STRUCTURE

The ventrolateral wall of the abdomen is composed of as many as 9 or 10 layers, although not all cover the entire extent. The skin is freely movable except over the coxal tuber. The cutaneous muscle is thick over the lower parts of the flank but thins dorsally and does not extend over the paralumbar fossa; it also leaves the abdominal floor bare except for detached fascicles that supply the male animal with cranial and caudal muscles of the prepuce. The cutaneous muscle extends through the flank fold to end in an aponeurosis over the lateral surface of the thigh (Figure 28–1, A).

The loose superficial fascia provides pathways for the cutaneous nerves and encloses certain lymph nodes. The elongated subiliac node lies vertically within the skin fold, pressed against the cranial margin of the thigh some distance above the patella; it can always be found on palpation. It drains the more superficial layers of the body wall as far forward as the caudal part of the thorax and also receives lymph coming from the skin and superficial muscles of the thigh and croup (see Figure 29–46). A number of smaller nodes within the paralumbar fossa drain the surrounding parts; they normally escape notice but appear as circumscribed swellings when enlarged. The subcutaneous abdominal (“milk”) vein runs forward over the abdominal floor from the udder (see Figures 29–34 and 29–44).

The deep fascia is transformed into an elastic tunica flava, attached to the underlying muscle and sharing in supporting the viscera. Ventrally it gives origin to the external spermatic fascia or the medial lamina of the suspensory apparatus of the udder.

The muscle layer is broadly arranged as in other species. In the flank it consists of a triple layer of flat muscles that take origin from the ribs, lumbar transverse processes, and ilium (see Figure 28–1). These are continued over the abdominal floor by aponeurotic tendons that enclose the rectus muscles to each side of the linea alba where the aponeuroses attach (see Figure 1–37). The linea alba runs from the xiphoid process of the sternum to the center of the prepubic tendon, where it blends with the end tendons of the recti.

The most superficial muscle of the flank, the external oblique, arises by fleshy serrations from the outer surfaces of the last eight ribs. Its most dorsal fibers run more or less horizontally toward the coxal tuber, but the greater number slope caudoventrally to find attachment to the linea alba (Figure 28–1, B). The gap that intervenes between the dorsal border and transverse processes is closed by a sheet of fascia. The fleshy part is succeeded by an aponeurotic tendon, and the transformation occurs along a line that first drops vertically, from a point roughly level with the coxal tuber, before sweeping cranially. A split within the aponeurosis provides the superficial opening (ring) of the inguinal canal.

The second muscle, the internal oblique, has a tendinous origin from the coxal tuber and the pelvic tendon of the external oblique and several independent fleshy origins from the tips of the lumbar transverse processes. It radiates to insert on the last rib and into the linea alba. Most fibers run cranioventrally, but the thicker, most caudal fascicles pass slightly behind the plane of the tuber. The muscle–tendon junction slopes caudoventrally, and only the most caudal strip is fleshy where the muscle crosses the margin of the rectus (Figure 28–1, C). The aponeuroses of the two oblique muscles become increasingly interwoven where they pass ventral to the rectus and together furnish the external layer of the rectus sheath. The flesh of the internal oblique forms the inner wall of the inguinal canal.

The third, the transversus abdominis, arises from the last ribs and the extremities of the lumbar transverse processes. Its craniodorsal triangle is tendinous, but most of the part covering the flank is fleshy; before reaching the edge of the rectus, the flesh gives way to an aponeurosis that crosses the dorsal face of the rectus to gain the linea alba, thus forming the inner layer of the rectus sheath. Most fibers run transversely, and none pass behind the plane of the coxal tuber; the dorsal surface of the rectus is thus left uncovered in its most caudal part (Figure 28–1, D).

The rectus abdominis muscle is interrupted in the usual way by several tendinous intersections (Figure 28–1/3). It arises from the outer surfaces of the lower ends of the last 10 ribs and continues as a wide band separated from its neighbor by the flattened linea alba; it narrows suddenly as it approaches the pubic brim, and the tendon that succeeds the flesh twists to form with its fellow and the linea alba a V-shaped trough that continues as the central part of the prepubic tendon. Before reaching the pubic brim, which it approaches almost vertically from below, the prepubic tendon is strengthened by joining the decussation formed by the contralateral parts of the pectineus muscles (each of which arises from both pubic bones) and by additional contributions from the aponeuroses of the abdominal oblique muscles. Ultimately, and after partial decussation, the rectus tendons end in common on the symphysial crest of the pelvis and on the medial symphysial tendon that arises here. A rounded median depression of the internal surface of the prepubic tendon is ascribed to the drag of the udder (see Figure 29–40).

