Rumen

The rumen, also known as a paunch, forms the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. It serves as the primary site for microbial fermentation of ingested feed. The smaller part of the reticulorumen is the reticulum, which is fully continuous with the rumen, but differs from it with regard to the texture of its lining.

The rumen is composed of several muscular sacs, the cranial sac, ventral sac, ventral blindsac, and reticulum.

The lining of the rumen wall is covered in small fingerlike projections called , which are flattened, approximately 5 mm in length and 3 mm wide in cattle. The reticulum (derived from the Latin for net [1]) is lined with ridges that form a hexagonal honeycomb pattern. The ridges are approximately 0.1 - 0.2 mm wide and are raised 5 mm above the reticulum wall. The hexagons in the reticulum are approximately 2–5 cm wide in cattle. These features increase the surface area of the reticulorumen wall, facilitating the absorption of volatile fatty acids. Despite the differences in the texture of the lining of the two parts of the reticulorumen, it represents one functional space.

Digested food (digesta) in the rumen is not uniform, but rather stratified into gas, liquid, and particles of different sizes, densities, and other physical characteristics. Additionally, digesta does not merely enter and exit the rumen without event, but it is subject to extensive mixing, and travels along complicated flow paths. Though they may seem trivial at first, these complicated stratification, mixing, and flow patterns of digesta are a key aspect of digestive activity in the ruminant and thus warrant detailed discussion.

After being swallowed, food travels down the oesophagus and is deposited in the dorsal part of the reticulum. Contractions of the reticulorumen propel and mix the recently ingested feed into the ruminal mat. The mat is a thick mass of digesta, consisting of partially degraded, long, fibrous material. Most material in the mat has been recently ingested, and as such, has considerable fermentable substrate remaining. Microbial fermentation proceeds rapidly in the mat, releasing many gases. Some of these gases are trapped in the mat, causing the mat to be buoyant. As fermentation proceeds, fermentable substrate is exhausted, gas production decreases, and particles lose buoyancy due to loss of entrapped gas. Digesta in the mat hence goes through a phase of increasing buoyancy followed by decreasing buoyancy. Simultaneously, the size of digesta particles–relatively large when ingested–is reduced by microbial fermentation and, later, rumination. Incomplete digestion of plant material here will result in the formation of a type of bezoar called Phythobezoars. At a certain point, particles are dense and small enough that they may “fall” through the rumen mat into the ventral sac below, or they may be swept out of the rumen mat into the reticulum by liquid gushing through the mat during ruminal contractions. Once in the ventral sac, digesta continues to ferment at decreased rates, further losing buoyancy and decreasing in particle size. It is soon swept into the ventral reticulum by ruminal contractions.

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Wikipedia