5.1.3 Structure and Function of the Stomach
The four parts of the ruminant stomach originate from the same primitive foregut found in other animals, with the rumen, reticulum, and omasum not simply being outgrowths of the esophagus.
The main structural difference in the ruminant digestive system lies in the presence of three additional compartments (rumen, reticulum, omasum) between the mouth and the true stomach (abomasum), where microbial fermentation occurs.
Digestion is thus divided into two phases: fermentation in the fore-gut (rumen, reticulum, omasum) and enzymatic digestion in the hind-gut (abomasum).
5.1.3.1. Rumen and reticulum
Food enters the rumen from the esophagus near the rumen-reticulum junction. Because of this, heavy foreign objects like nails or stones often settle at the bottom of the reticulum. The contents of the rumen and reticulum move freely between the two, which is why they are frequently referred to together as the reticulo-rumen.
Muscular pillars run across the rumen, dividing it into dorsal and ventral sacs. Their rhythmic contractions mix the contents thoroughly.
The mucosa of the rumen, especially in the ventral sac, is covered with ruminal papillae-finger-like projections that increase the surface area for absorption.
The reticulum lies against the diaphragm and connects to the omasum via the reticulo-omasal orifice. Its lining has a distinctive honeycomb structure.
5.1.3.2. Omasum
The omasum is spherical and communicates with the abomasum through the omaso-abomasal orifice. Inside, it contains many leaf-like structures called laminae, of varying sizes, which help in grinding and water absorption. An omasal groove or canal channels ingesta from the reticulo-omasal orifice to the omaso-abomasal opening.
5.1.3.3. Abomasum
The abomasum, or true glandular stomach, has two regions:
The fundic region, lined with folds and containing gastric glands similar to those of simple-stomached animals.
The pyloric antrum, which is more muscular and contains pyloric glands.