A thin fascia covers the abdominal muscles internally and supports the parietal peritoneum. The largest deposits of fat in the subperitoneal tissues are encountered toward the pelvic inlet. The wholly tendinous nature of a region of the abdominal wall, along the border of the rectus in front of the stifle, merits emphasis.

The inguinal canal resembles that of the horse (p. 549) so closely that a separate description is unnecessary. Inguinal hernias are infrequent in cattle but common in male sheep, although there are no obvious differences in the adult anatomy. It is probable that the frequent incidence in rams is connected with inherited anomalies in gubernacular development.

INNERVATION AND VASCULARIZATION

The most important nerves of the abdominal wall are the last thoracic (T13) and the first and second lumbar nerves, although the floor ventral to the costal arch is served by continuations of the caudal intercostal nerves. A knowledge of the topography and distribution of the nerves to the flank is of practical importance in obtaining local anesthesia.

The skin of the abdomen is supplied by branches from both dorsal and ventral primary rami, but the muscles and other deep structures are supplied by ventral rami alone (see Figure 1–37). The skin is divided into bands (dermatomes) that encircle the trunk, and each is the territory of a particular spinal nerve. The peritoneal regions supplied by spinal nerves correspond very closely to the dermatomes.

The dorsal rami (Figure 1–37/4) of the thoracic and lumbar nerves supply the epaxial muscles and the strip of skin extending from the dorsal midline roughly to the level of the patella. Below this line the skin is supplied by two tiers of branches from the ventral rami (Figure 1–37/5).

The ventral rami are much widened where they enter the flank between the internal oblique and transverse muscles. Each possesses a rather constant relationship to the skeleton that is a useful guide when the nerves are blocked with anesthetics. These nerves run obliquely, deviating in an increasingly caudal direction (Figure 28–2). The last thoracic ventral branch usually passes below the tip of the first lumbar transverse process, the first lumbar branch (iliohypogastric nerve) passes below the tip of the second, and the second lumbar branch (ilioinguinal nerve) passes below the tip of the fourth (Figure 28–3). Most variations affect the last of these three nerves, which sometimes passes below the transverse process of the third lumbar vertebra.

An exception to the general pattern of innervation of the abdominal wall is provided by the nerve to the cutaneous muscle; this is supplied by a branch from the brachial plexus.

It can now be appreciated that incisions of the upper flank require blockage of both dorsal and ventral branches. Anesthesia is most conveniently obtained by paravertebral injection of the relevant nerves close to their foramina of emergence from the vertebral canal. Anesthesia of the lower flank and abdominal floor requires blockage of the ventral branches only, and these are most conveniently reached where they pass close to the tips of the lumbar transverse processes (paralumbar block). Variation in topography makes the procedure less reliable than might be wished, unless the anesthetic agent is diffused rather widely. Lumbar epidural injection provides an alternative procedure. The specific innervation of the cutaneous muscle must be kept in mind regardless of the method chosen.

The abdominal wall receives blood vessels from several sources. The ventral part obtains its supply through the cranial and caudal epigastric arteries, which are branches of the internal thoracic and external pudendal arteries, respectively. The flanks are supplied from parietal branches of the aorta, of which the most important surgically is the deep circumflex iliac artery, which comes from the external iliac to pierce the flank a little cranial to the coxal tuber. The veins are initially satellite, but in the parous cow the arrangement is modified with the formation of the “milk vein” (p. 723).

GENERAL CONSIDERATIONS

The stomach is composed of four chambers—rumen, reticulum, omasum, and abomasum—through which the food passes successively (Figure 28–7). The first three, collectively known as the forestomach (proventriculus), are developed to cope with the complex carbohydrates that form so large a part of the normal diet of ruminants, and only the last chamber is comparable in structure and function to the simple stomach of most other species. All are derived, however, from the gastric spindle of the embryo (Figure 28–8).

