The Rumen and Reticulum
The rumen and reticulum together form the vessel that ferments and breaks down the incoming complex and rough food to prepare it for conventional digestion lower in the digestive tract while it also absorbs some nutrients.
The rumen is laterally compressed and extends from the cardia— which lies a little way above the middle of the seventh intercostal space or eighth rib—to the pelvic inlet, from the abdominal roof to the floor, and from the left body wall across the midline, especially caudally and ventrally, where it may reach the lower right flank (Fig. 28.12). The much smaller reticulum lies cranial to the rumen under cover of the sixth to eighth ribs and mainly to the left of the median plane. It reaches from the cardia to the most forward part of the diaphragm and occupies the full height of this shallower part of the abdomen. The reticulum also crosses the midline, especially ventrally, where it lies above the xiphoid process of the sternum (Fig. 28.4/3 and see Fig. 27.7/8). This position allows the application of external pressure in the expectation of eliciting pain when the reticulum is diseased.Because of the integrated structure and function of the rumen and reticulum, many prefer to describe it as a combined ruminoreticular compartment. There is much in favor of this convention. The division of the rumen from the reticulum, though more complete, is achieved similar to the subdivisioning of the rumen—namely, by the inflection of the walls to form a series of pillars (pilae) that project internally (Fig. 28.4B). The whole thickness of the stomach wall, except the peritoneum, participates in these formations. The rumen and reticulum communicate over the U-shaped ruminoreticular fold. The principal ruminal pillars encircle the organ, dividing dorsal and ventral major sacs, while lesser coronary pillars mark off the caudal blind sacs.
The cranial pillar has an oblique direction that partially divides the cranial extremity from the remainder of the dorsal sac, emphasizing the association of the former part (atrium ruminis) with the reticulum. External grooves correspond to the positions of all these folds. The relative proportions of the compartments vary among the domestic ruminants. The smaller size of the dorsal sac and the extensive caudal projection of the ventral blind sac give the rumen of sheep and goats an unbalanced appearance when compared with the more symmetrical bovine rumen. There are also differences in the development of the grooves that are visible externally, but these are altogether without significance.
FIG. 28.10 Transverse section of the bovine trunk at the level of the 13th thoracic vertebra. 1, Aorta; 2, right crus of diaphragm; 3, caudal vena cava; 4, dorsal sac of rumen; 5, ventral sac of rumen; 6, abomasum; 7, omasum; 8, duodenum; 9, gallbladder; 10, liver; 11, cranial pole of right kidney; 12, right adrenal gland; L, left side; R, right side.
The serosa covers the entire surface of the rumen and reticulum, except dorsally where the ruminal wall is directly adherent to the abdominal roof from the esophageal hiatus of the diaphragm to the level of the fourth lumbar vertebra (Fig. 28.13/12), and over certain grooves where it is reflected to continue into the greater omentum. The limited attachment allows the ruminoreticulum the freedom necessary for the incessant and reciprocal contractions and enlargements of its various parts.
The relationships are most easily studied by reference to the illustrations (Figs. 28.4A and B, 28.7, and 28.10). The most important points are contact between the reticulum and the diaphragm and liver cranially; insinuation of the abomasum between the two chambers (ventral sac of rumen and reticulum) ventrally; relation of the right surface of the rumen to the intestinal mass, omasum, abomasum, pancreas, and kidneys; and intrusion of the superficial wall of the greater omentum between the ventral sac of the rumen and the abdominal wall.
