While dissecting the abdominal wall it is important to realize some of the demands it is placed under, especially in the large herbivore. The floor of the abdomen is not supported by any bony structure and all of the weight of the abdominal contents (several hundred pounds in the horse an ox) are held in a sling like fashion. This has lead to many of the muscles found in the abdominal area to develop large areas of aponeurotic insertion and the incorporation of a strong band of elastic connective tissue into the construction of the abdominal wall (the tunica flava abdominis).

Begin your dissection in the paralumbar fossa, which is more pronounced in the large ruminant than the horse. The dissection will occur on both right and left sides. At this time both sides of your animal should be skinned all the way to the tuber coxae. Before beginning the dissection of the abdominal wall find the subiliac lymph nodes located in the flank cranial to the stifle joint. On one side try the instructions described in bullet point 1 below and than follow to point 2 for both sides.

1. This is similar to the approach that is commonly used to gain surgical access to the abdominal viscera (especially in the ox). Make a vertical incision in the middle of the paralumbar fossa. Cut through the external abdominal oblique noticing both its fiber direction and thickness in this area. Next continue the incision through the internal abdominal oblique noting both its fiber direction and thickness in this area. The next muscle layer to be encountered is the transverses abdominis note its fiber direction and thickness in this area (it is a thin layer so incise it carefully to leave the peritoneum intact. Finally the tough, translucent layer of peritoneum is visible. In a surgical setting this is often broken bluntly to avoid inadvertently cutting viscera. To close this incision the peritoneum and transverses abdominis are commonly sutured together as one layer, the internal abdominal oblique (layer 2 of closure), the external abdominal oblique (layer 3 of closure), followed by the skin. This is why it is important to be able to tell the layer by the direction of the fibers. To gain full access to the cavity for dissection purposes you will now need to proceed to step 2.

2. Begin by cutting the attachments of the external abdominal oblique muscle from the tuber coxae and the ribs. This will allow you to reflect this muscle ventrally. Next, reflect the internal abdominal oblique ventrally by freeing its attachments to the last rib and tuber coxae. This will expose the transverses abdominis muscle and the ventral branches of spinal nerves T13-L4 in the ruminant and T18-L4 in the horse. Both the internal and external abdominal oblique muscles should be reflected far enough ventrally to demonstrate their contribution to the external rectus sheath. Carefully reflect the transverses abdominis and than the peritoneum ventrally as well. At this time it is important to study the location of the organs and how they relate to the body wall. Be sure that everyone in your area has seen this before continuing!

Before continuing with dissection, make sure that everyone in your area has seen the organ relationships to the body wall.