The topography of the ruminant abdomen is dominated by the enormous development of the stomach, which in adult cattle almost fills the left half of the cavity and occupies a substantial portion of the right (Figure 27–1 and Figures 28–9, 28–10, 28–11, and 28–12). Its capacity measures about 60 L. This figure, which is much more modest than many estimates, may be apportioned between the various chambers as follows: rumen, 80%; reticulum, 5%; omasum, 8%; and abomasum, 7%. The proportions in small ruminants are somewhat different, being perhaps 75% rumen, 8% reticulum, 4% omasum, and 13% abomasum. The relative volumes are fairly constant in the short term because the enormous storage capacity of the first chambers and the more or less continuous passage of ingesta into the distal parts minimize the effects of intermittent feeding.

The different chambers are identifiable as expansions of the foregut spindle in the early embryo. They increase at unequal rates throughout the embryonic and fetal periods, as first one takes the lead and then another. At one stage the fetal stomach has an almost adult configuration, but during the last months of intrauterine life the abomasum outstrips the others; at birth it accounts for more than half the weight and capacity of the entire organ—which is appropriate because it is the only part that has an immediate function to perform. The postnatal changes through which the adult proportions and topography are acquired are described later (p. 692).

THE RUMEN AND RETICULUM

The rumen and reticulum together form the vessel in which the unpromising food material, invulnerable to attack by mammalian digestive enzymes, is reduced by processes of microbial fermentation. Some of the simpler products are assimilated directly, while others are susceptible to conventional digestion lower in the digestive tract.

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 (see Figure 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; it also passes across the midline, especially ventrally, where it lies above the xiphoid process of the sternum (see Figures 27–7/8 and 28–4/3). This position allows the application of external pressure in the expectation of eliciting pain when the reticulum is diseased.

The rumen and reticulum are so intimately related in structure and function that many now prefer to describe 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 in exactly the same way as the subdivision of the rumen, namely by the inflection of the walls to form a series of pillars (pilae) that project internally. The whole thickness of the stomach wall, except the peritoneum, participates in these formations. The divisions and the pillars that bound them are illustrated in Figure 28–4, B. 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 visible externally, but these are altogether without significance.

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 (Figure 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 (see Figures 28–4, A-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 the 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 (Figure 28–14/4,5). 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.

The ruminoreticular mucosa is lined by a harsh stratified cutaneous epithelium (Figure 28–15, A-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” (see Figures 27–7/8 and 28–16, B). 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 (Figure 28–15, A). The ruminal papillae vary in prominence according to age, diet, and location (Figure 28–16, A, and Figure 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 tonguelike forms to flattened leaves about 1 cm long. The ruminal epithelium resembles that of the reticulum. A thick lamina propria beneath the epithelium forms the core of the papilla; apart from collagen, elastic, and reticular fibers, it includes 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 (Figure 28–15, B).

The rugose nature of the ruminoreticular lining was formerly interpreted as an adaptation for the mechanical disruption of the macerating ingesta. Since it became known that the volatile fatty acids produced by microbial fermentation are absorbed in the rumen and reticulum, it has been regarded as primarily a device to increase the epithelial surface. Papillary development is stimulated by these acids (especially butyric), and their absorption is facilitated by the very rich subepithelial capillary plexus. In some wild ruminants, striking changes in papillary prominence and size, and thus in the ruminal surface area (Figure 28–16, A), accompany seasonal changes in forage quality.* Changes in papillary development tend to be more restrained in domestic species, whose diet is subject to human influence to a greater or lesser degree.

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 (Figure 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.

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 in which the reticulum does not participate are substituted 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; they 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: food of recent ingestion is 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 (Figure 28–17).

Cattle are notoriously careless feeders and often ingest foreign bodies, especially pieces of wire, 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 (traumatic reticulitis—”hardware disease”). Common sequelae include abscessation of the liver and possibly other abdominal organs and, more critically, a purulent pericarditis when the object penetrates the diaphragm. Some of these bodies corrode, while others may be immobilized by introducing a magnet through the mouth (Figure 28–18/2 and inset).