The rumen also has a variable relationship to the uterus and other organs at the entrance to the pelvis, where the dorsal sac may be palpated per rectum. The direct contact of the dorsal sac with the upper part of the left flank makes auscultation and palpation simple. It also facilitates trocarization for the relief of tympany.The interior of the ruminoreticulum communicates with the esophagus and omasum through openings placed at the extremities of the reticular groove, a prominent gutter that descends from the cardia over the right face of the reticulum toward the fundus (Fig. 28.14/4 and 5). The groove is bounded by spiral fleshy lips; the upper end of the left (cranial) lip is expanded to overhang the slitlike cardiac opening, while a similar thickening of the lower end of the right (caudal) lip partly conceals the round exit into the omasum. The cardia is placed at the junction of the rumen and reticulum and discharges into both chambers. In the unweaned animal the reticular groove may be converted into a closed tube, forming a channel that conveys milk directly from the esophagus to the omasal canal, whence it drops into the abomasum. The muscular contractions that draw the lips together are reflexly stimulated by sucking from the dam or by the presentation of suitable bucket feeds. As the animal matures, alterations in diet and feeding regimen result in decreasing use of this route, although even in the adult a portion of the soluble nutrients released into the saliva during mastication succeeds in bypassing the ruminoreticulum. The groove reflex is stimulated by antidiuretic hormone (ADH), which indicates that the reflex may have some function in adult life. ADH is produced in response to dehydration or an increase in plasma osmolality. ADH is associated with thirst, and its effect on the reticular groove may cause a portion of the water drunk by dehydrated animals to bypass the ruminoreticulum. Closure of the groove can be stimulated by certain chemicals (e.g., copper sulfate).
This provides a useful strategy when it is desirable to introduce drugs to the abomasum without prior dilution in the forechambers.
FIG. 28.11 Transverse section of the bovine trunk at the level of the 3rd lumber vertebra. 1, Aorta; 2, caudodorsal blind sac; 3, dorsal coronary pillar; 4, caudal pillar; 5, left longitudinal pillar; 6, ventral coronary pillar; 7, caudoventral blind sac; 8, descending duodenum; 9, left kidney; 10, caudal vena cava;
11, milk vein; 12, intestinal mass; L, left side; R, right side.
FIG. 28.12 Transverse section of the bovine trunk at the level of the 5th lumbar vertebra. 1, Bifurcation of aorta and formation of caudal vena cava; 2, right dorsal coronary pillar; 3, caudal pillar; 4, caudodorsal blind sac; 5, caudoventral blind sac; 6, colon; 7, psoas minor; 8, psoas major; 9, internal abdominal oblique; 10, external abdominal oblique; 11, milk vein; L, left side; R, right side.
FIG. 28.13 Schematic transverse section of the abdominal cavity to show the disposition of the greater omentum. 1, Dorsal sac of rumen; 2, ventral sac of rumen; 3, superficial wall of greater omentum; 4, deep wall of greater omentum; 5, omental bursa; 6, descending duodenum; 7, intestinal mass; 8, right kidney; 9, aorta; 10, caudal vena cava; 11, supraomental recess; 12, retroperitoneal attachment of rumen.
The ruminoreticular mucosa is lined by a harsh stratified cutaneous epithelium (Fig. 28.15A and B) that is stained a greenish brown. The floor of the reticular groove, however, is smooth and pale. The reticular mucosa has a distinctive pattern formed by ridges about 1 cm high that outline four-, five-, and six-sided "cells" (Fig. 28.16B and see Fig.
27.7/8). These ridges and the cell floors between them carry low papillae. The reticulate pattern becomes less regular toward the junction with the rumen and gradually modifies to merge with the papillated surface of this chamber. The upper keratinized layer of the epithelium protects against abrasion by the rough, fibrous diet, whereas the deeper layers metabolize volatile short-chain fatty acids. Histologically, the epithelium shows many similarities with the epidermis. The lamina propria-submucosa, formed by a network of collagen and elastic fibers, includes bands of smooth muscle within the distal parts of the reticular ridges (Fig. 28.15A). The ruminal papillae vary in prominence according to age, diet, and location (Figs. 28.16A and 28.14). Normally they are largest and most densely strewn within the blind sacs, fewer and less prominent in the ventral sac, and least developed over the center of the roof and toward the free margins of the pillars. Individual papillae vary from low rounded elevations through conical and tongue-like forms to flattened leaves about 1 cm long. The ruminal epithelium resembles that of the reticulum. A thick subepithelial lamina propria forms the core of the papilla through collagen, elastic, and reticular fibers and also contains a dense capillary network. There is no muscularis mucosae. The looser submucosa is located directly against the lamina propria and also contains a vascular network (Fig. 28.15B).
FIG. 28.14 Paramedian section of part of the trunk of a goat. 1, Heart; 2, diaphragm; 3, atrium ruminis; 4, reticular groove; 5, reticulum; 6, ruminoreticular fold; 7, abomasum; 8, ventral sac of rumen.
