Lab Instructions

For this lab, you will be studying instructor prosections and wet and dry specimens around the lab. Please refer to these materials when reading this guide and be sure to use a diversity of materials in your studies. Because these materials are limited, please feel free to jump to different sections of this lab guide to make the most of your time in lab. This guide is organized by species, allowing you to quickly scroll from section to section rather than having to read through text regarding a species that you may not be standing at.

External Topography and Landmarks

The lateral part of the abdominal wall that is not protected by the rib cage, pelvis, or thigh is called the flank. In the horse, the last rib is very close to the tuber coxae, leaving only about one hand’s-width space in the upper flank. Surgical intervention in this area is therefore limited in the horse, however laparoscopy is readily performed via the flank. The ruminant has a large flank area, and this allows for convenient access to the abdominal cavity.

Paralumbar Fossa: This triangular depression in the upper part of the flank is especially prominent in the standing domestic ruminant, in which it is an important surgical area. It is bound dorsally by the transverse processes of lumbar vertebrae L2 to L5, cranially by the last rib, and caudoventrally by the tense band or ridge of the ventral part of the internal abdominal oblique (IAO) m., created by the weight of the abdominal viscera. In the living horse and pig, the paralumbar fossa is small in size and often not a fossa at all, so it is less apparent.

The flank fold is a skin fold that extends from the caudoventral abdominal wall to the craniomedial aspect of the thigh near the stifle. The cutaneus trunci m. is contained within this fold, which accounts for the active twitching of the fold, especially in the horse (remember the cutaneous trunci m.??). The fold can also be used as a handhold in the restraint of calves and small ruminants- hence, the term “flanking a calf down.”

Review of Peritoneum

The peritoneum is the serous membrane that lines the walls of the abdominal cavity, extends partially into the pelvic cavity, envelops the descended testes of the male, completely or partially covers all organs in the abdominal cavity, and forms multiple connections among structures. Note that the word is singular- it is one continuous membrane that does all the above. Anatomists apply regional names to specific parts of this membrane. Here is the way Dr. Smallwood “outlines” the peritoneum:

• Parietal Peritoneum– lines the walls of the abdominal cavity; extends into the pelvic cavity, and outside the cavity in the male (and bitch) through the inguinal canals (derives from somatic mesoderm).
• Visceral Peritoneum– closely invests an organ’s surface; adherent to that surface (derives from splanchnic mesoderm).
• Connecting Peritoneum– double folds of peritoneum that connect things:
  • Mesentery – connects the bowel to the body wall (e.g., mesoduodenum, mesocolon); in the broader usage, connects some other organ to the body wall (e.g. mesovarium).
  • Omentum – attaches the stomach to something; the greater omentum connects the stomach to the body wall (embryologically it was the dorsal mesogastrium), and the lesser omentum attaches the stomach to the liver (embryologically, part of ventral mesogastrium).
  • Fold – connects different parts of the bowel or another organ to one another (e.g., ileocecal fold, genital fold).
  • Ligament – connects organs other than the bowel to the body wall or to the bowel (e.g., broad ligament, falciform ligament [liver to body wall, seen in calf], gastrosplenic ligament).

Review of the Embryological Development of the Gut

Embryologically, the stomach primordium and proximal duodenum are attached to the dorsal and ventral body walls by the dorsal and ventral mesogastria. The liver primordium grows ventrally from the duodenum and develops between the layers of the ventral mesogastrium. That part of the ventral mesogastrium between the liver and the body wall becomes the falciform ligament, and that part between the stomach and liver becomes the lesser omentum.

The dorsal part of the stomach primordium grows faster than the ventral part, thus creating greater (dorsal) and lesser (ventral) curvatures of the stomach. As the greater curvature (dorsal part) rapidly expands, it becomes “top- heavy” and “flops over” to the left side, carrying with it the dorsal mesogastrium, which becomes the greater omentum.  After it flops over, the greater curvature, which now faces to the left, continues to grow faster so that the stomach now rotates around a dorsoventral axis, carrying the pylorus to the right and cranially.

In the ruminant, continued expansion and modification along the proximal part of the greater curvature, now facing the left, results in the formation of the rumen and reticulum. Similarly, an outgrowth and modification near the lesser curvature results in formation of the omasum. Knowing this, it makes sense that, in the adult animal, the greater omentum is closely associated with the rumen and reticulum and greater curvature of the abomasum, whereas the lesser omentum is related to the omasum and lesser curvature of the abomasum. Conversely, the attachments of the greater and lesser omenta in the adult animal mark the embryonic greater and lesser curvatures of the stomach.