THE OMASUM

The omasum lies within the intrathoracic part of the abdomen to the right of the midline, between the rumen and reticulum to the left and the liver and body wall to the right (Figures 28–9/7 and 28–7/2). It is bilaterally flattened and displays a long convex border that faces dextrocaudally and a much shorter lesser curvature that looks in the opposite direction. The long axis is more or less vertical in the cadaver, but the position and orientation of the living organ alter constantly. Most of the omasum lies under cover of the eighth to eleventh ribs, but in cattle the lower pole generally projects onto the abdominal floor below the costal arch (Figure 28–19/5). Although its position places most of the omasum beyond direct manual reach, the organ may be examined by auscultation and percussion. The lower pole of the omasum has an extensive attachment to the fundic region of the abomasum around the omasoabomasal orifice. Much of its right surface is covered by and partly connected to the lesser omentum (Figure 28–4, C/13).

The omasum is relatively smaller in sheep and goats, in which it is bean-shaped. It maintains an almost vertical position when the stomach is at rest. It projects on the eighth and ninth ribs but, because of the intervention of the liver, makes no direct contact with the body wall.

The interior is occupied by about a hundred crescentic laminae that arise from the sides and greater curvature and project toward the lesser curvature, where there is a more open passage, the omasal canal (see Figure 28–9). The laminae are of several lengths, and those of different sizes alternate so as to divide the lumen into a series of narrow and fairly uniform recesses (Figure 28–10/7). The reticulo-omasal orifice is situated at the upper end of the short canal; the large, oval omasoabomasal opening (Figure 28–10/6) at the other extremity is partly occluded by the prolapse of abomasal folds. The floor of the canal (known as the omasal groove) is smooth except for a few low ridges that run along its length and a scattering of clawlike projections that guard the upper opening.

The keratinized stratified squamous epithelium over the laminae is raised to cover numerous papillae. Most are small and lenticular, but there are a few larger, conical projections that point distally and perhaps promote the onward movement of the ingesta. The mucosa is further characterized by a lamina propria that includes a dense subepithelial capillary network and encloses a thick muscularis mucosae consisting of a thin outer longitudinal layer and a thicker inner circular layer. The inner layer is continuous with the muscle of the omasal wall. The contents of the omasal recesses are finely divided and rather dry; they impart a firmness to the organ that allows it to be readily recognized on palpation at laparotomy, directly, or from within the rumen after opening that chamber.

Omasal contractions are biphasic. The first phase squeezes ingesta from the omasal canal into the recesses between the laminae; the second phase is a mass contraction. The principal effect is to squeeze fluid from the material within the recesses, which is a process essential to the continuing movement of ingesta to the abomasum. These contractions occur at a much slower and more deliberate tempo than those of the ruminoreticulum. Although the rough surfaces and muscular cores of the laminae suggest that these folds triturate the food by rubbing against each other, there is no evidence of such activity. Absorption is continued in the omasum.

THE ABOMASUM

The abomasum lies flexed on the abdominal floor, embracing the lower pole of the omasum from behind (Figure 28–7/3). The larger of the two limbs forms a piriform sac that reaches forward to the left to make contact with the body wall between the reticulum and the atrium and ventral sac of the rumen (Figure 28–4, A/6). This limb is divided by analogy with the simple stomach into fundus and body, but the boundary between these parts is imprecise. In fact, the location of the omasoabomasal opening in the living animal is not known with certainty; it is possible that it is terminal, and in that case no blind diverticulum and therefore no true fundus exists. The cranial part of the fundus is extensively connected to the reticulum, atrium, and ventral sac by muscle bundles.

The narrower and more uniform distal limb constitutes the pyloric part of the organ. It passes transversely, or with a slightly cranial inclination, toward the right body wall and ascends to terminate at the pylorus, caudal to the lower part of the omasum (Figure 28–4, D/15). The abomasum does not usually come into contact with the liver in adult cattle.

The abomasum of the sheep and the goat is relatively large. In contrast to the situation in adult cattle, it is usually allowed direct contact with the liver by the smaller size of the omasum.

