 VIVO Pathophysiology |
Herbivores |
Digestive Anatomy in Ruminants
The stomach of ruminants has four compartments: the rumen, reticulum, omasum and abomasum, as shown in the following diagram:
Collectively, these organs occupy almost 3/4ths of the abdominal cavity, filling virtually all of the left side and extending significantly into the right.
The rumen, far and away the largest of the forestomaches, is itself sacculated by muscular pillars into what are called the dorsal, ventral, caudodorsal and caudoventral sacs and is connected to the reticulum by the rumino-reticular groove.
The reticulum lies against the diaphragm, nestled in the ventro-cranial part of the abdomen across the diaphragm from the heart. Although the reticulum has a distinct morphology, it is functionally an extension of the rumen and can be considered a "cranioventral sac" of the rumen. Heavy material from the esophagus tends to fall into the reticulum.
The omasum is a spherical organ connected to the reticulum by a short tunnel of tissue. It retains large fragments of feed, absorbs water and water-soluble nutrients, and passes small particles to the abomasum.
The abomasum is the ruminant's true or glandular stomach. Histologically, it is very similar to the stomach of monogastrics.
In young ruminants, prior to maturation of the rumen, ingested milk bypasses the rumen and is delivered directly to the abomasum via what is called the esophageal groove.
The interior of the rumen, reticulum and omasum is covered exclusively with stratified squamous epithelium similar to what is observed in the esophagus. Each of these organs has a very distinctive mucosa structure, although within each organ, some regional variation in morphology is observed. The images below are from a sheep.
| The interior surface of the rumen forms numerous papillae that vary in shape and size from short and pointed to long and foliate. |  |
| Reticular epithelium is thrown into folds that form polygonal cells that give it a reticular, honey-combed appearance. Numerous small papillae stud the interior floors of these cells. |  |
| The inside of the omasum is thrown into broad longitudinal folds or leaves reminiscent of the pages in a book (a lay term for the omasum is the 'book'). The omasal folds, which in life are packed with finely ground ingesta, have been estimated to represent roughly one-third of the total surface area of the forestomachs. |  |
The anatomic features described above are exemplified by cattle, sheep and goats. Certain other animals are also generally called ruminants, but have slightly different forestomach anatomy. Camelids (camels, llamas, alpacas, vicunas) have a reticulum with areas of gland-like cells, and an omasum that is tubular and almost indistinct. These animals are occasionally referred to as pseudoruminants or as having "three stomachs" rather than four.
A final note on anatomy. Stratified, squamous epithelium such as found in the rumen is not usually considered an absorptive type of epithelium. Ruminal papillae are however very richly vascularized and the abundant volatile fatty acids produced by fermentation are readily absorbed across the epithelium. Venous blood from the forestomachs, as well as the abomasum, carries these absorbed nutrients into the portal vein, and hence, straight to the liver.
 Fermentation Chemistry |
Rumen Physiology and Rumination  |
Updated June 2026. Send comments to Richard.Bowen@colostate.edu