The rugose nature of the ruminoreticular lining was formerly interpreted as an adaptation for the mechanical disruption of the ingesta. Now it is regarded as a mechanism to increase the surface area for the absorption of the volatile fatty acids produced by microbial fermentation.
Volatile fatty acids, especially butyric, stimulate papillary development, and their absorption is facilitated by the very rich subepithelial capillary plexus. In some wild ruminants but not in domestic species, striking changes in papillary prominence and size, and thus in the ruminal surface area (Fig. 28.16A), accompany seasonal changes in forage quality.*The reticulum of the small ruminants is relatively larger than that of cattle. Although it extends farther caudally, its contact with the abdominal floor is subject to much functional variation (Fig. 28.14/5). There are conspicuous species differences in its lining. The ridges that bound the reticular "cells" are relatively much lower and have more prominently serrated margins. The papillated "ruminal" mucosa also extends over a larger part of the reticular wall.
The smooth muscle of the ruminoreticular wall is arranged in two coats that continue the striated muscle of the esophagus. The thin outer coat runs craniocaudally over the rumen but has an oblique course on the reticulum. Most bundles of the much thicker inner layer run more or less at right angles to the superficial coat and thus encircle the long axis of the rumen. They extend into the pillars and form the bases of these structures. The thicker parts of the ruminoreticular muscle are sold for consumption as tripe.
FIG. 28.15 (A) Reticulum of a goat (magnification ?28). (B) Rumen of a goat (magnification ?28). 1,
Stratified squamous epithelium; 2, lamina propria; 3, lamina muscularis mucosae; 4, submucosa; 5, muscularis interna.
The regular sequence of ruminoreticular contractions mixes and redistributes the stomach contents. The cycle consists of a biphasic reticular contraction (relaxation between contraction phases is more consistent in cattle than in sheep), which throws the reticular contents into the atrium ruminis, followed by contraction of first the dorsal and later the ventral rumen sacs. The wave of contraction passes over each in a craniocaudal direction. The process is centrally regulated, and the tempo and vigor are adjusted according to information supplied by intramural receptors that are stimulated by stretching of the wall and by contact with floating fragments. Both the sensory and the motor pathways travel within the vagus nerves.
Regurgitation of food for remastication requires the coordination of the stomach movements with those of the thoracic wall and throat. It is preceded by an additional reticular contraction that floods the cardiac region. The ingesta are drawn into the esophagus on expansion of the thorax with a closed upper airway and are then carried orally by an antiperistaltic wave. The heavy remasticated cud, now further sodden and divided, tends to drop from the cardia into the reticulum.
In eructation (the discharge of gas through the esophagus), ruminal contractions without the reticulum's participation substitute for the normal pattern of activity. These contractions originate in the ventral sac and generally spread to the dorsal sac, where they begin caudally and extend cranially. These contractions force the ruminal gas forward to the cardiac area whence it is aspirated into the esophagus, through which it is hurried orally by an antiperistaltic wave. It then passes through the relaxed pharyngoesophageal sphincter into the pharynx. Some escapes from the mouth, but part is directed to the lungs.
The content of the rumen shows some stratification, with the recent ingesta piled above the heavier, more sodden remasticated material. It is therefore the lighter material that is most liable to be regurgitated for further mastication and insalivation (Fig. 28.17).
FIG. 28.16 (A) Rumen papillated mucosa taken from a Waterbuck (left) and a lesser Kudu. (B)
Reticulum: mucosal ridges outlining “cells” characteristic of the reticular mucosa (cow).
Inflammation of the rumen and the reticulum in cattle can occur through mechanical or chemical (e.g., lactic acidosis) causes. Cattle are notoriously careless feeders and often ingest foreign bodies, especially pieces of wire and nails, with their forage. These bodies tend to collect within the reticulum and, when sharp, may be driven through the reticular wall by the contractions of this organ. This leads to inflammation of the reticulum at the site of the perforation (traumatic reticulitis, or "hardware disease") and peritonitis or pericarditis if the nail penetrates the diaphragm. The inflammation in the forestomach leads to pain in the anterior abdomen and a hunched posture with abducted elbows. Some of these bodies corrode, while others may be immobilized by introducing a magnet through the mouth (Fig. 28.18/2 and inset).