The spleen develops in the dorsal mesogastrium of the embryo. Thus, it makes sense that the spleen is associated with the greater curvature (and greater omentum) of the stomach in the adult. That part of the greater omentum between the stomach and the spleen is referred to as the gastrosplenic ligament. In the ruminant, the gastrosplenic ligament has been replaced by an area of direct attachment to the body wall. This is a consequence of the massive expansion and direct contact of the ruminant stomach (specifically the rumen and reticulum parts, commonly referred to as the ruminoreticulum) to the craniodorsal abdominal wall, such that the peritoneum receded back out of the area, leaving a direct adhesion instead of a mesenteric attachment. Also, as a result of the expansion of the ruminoreticulum, the spleen of the ruminants was separated away from its embryonic association with the greater omentum.

A few comments on the omenta: In those ungulates with a simple stomach (horse and pig), the omenta remain relatively simple, whereas in the ruminant, the stomach and the greater omentum are more complex. In the horse and the pig, the greater omentum, like that of the dog/cat, consists of a simple peritoneal fold that extends from the greater curvature of the stomach to the dorsal body wall. The superficial wall of the greater omentum extends caudally a variable distance from the greater curvature before turning back on itself as the deep wall, which continues cranially, then dorsally, to the body wall. Between the superficial and deep walls is an enclosed part of the peritoneal cavity known as the omental bursa. Commensurate with its small stomach, the greater omentum of the horse is not really so great (poorly developed) and does not typically extend back to cover the intestinal mass. It is interesting to note that, among our common domestic mammals, the horse has the most poorly developed greater omentum and is most susceptible to peritonitis. One recognized function of the greater omentum is that it quickly adheres to any area of peritoneal insult and seals it off. Its vessels then serve as an avenue of attack for inflammatory cells to combat any invasion of the peritoneal cavity by foreign material (e.g., bacteria, intestinal leakage). Have you surmised? Yes, ruminants, with their spectacularly robust greater omentum, also have relatively excellent ability to combat peritonitis and to “wall off” focal areas of insult.

OK, end of theory – let’s apply it, but not a bad idea to refer back frequently, particularly to the figures!

Lindsey and Dr. Gallenstein created this stunning chicken wire, paper mache, fiberglass and fishnet piece of art in 2018 after seeing students struggle with understanding the attachments and arrangement of the ruminant greater omentum. This model is intended to help you transfer that understanding to the cadavers, NOT to be testable material on this model. 

Omentum and Associated Structures

Omentum of the Calf

The attachments of the greater omentum of the ruminant can best be understood by first studying that of the newborn calf.

Thus, the ventral sac of the rumen is enveloped by the omental bursa, but it is not within the bursa, because the bursa is a potential space between the two walls of the greater omentum. The ventral sac is covered by visceral peritoneum, which is continuous with the greater omentum along its line of attachment. The relationship of the ventral sac of the rumen to the omental bursa is analogous to the relationship of the heart to the pericardial sac. The deep and superficial walls come back together in the cranial groove of the rumen, and both walls continue along the greater curvature of the abomasum.

Congratulations, you have just found the epiploic foramen, and you now have your finger in the omental bursa. As you can see, the peritoneal cavity proper communicates with the omental bursa through this opening.

Omentum of the Goat

Surgeons manipulate this fold to allow the organ of interest (e.g., uterine horn) to slip out of the supraomental recess, and then release the fold to “sack up” and keep contained wandering loops of bowel.

On the right side, an incision was made about 5 cm ventral to the mid part of descending duodenum. Look through this window at the exposed deep wall. The omental bursa is opened when this window is created in the superficial wall.

Now move around to the left side, and note the attachment of the superficial wall along the left longitudinal groove of the rumen. Again, locate the caudal fold and trace it back toward the right side of the animal. A better understanding of the omental attachments can be gained by reference to a transverse schematic drawing.

The Stomach

In addition to the instructor prosections, please also refer to plastinated, dry and wet models of stomachs. You will be expected to be able to identify these structures in any of these contexts.

Based on what in the lab is available to you at the moment, feel free to jump to the section of any species and come back to the others when you can.

Ruminant Stomach (Calf and Goat)

The ruminoreticulum (i.e. the rumen and reticulum compartments), occupies the entire left half (and more) of the abdominal cavity of an adult ruminant. It is difficult to observe all the grooves and other features in a large animal, but this can easily be accomplished in the newborn calf. These potential ruminants (physiologically not fully developed to ruminate yet) have all the right parts, they just haven’t gotten big and bulky yet.

The following structures described below are found inside the ruminant rumen.

