Anatomy of the Equine Abdomen

Understanding the anatomy of the equine abdomen is crucial for diagnosing and treating colic (abdominal pain) in horses.
Colic affects up to 10% of horses, requiring veterinary expertise in abdominal structures.

Knowledge of abdominal muscle anatomy aids in diagnosing and treating abdominal pain (colic).
The abdominal wall consists of several muscles covered by a yellow, shiny abdominal tunic, which is deep fascia in nature.

Located:
- Caudal and ventral ends of the cutaneous trunci.
- Dorsal to the stifle joint and cranial to the tensor fascia latae.
Function:
- Drain superficial parts of the hip, thigh, and flank.
- Send efferent lymph vessels to the lateral iliac lymph nodes in the abdomen.

External Abdominal Oblique (EAO) (Figure 1)
- Origin: Lateral surface of the ribs and thoracolumbar fascia from the triceps muscle to the 18th rib.
- Insertion: Tuber coxae, linea alba (LA), prepubic tendon, iliac fascia.
- Fiber Direction: Caudal and ventral.
- Musculo-Tendinous Junction: Clear distinction between muscle and tendon; hypertrophies in older horses with COPD, forming a line called "heave line".
Internal Abdominal Oblique (IAO) (Figure 2A)
- Origin: Tuber coxae and dorsal part of the pelvic tendon of the EAO (inguinal ligament).
- Insertion: Last rib, costal cartilages, linea alba cranially, prepubic tendon caudally.
- Fiber Direction: Cranial and ventral.
- Inguinal Canal: Space between EAO and IAO; can lead to intestinal entrapment.
- Deep Inguinal Ring: Contains cremaster muscle, testicular artery and vein, ductus deferens.
- Superficial Inguinal Ring: Area close to the skin.
Transversus Abdominis (TA) (Figure 2B)
- Origin: Lumbar vertebrae and medial aspect of the last rib.
- Insertion: Linea alba, forms internal lamina of the RA muscle.
Rectus Abdominis (RA) (Figure 2B)
- Origin: 4th to 9th costal cartilages and sternum.
- Insertion: Prepubic tendon and accessory ligament.

Most abdominal surgeries in equine medicine are performed through the linea alba, which is a strong holding layer and similar in function to the yellow abdominal tunic.

The gastrointestinal (GI) tract and accessory digestive glands (pancreas and liver) occupy most of the abdominal cavity.
Other organs include the spleen, kidneys, and parts of the urogenital organs.

Location: Left side of the abdominal cavity, consists of base, body, and apex (Figure 6).
The base is located against the last three ribs and can be palpated rectally.
Attaches to the left kidney via nephrosplenic (renosplenic) ligament.
Potential space for large intestinal entrapment (pelvic flexure) created by its position.

Left Kidney (L.Kd) (Figure 6): Palpable rectally, located ventral to the last rib and lumbar transverse processes.
Right Kidney (R.Kd): More cranial than left kidney; not palpably accessible rectally.

Location: Right side of the abdominal cavity, ventral to the 7th/8th rib.
Non-palpable on rectal examination.
Contains space above the portal vein called the epiploic foramen, which is significant for small intestinal entrapment.
Boundaries of epiploic foramen identified:
- Dorsally: Caudate process of the liver, caudal vena cava.
- Ventrally: Right lobe of pancreas, portal vein.
- Cranially: Hepato-duodenal ligament.

Types of Entrapments: Epiploic Foramen
- Direction of entrapment: Lateral to medial, medial to lateral; medial more common.
- Complications: Tearing of the omentum in medial cases, longer intestinal travel in lateral cases.

Stomach: Resides left of midline; boundaries between glandular and non-glandular regions.
Duodenum: Transits right side before reaching base of the cecum, contains major and minor duodenal papillae responsible for draining bile and pancreatic secretions (4-5 cm apart).
Jejunum: Long, coiled, occupies the left side of the abdomen; at risk for epiploic foramen entrapment.
Ileum: Short, contracted, links to the cecum at the ileo-cecal orifice.

Cecum: A comma-shaped blind sac with base on the right, body and apex beneath the right ventral colon.
- Allows for telescoping of 4 meters of small intestine, potential impaction at the ceco-colic junction.
Colon: Right ventral colon (RVC) to left ventral colon (LVC), via sternal (sternal flexure) and pelvic flexures.
Sacculation and banding: Key features for motility and absorption.
- Formulas for band formation:
- Cecum, RVC, LVC: 4 bands each.
- LDC: 1 band.
- RDC: 3 bands.
- TC: 2 bands.
- Small colon: 2 bands.

Cecum: Commonly affected by conditions such as cecal tympany and intussusception.
Root of the mesentery: Contains cranial mesenteric artery; infection can lead to colic.

Definition: Abdominal pain resulting from GI or non-GI causes.
Anatomical Factors Predisposing Causes:
- Presence of openings (nephrosplenic area, epiploic foramen).
Non-Anatomical Factors:
- Sand ingestion, enterolith formation, parasitic infestations (e.g., Strongylus).
Types of Colic:
- Medical (most common) and surgical.
- Clinical signs include rolling, sweating, looking at flank, changes in pulse.

Rectal Examination: Determines the health of GI structures; risks of rectal tears exist.
Abdominocentesis: Important for analyzing peritoneal fluid, can collect fluid via needle/catheter inserted properly.

Knowledge of anatomical layout is crucial for successful surgery.
Possible complications include peritonitis, adhesions, hernia, and suture failure.