The position and relations of the abomasum depend on the fullness of the different parts of the stomach, intrinsic abomasal activity, and, most importantly, the contractions of the rumen and reticulum to which the abomasum is attached. Age and pregnancy are also an influence (Figure 28–20). Although it is difficult to specify abomasal relations exactly, it is vital to appreciate that there are limits beyond which deviations produce digestive disturbance and may endanger life. Abomasal displacement, which may be to the right or left, is a well-recognized disorder, particularly in dairy cows (see further on).

The abomasum is lined by a pink, slime-covered, glandular mucosa that is in striking contrast to the harsh lining of the forestomach. At the omasoabomasal junction the epithelium changes abruptly to a simple columnar epithelium with occasional goblet cells. The lamina propria is less dense than that of the omasum, and frequently, solitary lymph nodules are observed at the junction with the epithelium. The mucosa of the abomasum has all the characteristics of that of the simple stomach (Figure 28–21, A-C). The area is increased about sixfold by the presence of almost a dozen large folds that arise around the entrance and course over the walls of the fundus and body before subsiding as the flexure is approached (Figure 28–22/2). Approximation of the proximal ends of these folds forms a mucosal valve or “plug” that discourages the reflux of ingesta into the omasum. The mucosa of the pyloric part is most remarkable for the large swelling or torus that projects from the lesser curvature to narrow the pyloric passage (Figure 28–22/6). The vascular arrangements within the torus suggest that it is capable of a form of erection, but the possible functional significance of this (and of the entire structure, for that matter) is unknown. The dark mucosa of the body and fundus contains true peptic glands; the glands of the lighter pyloric part secrete mucus alone.

The abomasal wall is relatively thin. The serous covering is deficient only at the attachment to the other stomach chambers and along the origins of the omenta. The muscle coat consists of longitudinal and circular strata. The longitudinal muscle is confined to the curvatures of the fundus and body but forms a thicker and wider covering for the pyloric part. The circular fibers provide a more complete layer that is better developed over the pyloric part, especially distally.

The movements of the adult abomasum are rather sluggish. They consist of general contractions of the proximal limb and more forceful peristalsis confined to the pyloric part. The latter activity often appears to be prompted by the tipping of the ingesta toward the pylorus when the fundic region is elevated by reticular contraction. It is possible that these normal alterations in position facilitate morbid displacements. Atony, with the accumulation of gas in the fundus, is a constant finding in these cases, and it may be that a slight initial displacement is worsened because this gas is denied its usual escape through the omasoabomasal opening when this comes to lie below the gas bubble.

Displacements are commonly related to the high proportion of concentrates to roughage in the ration of stabled cows, which leads to atony of the abomasum and accumulation of liquid ingesta and gas. Pregnancy may be a predisposing factor (Figure 28–20, C). Because the abomasum is well fixed proximally to the heavy omasum and distally by the lesser omentum, it is its middle part that travels farthest from its usual position on the abdominal floor. Contractions of the ruminoreticulum may allow the abomasum, buoyed by the gas within, to work its way under the atrium of the rumen and up on the left side. The loop formed by the middle part of the abomasum eventually comes to lie between the rumen and the left abdominal wall, deep to the last three or four ribs, where it can be identified by simultaneous percussion and auscultation (left displacement of the abomasum; LDA). In right displacement (RDA) the loop formed by the middle part of the abomasum slides to the right and lies between the right abdominal wall and the intestines and liver. Displacements to the right are often complicated by twisting of the loop. Treatment of uncomplicated displacements consists of returning the abomasum to its normal position by placing the cow on her back, by deflating the organ through a paramedian incision of the abdominal wall, and by including its muscular coat in the closing of the incision (abomasopexy).

THE OMENTA

The attachment of the greater omentum begins dorsal to the esophagus. The two serosal sheets of which it is composed pass directly onto the rumen but are so widely separated that the immediately postcardiac part of the rumen roof is enabled to attach directly to the abdominal roof (Figure 28–13/12). This retroperitoneal space is closed caudally where the two serosal sheets come together halfway along the right longitudinal groove to form a conventional duplicature attaching to the stomach. The attachment of this fold may be traced along the right longitudinal groove, through the caudal groove between the caudal blind sacs, and then forward along the left longitudinal groove. It now crosses the atrium ruminis and widens to make a broad attachment to the reticulum before bending sharply to the right, ventral to the ruminoreticulum, to reach the greater curvature of the abomasum (Figures 28–4, A,C and 28–8/8). It follows this to the pylorus and continues onto the caudal aspect of the first (vertical) part of the duodenum from which it extends onto the descending duodenum and later the mesoduodenum. The omental attachment is reflected where the duodenum turns cranially, and it retraces its attachment along the descending duodenum until carried back to the cranial duodenal flexure at the porta of the liver. It then returns to the right face of the rumen via the pancreas.