The grooves on the outside of the rumen correspond with the pillars on the inside, which carry the same designations. The ruminal pillars represent in-folding and thickening of the muscular wall.

The recess of the rumen is the part of the ventral sac below the cranial pillar. The cranial sac is relatively smaller in the sheep and the goat. Although a caudodorsal blind sac is usually described in small ruminants, the absence of dorsal coronary pillars in the sheep and the goat makes this a poorly defined area in these animals. It is because the ruminoreticular opening is so large that these two compartments are often considered together, both anatomically and physiologically, as the ruminoreticulum.

The non-glandular, stratified squamous mucosa of the rumen is characterized by the presence of closely packed, tongue-shaped ruminal papillae, which give it the appearance of Astroturf (or “gastroturf”). The papillae vary in shape and number throughout regions of the rumen. The reticulum mucosa has a very distinct honey-comb pattern.

The cardia is located in the reticulum, not in the rumen, and therefore the esophagus actually enters the reticulum. The momentum of the coarse, light material that is propelled through the cardia with reasonable velocity undoubtedly clears the ruminoreticular fold and enters the rumen. On the other hand, heavy objects such as nails, spark plugs, or flashlight batteries more probably fall out of the cardia into the reticulum, where they may reside for life. Sharp objects may also penetrate the wall of the reticulum and cause traumatic reticulitis and may continue to migrate into liver, or through the diaphragm into lungs, pericardial sac and even the heart (traumatic reticulopericarditis).

Note the distinct groove, the reticular groove, beginning at the cardia and spiraling down the reticular wall, to lead to the reticulo-omasal opening. This groove is the first part of the gastric groove. Do not confuse the ruminoreticular groove, an external feature of the ruminant stomach, with the reticular groove, an important internal structure.

The reticular groove plays an important role in the suckling animal and thus the calf should receive special attention here. When the young ruminant suckles milk, it initiates a strong (vagal) reflex closure of the muscular lips of the reticular groove, converting it from a groove to a tube. Simultaneously, there is dilation of the reticulo-omasal opening and omasal groove so that, ideally, the milk completely bypasses the forestomach and is delivered directly to the abomasum, where the milk-digesting enzyme rennin is secreted.

Note the normal position of the body of the abomasum and observe that it is not firmly fixed in this position (i.e. can readily be displaced). Then look for a removed section of the wall of the abomasum.

In the newborn calf, coagulation of the milk results from the action of the enzyme rennin, which accounts for a common name for the abomasum, the rennet. Calling it the “true stomach” is a tad ridiculous, because there is only one stomach per animal and the other parts are real too.

Ruminant stomach clinical case

Ruminant emergency clinical case

Ruminant stomach mucosa

Horse Stomach

The fundus is enlarged to form a saccus cecus (blind sac), which is lined by a non-glandular, stratified squamous epithelium. The junction of the non-glandular (squamous) and glandular regions is marked by a ridge, the margo plicatus. The larvae of the common botfly, Gasterophilus intestinalis, may be found attached to the stomach wall. The glandular part of the stomach is divided into three regions.

Not so easy, right? The horse has a very strong cardiac sphincter. This sphincter, combined with the strong muscular tunic of the more distal part of the esophagus, makes it practically impossible for the horse to vomit or even eructate (belch). Excessive, and quick, ingesta/gas accumulation in the equine stomach can cause complete rupture of the organ! (a life-threatening problem that often leads to death/euthanasia of the horse).

From the body of the stomach, the organ narrows distally to form the pyloric part of the stomach (first the pyloric antrum, which then leads to the pyloric canal). At the end of the pyloric part of the stomach, is the pyloric sphincter (aka pyloris), which is well developed in the horse.

The Pig Stomach

The most obvious modification of the pig stomach is the small out-pocket at the apex of the fundus, termed the gastric diverticulum.

The non-glandular part of the stomach is small and is represented by a narrow strip of stratified squamous epithelium that extends from around the cardia into the gastric diverticulum, anatomically the pars cardiaca, and most commonly called the pars esophagea in the literature/clinically. This non-glandular area is where gastric ulcers typically occur in pigs. Gastric ulcers are a common problem in the modern swine-production industry, and can result in significant economic loss to the producer. The glandular regions of the stomach are large and can be distinguished in part by mucosal color and prominence of mucosal folds.

Internally, at the pyloric sphincter note the musculofatty protuberance from the lesser curvature, the torus pyloricus. With the surrounding circular muscle fibers, the torus pyloricus forms a sort of ball-and-seat valve for more efficient, watertight closure of the pylorus (similar to ruminants).