Left side: Left dorsal and ventral colon, pelvic flexure, spleen, kidney, small intestine and descending colon. Right side: Cecum, right dorsal and ventral colon, liver, kidney, small intestine (descending duodenum) and descending colon.
It is important to understand the orientation of the equine ascending colon and all of the flexures that are associated with it. Also note the location of the cecum and its in situ orientation.
Left side: Rumen, reticulum, greater omentum, and spleen Right side: Greater omentum, small intestine (descending duodenum) spiral colon, cecum, abomasum, liver
In the adult ruminant the left part of the abdominal cavity is dominated by the rumen. The caudodorsal blind sac should be noted in the area of the paralumbar fossa (this sac is poorly developed in the small ruminant). The ventral sac and the caudoventral blind sacs of the rumen are with in the omental bursa. The superficial leaf of the greater omentum can be seen covering these sacs on the left (it is the heavily fat laced tissue covering the ventral half of the rumen). On the right, the picture is different. The first thing that you should note is a wall of fat laden tissue with a loop of bowel coursing through it ? this is the descending duodenum. Ventral to the descending duodenum the fat laden tissue is the superficial leaf of the greater omentum and dorsal to it the mesoduodenum. The majority of the abdominal viscera can be found in the omental recess (dorsal and medial to the deep leaf of the greater omentum). To gain access to the omental recess slide your hand caudally ? this will be more difficult in the goat.
We see many modifications in the gastrointestinal tract, with most of these being dictated by the animal?s diet. Carnivores have the ?simplest? GI tract when we compare it to other species. This should make some sense when we realize that their diet is based on meat which is easily digested by mammals. The most ?complex? GI modification occurs in the herbivore, which is dependent on microbial fermentation to digest a large portion of its food. Among the herbivores, two basic patterns have evolved to allow for this to happen, and we can divide them into foregut and hindgut fermenters.
The ruminant is an example of a foregut fermenter. The stomach has evolved into a multi-chambered structure to allow for mixing, fermentation, buffering and volatile fatty acid (VFA) absorption. The rumen and reticulum can be discussed together from both an embryological and functional standpoint. The ruminoreticular complex is a non-glandular portion of the stomach and the site for most of the mixing, fermentation, buffering and a majority of the VFA absorption. The omasum is another non-glandular portion of the stomach and is joined to the reticulum. Within the omasum many lamina present are present and these increase the surface area of the lumen allowing for more absorption of water and VFA. The last compartment of the stomach is the abomasum, which is the only glandular compartment of the ruminant stomach, and connected to the omasum. In addition to the compartments of the stomach, the ruminant also has modifications in the ascending colon and some in the cecum.
The horse is an example of a hindgut fermenter. The cecum and ascending colon of the horse have become greatly elaborated to allow for fermentation. The cecum of the horse measures approximately 3 feet in length and it extends from its base in the right paralumbar fossa region ventrally and medially to the apex ending inches caudal to the xiphoid cartilage. The ascending colon has been modified in such a way that it courses from the right half of the abdomen cranially to the sternum where it turns to the left, courses caudally towards the pelvic inlet, reflects on to itself and course cranially to the diaphragm and once again turns to the right (it sounds confusing but once you see it laid out it is not that bad).
Dissection:Once everyone has been oriented to the normal topography of the viscera continue your dissection. For both the ruminant and the horse it will be necessary to empty the gastric contents. Horse: To open the stomach, make an incision near the greater curvature near the attachment of the greater omentum. Carefully remove the ingesta from the stomach, flush the inner wall of the stomach with water. Ruminant: Make an incision in the caudal portion of the dorsal sac of the rumen (ie the area near the left paralumbar fossa) and remove the ingesta from the stomach.
Horse:The stomach of a horse is surprisingly small for an animal of its size (approximately 3 gallons) and it does have a glandular and a non-glandular portion but they are not compartmentalized. However, they are divided by a ridge known as the margo plicatus.
Ruminant:The incision made in the rumen will allow access to the entire rumen and reticulum. Compare the mucosa of the dorsal and ventral sacs of the rumen. Note that in the extreme dorsal portion of the dorsal sac the papillae are blunted and not as prominent compared to the ventral sac (you should know why this is). Also, observe all of the grooves and pillars associated with the rumen. The reticulum is characterized by a pattern of ridges that form 4-6 sided cells. In the reticulum, observe the reticular portion of the gastric groove.
Dissection:In the horse, incise the base of the cecum and to remove the cecal contents. Note how the ileum joins the base of the cecum and also find the cecocolic ostium. It is not necessary to do this in the ruminant, but it is important to note that the ileum joins the cecum and colon at their junction.
Learn the pattern of banding on the equine colon and observe the modifications in the ascending colon across species.
Observe the liver in situ. Caudal vena cava, portal vein, hepatic artery, bile duct, lobes and renal recess are of importance. Once this is done remove the liver. Transect the caudal vena cava, hepatic artery and portal vein, and bile duct. Try to find the round ligament located between the left and quadrate lobes of the liver ? attempt to leave a stump of this with the liver. Next free the liver from the diaphragm or remove a portion of the diaphragm with the liver.
If you have a male animal attempt to the find the vaginal ring in the caudodorso lateral area of the abdominal wall.