The lesser omentum arises from the visceral surface of the liver, between the porta and the esophageal impression (Figure 28–23), and passes to the region of the reticular groove, the right face of the omasum, and thence along the lesser curvature of the abomasum to the first part of the duodenum, which returns it to the liver (Figure 28–4, C).

The omental sheets enclose a space, the omental bursa, which is completely divided from the greater peritoneal cavity except at the epiploic foramen near the porta of the liver. The bursa is a mere capillary cleft in life, but it is simpler for descriptive purposes to envisage it as distended. A first impression of its topography may be obtained from the schema, in which it can be seen that the ventral sac of the rumen projects into it (Figure 28–24, B/6,2′). Of the omental sheets that run transversely across the abdomen, one lies against the abdominal wall and the other lies against the viscera (chiefly, the intestines) (Figure 28–13/3,4). The superficial and deep sheets pass into each other caudally and, in this way, close the bursa behind (Figure 28–24, A). The omasum, abomasum, and lesser omentum provide most of the cranial bursal wall. The entrance to the bursal cavity, the epiploic foramen, is situated dorsocranially between the liver and the duodenum or, more precisely, between the caudal vena cava dorsally and the portal vein ventrally.

The greater omentum is an important store of fat that is first deposited along the small vessels that ramify between the peritoneal layers; usually the fat is present in such large amounts that the whole omentum becomes thickened and opaque. (In many cows one such thickening forms a short offshoot near the pylorus known as “pig’s ear”; it can be palpated during surgery and marks the position of the pylorus.) The superficial sheet screens the ventral sac of the rumen from view when the lower left flank is opened, and both superficial and deep sheets intervene between the organs that lie ventral to the duodenum and the right flank (Figure 28–4, A,C). The intestines are closeted in the space above the bursa and to the right of the rumen, which is known as the supraomental recess (Figures 28–24/7 and 28–13/11); it is freely open behind and is often entered by the pregnant uterus.

INNERVATION AND VASCULARIZATION

The principal gastric nerves, parasympathetic efferent and afferent, run in the trunks formed along the esophagus by the regrouping of vagal fibers (see Figure 27–3/19,20). The sympathetic nerves that reach the stomach through periarterial plexuses have a subordinate role.

Section of both vagal trunks abolishes all motor activity of the forechambers. Section of the dorsal trunk alone results in almost complete but not necessarily permanent paralysis of the rumen, while the effect on the reticulum is generally less marked. The effects of division of the ventral trunk are unpredictable and range from little or no discernible change to almost complete paralysis of the forechambers. It is presumed that these inconstant results can be explained by differences in the regrouping of fibers where the vagus nerves combine to form the dorsal and ventral trunks and by the later assumption of part of these functions by association neurons in the stomach wall.

Abomasal contractions are greatly reduced after bilateral vagal section but are not wholly interrupted, possibly because some intrinsic control is vested in a submucosal nerve plexus present in the wall of this chamber alone. Division of the splanchnic nerves brings only slight alteration to the gastric movements. Clinically, disturbances of stomach function may follow involvement of the vagus nerves at any point along their courses from the brainstem; the most common causes are mediastinal infections and traumatic reticulitis.

The stomach is supplied with blood through several branches of the celiac artery. The large right ruminal artery runs caudally in the right longitudinal groove and continues into the left groove by passing between the dorsal and ventral blind sacs. It supplies most of the rumen wall and ends in anastomosis with the left ruminal artery, which follows the cranial groove (between atrium and ventral sac) to supply adjoining parts of the rumen and reticulum. The omasum and abomasum are supplied by the left gastric and left gastroepiploic arteries that follow their curvatures.

The veins are mainly satellite to the arteries. The left ruminal vein joins veins draining other chambers of the stomach; the right one, the veins leading from the spleen; their union produces a major radicle (splenic vein) of the portal vein.

