The rumen, also known as a paunch, forms the larger part of the reticulorumen

Reticulorumen

The reticulorumen represents the first chamber in the alimentary canal of ruminant animals. It is composed of the rumen and reticulum. The reticulum differs from the rumen with regard to the texture of its lining. The rumen wall is covered in small, finger-like projections called papillae, which...

, which is the first chamber in the alimentary canal of ruminant

Ruminant

A ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud, and chewing it again...

 animals. It serves as the primary site for microbial fermentation

Fermentation (food)

Fermentation in food processing typically is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination thereof, under anaerobic conditions. Fermentation in simple terms is the chemical conversion of sugars into ethanol...

 of ingested feed. The smaller part of the reticulorumen is the reticulum

Reticulum (anatomy)

The reticulum is the second chamber in the alimentary canal of a ruminant animal. Anatomically it is considered the smaller half of the reticulorumen along with the rumen. Together these two compartments make up 84% of the volume of the total stomach. The reticulum is located between the rumen and...

, 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 papillae, which are flattened, approximately 5 mm in length and 3 mm wide in cattle.