For the most part, the digestive system of ruminants is very similar to that of other mammals, but the stomach is considerably different from the so-called "monogastric" condition. The word "ruminant" comes from the Latin ruminare, to think; and as wildly laughable as the idea of a cow "thinking" may be, nevertheless, the contemplative and placid expression on a cow's face as she "chews her cud" does sort of give the impression that she's dealing with serious world problems.
You may have heard it said that cows have "four stomachs." In an anatomic sense this is incorrect; there really is only one stomach, but it does have divisions. The first three divisions of the ruminant stomach are sometimes considered to be diverticula of the esophagus; there is considerable debate on this point, however, and some authorities consider them derivatives of the stomach. In any event, they are all derived from the primitive embryonic foregut.
In monogastric animals, the stomach's functions are limited to temporary storage and preliminary mastication of the food into a liquid mass; little or no absorption of nutrients takes place. In ruminants, however, the stomach has an absorption function in addition to the usual functions of mastication and acidification. The evolutionary success of the ruminants as a group is based on the efficiency of their digestive system in extracting nutrition from low-quality food. The key to the ruminant digestive system's efficiency is the presence of symbiotic microorganisms in it, particularly in the forestomachs. These inhabitants and their capabilities turn a cow into a mobile fermentation plant for cellulose.
The symbiotic microorganisms produce cellulase enzymes, something no mammal can do on its own. The cellulase breaks down the cell walls of plant materials to release fatty acids into the gut. These fatty acids are absorbed by the cow, and constitute a significant portion of her overall energy budget input. Later, when the food passes into the intestine, absorption of amino acids, lipids, carbohydrates, etc., takes place as in other animals. This "double digestion" system of ruminants allows them to exploit an energy sourcecellulosedenied to other herbivores. Ruminants can usually survive on relatively poor ground unsuitable for other species.
The four divisions of the ruminant stomach are the rumen, the reticulum, the omasum, and the abomasum. In a functional sense, these operate as parts of a "stomach system" and we can correctly speak of a cow having "forestomachs," if not "four stomachs."
The rumen is an enormous space filled with chewed and half-chewed materials the cow has ingested, swallowed, regurgitated, and swallowed again (often several times). A combination of mechanical mastication and cellulase enzymatic action on the hard cellulosic material of the bovine diet permits the breakdown of these otherwise-indigestible materials. The cow who "chews her cud" is methodically grinding the food into smaller and smaller bits, allowing the symbionts more and more surface area on which to work.
Slide 136 shows a section of a rumen. If you examine it first by holding it up to the light you'll see that projecting from the wall are numerous finger-like projections. These are the ruminal papillae, which are somewhat conical in three dimensional view.
Observing the papillae at low power, you should see several interesting histological features. First, they're covered with stratified squamous epithelium. Moreover, it's keratinized, which is a little unusual by the standards of non-ruminant animals, but it's normal here. Despite keratinization, absorption occurs across the epithelium in the rumen. Note that there is no muscularis mucosae in the rumen: the lamina propria and the submucosa combine to form a single underlying CT support for the epithelium. Nor are there any glands present in either the mucosa or submucosa.
The other features of the slide are those which you should expect to see in a typical tubular organ: a tunica muscularis with inner and outer layers, and an overlying serosa.
To ruminate on the structure of the rumen, click here.
Slide 130 is a section of reticulum. It's difficult to appreciate the gross appearance of this organ from a slide; it's sometimes described as a "honeycomb." You can buy reticulum in the supermarket, where it's sold as "tripe," and if you have ever eaten it, you know what the gross appearance is. In sections, it's what you might expect to get if you were to cut through a waffle.
Notice that in the reticulum, there is a muscularis mucosae, of rather odd type. It consists of bands of smooth muscle which run through the tops of the ridges of the honeycomb, and which are more or less isolated from the lower levels, nearer the wall. It can be shown in gross specimens that this smooth muscle is continuous with that of the esophagus. The contraction of the muscularis mucosae serves to contract the openings of the honeycomb, somewhat like a purse-string closes a purse.
To see the layout of the reticulum and this type of muscularis mucosae, click here.
Again, the epithelium on the surface is a keratinized stratified squamous type, and there are no glandular elements.
Slide 129 is an example of the omasum. This organ has a number of colorful layman's names applied to it, among them "many-plies" and "the butcher's Bible," the latter referring to its similarity to the leaves of a book in gross appearance.
The omasum has a muscularis mucosae which underlies the epithelium; but in addition to muscularis mucosae, there is an excursion of smooth muscle from the tunica muscularis up between the folds of the muscularis mucosae. What at first glance appears to be a "spine" of muscle running up through each of the "leaves" of the mucosa can be resolved on careful examination as a three-layered structure: muscularis mucosae on the outside, with tunica muscularis inside it. If you follow the central strand of muscle outwards you should find where it comes off the inner layer of the tunica muscularis. There is a very scanty bit of submucosa separating the two types of smooth muscle, but it's so tenuous that you may not be able to make it out.
To view the arrangements of the omasum, click here.
If you were to pass a rod through a mucosal fold from side to side, you would go through the following sequence: epithelium / lamina propria / muscularis mucosae /submucosa/ tunica muscularis / submucosa / muscularis mucosae / lamina propria / epithelium.
To see this in detail, click here.
As with the previous divisions, the epithelium is stratified squamous, and there are no glands.
Turn now to slide 131. This is a section of abomasum, and this portion is the true glandular stomach of ruminants. Its histology is very similar to the fundic region of the stomach of monogastric animals, and you will find it instructive to compare it to some of those slides used in the exercise on esophagus and stomach.
The surface epithelium here is simple columnar, not stratified squamous. There are gastric pits (foveolae) and below those, there are gastric glands of the fundic type. The glands contain parietal cells (which make hydrochloric acid) and chief or zymogenic cells which make digestive enzymes.
To see the abomasum, click here.
The submucosa of this region is much more developed than in the previous divisions of the stomach. The abomasum, unlike the rumen, reticulum, and omasum, does not absorb nutrients. As with monogastric animals, it prepares food for enzymatic breakdown and absorption in the small intestine.
Cattle aren't the smartest creatures God ever made, and they're prone to eating an astonishing variety of things that aren't edible. High up on the list of junk that cows will ingest is bits of wire, usually clipped off a hay bale's binding. These indigestible sharp objects can easily cause serious problems, as you might imagine, when they puncture the wall of the forestomach. One way to deal with this is to have the cow swallow a magnet, which attracts and holds the ferrous metals and prevents them from migrating to a place where they can do damage as they slosh around in the vast caverns of the forestomach. There's a brisk trade in recycling these "cow magnets," which are recovered at slaughter and re-used. The nickname for injuries and illness caused by ingested material is "hardware disease," and you can see a nice example of it if you click here.