Ruminant and Horse Stomach Anatomy

Ruminant Stomach

• The proximal three compartments (rumen, reticulum, and omasum) are non-glandular and collectively referred to as the forestomach or proventriculus.
• The distal compartment (abomasum) is glandular and similar to the simple stomach of other species, hence called the true stomach.

Simple Stomach

• The simple stomach is a J-shaped curved sac composed of:
  • Greater curvature: the long convex surface extending from the cardia to the pylorus, where the greater omentum attaches.
  • Lesser curvature: the short concave surface extending from the cardia to the pylorus, where the caudal edge of the lesser omentum attaches.
  • Cardia: the fixed part of the stomach consisting of the cardiac opening (between the esophagus and stomach) with circular smooth muscle fibers.
  • Fundus: a blind, expanded portion on the left side adjacent to the cardia, homologous to the forestomach in ruminants.
  • Body: the largest part of the stomach extending from the cardia to the pylorus.
  • Pylorus: the distal opening of the stomach surrounded by a strong band of circular smooth muscle fibers (pyloric sphincter), through which stomach contents are emptied into the duodenum.

Gastric Glands

• The stomach can be divided into three regions based on specific distribution of gastric glands:
  • Region of cardiac glands: secretes mucus for protection.
  • Region of fundic (gastric) glands:
    • Neck cells produce mucus.
    • Chief cells produce pepsinogen.
    • Parietal cells produce chloride and hydrogen.
  • Region of pyloric glands.

Species Differences

• Dog:
  • The stomach is C-shaped.
  • The cardia is wide, which may be related to the ease of vomiting.
  • Gastric volvulus is relatively common, especially in large breeds.
• Cat:
  • The stomach is C-shaped but has a narrower lumen than the dog.
  • The angular notch is deep, making it difficult during endoscopy.
• Horse:
  • The stomach is small, with a capacity of 5-15 L.
  • A blind sac (saccus caecus) is lined by non-glandular mucosa, homologous to the forestomach of ruminants.
  • Some horses have non-glandular regions marked by scars from larvae of gastrophilus.
  • The cardiac sphincter is well-developed, which, along with the oblique entrance of the esophagus, is thought to be responsible for the reputed inability to vomit.

Compound Stomach

• The stomach of domestic ruminants is composed of four chambers:
  • Rumen
  • Reticulum
  • Omasum
  • Abomasum
• Rumen, Reticulum, and Omasum are referred to as the forestomach (proventriculus) with non-glandular mucosa, responsible for enzymatic destruction of carbohydrates with the help of microbes.
• The last chamber, the abomasum, has a glandular mucosa and is comparable to the simple stomach of horses and dogs.