Digestive Anatomy in Ruminants

The stomach of ruminants has four compartments: the rumen, reticulum, omasum and abomasum, as shown in the following diagram:



The ruminant stomachs, as seen from the right side
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 reticulum lies against the diaphragm
and is joined to the rumen by a fold of tissue. 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. In many respects, the reticulum can be considered a
"cranioventral sac" of the rumen; for example, ingesta flows freely
between these two organs. The reticulum is connected to the spherical
omasum by a short tunnel.


The abomasum is the ruminant's true or glandular stomach. Histologically, it is very similar to the stomach of monogastrics.


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.