:
- Right Ventral Colon (for those with an RVC bias remember, "the RVC comes first!")
- Passes out of the caecocolic orifice on the right side of the abdomen and continues cranially to the xiphoid region.
- Sternal Flexure
- Passes across the midline from right to left.
- Left Ventral Colon
- Runs caudally on the left ventral abdominal floor.
- Pelvic Flexure
- Turns dorsally just cranial to the pelvic inlet and then runs cranially to the diaphragm.
- Left Dorsal Colon
- Runs cranially, parallel and dorsal to the left ventral colon.
- Diaphragmatic Flexure
- Turns caudally at the diaphragm.
- Right Dorsal Colon
- Continues caudally on the right. It is the shortest limb of the ascending colon.
- Right Ventral Colon (for those with an RVC bias remember, "the RVC comes first!")
- The transverse colon continues on from the right dorsal colon as the right dorsal colon turns medially.
- The right dorsal colon is attached by a mesentry to the dorsal abdominal wall, the base of the caecum, the root of the mesentry and the pancreas.
- This anatomical arrangement of mesentry allows the left ascending colon to twist and is a common cause of colic (colonic torsion).
- The ventral parts of the ascending colon are attached to the dorsal parts by a short mesocolon.
- The mesocolon houses the blood vessels, nerves and lymphatics.
- In the ventral colon many important digestive and absorptive functions take place, whilst the dorsal colon is mainly responsible for transportation of ingesta.
- Taenia are present.
- Different parts of the colon can be distinguished by the number of taenia present:
- The right and left ventral colon and the sternal flexure have four taenia.
- The left dorsal colon and pelvic flexure have one taenia.
- The right dorsal colon and diaphragmatic flexure have three taenia.
- Different parts of the colon can be distinguished by the number of taenia present:
Transverse Colon
- Short
- Passes from across the midline from right to left. It passes cranial to the root of the mesentry
- The transverse colon has two taenia.
- Turns caudally to become the descending colon at the level of the left kidney.
Descending Colon
- Between 2-4m long.
- Suspended by a long mesentry; mesocolon descendens.
- The descending colon has two taenia.
- Between the two taenia are distinct sacculations that house the faecal balls.