Many small lymph nodes are scattered over the stomach, particularly in the ruminal grooves and over the omasal and abomasal curvatures. Lymph from the forechambers leads, after serial passage through these peripheral nodes, to a number of large atrial nodes situated between the cardia and omasum and thence to the visceral root of the cisterna chyli. The nodes placed along the abomasal curvatures direct their efferent vessels to the hepatic lymph nodes.

POSTNATAL DEVELOPMENT

At birth the ruminant stomach is prepared for the digestion of milk. The abomasum predominates and is remarkable not only for its size, which surpasses the combined capacity of the other chambers, but also for the degree of structural maturity that it has attained. Its full extent is apparent directly after the consumption of a generous feed, when it extends from the liver and diaphragm to the pelvic entrance, from one flank to the other, and from the floor well into the upper half of the abdomen (Figures 28–20, A, and 28–25/4). Its capacity may already exceed 60% of the adult measure. So large an organ inevitably impinges on almost all other abdominal contents, but only the extensive contact with the liver, which in the neonate reaches far across the median plane, need be mentioned. The abomasal mucosa is at first not quite mature, and a few days elapse before the fundic glands become fully active; presumably this is a provision to allow the absorption of unaltered antibodies from the colostrum.

In contrast to the abomasum, the rumen and reticulum of the newborn calf are very small. They are confined to the left dorsal and cranial corner of the abdomen and are generally found crumpled and collapsed (Figure 28–25/2,3); they are bypassed by milk feeds and normally contain only a small amount of fluid—secretions of the respiratory tract (swallowed in utero) in the youngest animals and saliva in those a little older. The omasum is also retarded in development and forms a relatively inconspicuous bridge between the reticulum and the abomasal fundus. The walls of the forechambers are thin and deficient in muscle, and while their mucosae possess the characteristic adult features, these are present in subdued form.

No striking changes in proportions and structure are to be observed before the young calf shows serious interest in solid food, generally from the time it is 2 or 3 weeks old. Thereafter the abomasum continues to increase at a slow but steady rate while the rumen and reticulum enter a period of spectacular growth. They have generally overtaken the abomasum by 8 weeks, and at 12 weeks they are more than twice as large. This unequal growth continues—but more slowly—until the time when the definitive topography and proportions are established. It is difficult to specify this age, for many variable factors are involved; some authors assert that the conformation is virtually adult after 3 months, but others believe that it does not become so until near the end of the first year.

Normal development depends on the availability of a normal diet of solid forage, but there remain some uncertainties concerning the precise stimuli that are involved. At one time it was thought that roughage not only stretched the stomach wall and stimulated its muscular growth but also promoted the differentiation of the mucosa. Later it was shown that many gross and microscopic features of the mucosa develop only with exposure to certain end products of microbial fermentation, notably butyric acid. Exposure to these stimuli must be continued for some time if development is to follow its normal course, and the return of a young, partly weaned calf to a wholly milk diet may result in the arrest and sometimes even reversal of the maturation processes.

The abomasum is initially the most vigorous chamber, but its activity diminishes as the raminoreticulum, first inert and then only spasmodically active, establishes a regular cycle of contraction by the second month. The feeding habits, the structural changes, and the motor and chemical activities of the stomach, when taken in conjunction, define three phases of development. A neonatal period, in which milk forms the sole diet, may last for 2 or at most 3 weeks and may be followed by a transitional period when the stomach is adapting to solid food. From the eighth week onward the anatomy and the processes of digestion may be essentially those of the adult. The chronology will clearly be different in dairy and suckler calves.

Changes in abdominal topography are not confined to the stomach. In the newborn the liver is relatively large and lies across the midline, extensively related to the abomasum. As the rumen and reticulum increase in size the liver is pressed toward the right and dorsally, and it rotates so that its left lobe comes to lie cranioventral to the right one and out of the reach of the abomasum. The intestines are simultaneously pushed away from the left flank and become confined to the right side; the expansion of the dorsal ruminal sac also displaces the left kidney, thrusting it across the midline until it comes to rest below and caudal to its fellow (Figures 28–11/9 and 29–9/10).