Part 3: Nerves and Vessels

On the left side of the animal, transect the costal (muscular) part of the diaphragm along the ribs and reflect it off of the ribs toward the liver. Transect the left crus (muscular lumbar part) of the diaphragm at the esophageal hiatus and reflect it toward the liver. Reflection of the left half of the diaphragm in this manner allows you to observe the continuation of the vagal trunks and to trace the sympathetic trunk on the left side. (See DG Fig. 4-27)

On the left side, within the thorax, use blunt dissection to re-identify and trace the thoracic part of the sympathetic trunk (also identified in Chapter 4). Recall that the sympathetic trunk is found dorsally, inside the chest, running longitudinally along the ventral surface of the necks of the ribs and is present on both left and right sides. (See DG. Figs. 3-20 and 4-27)

• Trace the left sympathetic trunk caudally; just beyond the level of the diaphragm, identify some of the splanchnic nerves.

  • Dissection Note: There are various splanchnic nerves: major, minor, and lumbar. The various types can be observed on the demonstration specimens, if visible.

    • • Near the cut left crus, the major splanchnic nerve can be observed crossing the ventral surface of the adrenal gland on its way to the celiacomesenteric ganglion and plexus (this same arrangement is present on the right side of the animal as well). (See DG Fig. 4-27)

      • There are generally one or two minor splanchnic nerves given off just caudal to the major splanchnic nerve. These also contribute to the celiacomesenteric plexus.

      • In the lumbar region, there are very small lumbar splanchnic nerves that contribute to various ganglia and plexuses, including the caudal mesenteric ganglion and plexus.

        Related Learning Objective

        • D5.8

         

On the left side, within the abdomen, just caudal to the diaphragm, locate the visible portion of the descending aorta. Note the two major vessels coming off of the aorta in this region (the celiac and cranial mesenteric aa., which will be dissected next). Observe the intricate web of nervous tissue covering these large vessels; this is the celiacomesenteric plexus & ganglia viewed from the left side. Use blunt dissection to carefully reveal this region. (See DG Fig. 4-27) Note that you would also have the same arrangement, with the celiacomesenteric plexus & ganglia, viewed from the right side.

• Comment: This center of nervous tissue is made up of the component parts listed below (but you need not identify the individual parts):

  • left celiac ganglion, right celiac ganglion and celiac plexus

    • • The celiac ganglia (recall that ganglia are enlargements of the nervous tissue containing neuron cell bodies) lie on the left and right surfaces of the celiac artery, very near its origin from the aorta. They are interconnected and have numerous nerves branching from the ganglia as the celiac plexus (plexus refers to the many observed nerve fibers).

  • cranial mesenteric ganglion and cranial mesenteric plexus

    • • The cranial mesenteric ganglion (enlargement of the nervous tissue) is just caudal to the celiac ganglia and wraps around the caudal surface of the cranial mesenteric artery in a “horseshoe” shape. This ganglion is connected to the celiac ganglia and also has numerous nerves branching from it as the cranial mesenteric plexus.

  • All together, the celiac plexus + celiac ganglia + cranial mesenteric plexus + cranial mesenteric ganglion = the celiacomesenteric plexus & ganglia.

    Related Learning Objective

    • D5.6

     

Just caudal to the diaphragm (near the aortic hiatus), identify the celiac a. leaving the aorta ventrally to supply the abdominal viscera. The celiac artery is very short and has 3 major branches you should identify: hepatic a., left gastric a., and splenic a.  Generally speaking, the celiac artery supplies the more cranial regions of the gastrointestinal tract including the liver, gall bladder, stomach, and spleen as well as part of the duodenum and pancreas. (See DG Figs. 4-27, 4-28, and 4-29)

• The hepatic a. is the first branch from the celiac a. Identify its origin as it courses toward the liver.

  • Comment: Near the dorsal aspect of the pylorus (of the stomach) the hepatic a. will give off several branches and then terminate as the right gastric a. and gastroduodenal a. (See DG Fig. 4-29)

    • • The right gastric a. is a small artery that extends along the lesser curvature of the stomach from the pylorus toward the cardia. (It anastomoses with the left gastric a. along the lesser curvature.) (See DG Fig. 4-29)

      • The gastroduodenal a. is a short branch from the hepatic a. that splits into two terminal branches, the right gastroepiploic a. and the cranial pancreaticoduodenal a. (See DG Fig. 4-29)

        • • The right gastroepiploic a. runs along the greater curvature of the stomach in the greater omentum. (It anastomoses with the left gastroepiploic a. – a branch from the splenic a. – along the greater curvature.) (See DG Fig. 4-29)

          • The cranial pancreaticoduodenal a. courses caudally alongside the descending duodenum and right lobe of the pancreas within the mesoduodenum. (It anastomoses with the caudal pancreaticoduodenal a. – a branch of the cranial mesenteric a.) (See DG Fig. 4-29)

• Return to the celiac a. and identify the origin of the left gastric a. branching from it.

  • Comment: The left gastric a. runs to the lesser curvature of the stomach, near the cardia, where it follows the lesser curvature toward the pylorus to anastomose with the right gastric a. as previously mentioned. (See DG Fig. 4-29)

• Return to the celiac a. and identify the splenic a. branching from it and coursing toward the spleen.

  • Comment: The splenic a. will branch several times and enter the visceral surface of the spleen. The splenic a. will also give off some short gastric arteries, pancreatic branches, and the left gastroepiploic a. (aa.)(all of which you need not identify).

    Related Learning Objective

    • D5.6

     

Return to the abdominal aorta. Just caudal to the celiac a., identify the cranial mesenteric a. leaving the aorta ventrally to supply the abdominal viscera. The cranial mesenteric a. has many branches supplying the abdominal viscera which we will not dissect, but will be covered in the Application portion of the course and commented on below. However, you should identify the jejunal aa. as they are readily identifiable within the mesentery. Generally speaking, the cranial mesenteric a. supplies the small and large intestine regions of the gastrointestinal tract, including the duodenum (and part of the pancreas), jejunum, ileum, cecum, and colon (ascending, transverse, and descending). (See DG Figs. 4-28, 4-29, and 4-31)

• Dissection Note: In the dog, the first branch from the cranial mesenteric a. is often a ‘common trunk’ that will give rise to the middle colic, right colic and ileocolic aa. described below. However, in the cat, only the separate branches are usually present (i.e., the cat does not usually have a common trunk). (See DG Fig. 4-31)

• The middle colic a. is typically the first branch leaving the cranial mesenteric a. (or common trunk if there is one). (See DG Figs. 4-29 and 4-31) The middle colic a. courses through the mesocolon along the transverse colon to the left colic flexure and descending colon. This branch of the middle colic will anastomose with the left colic a. from the caudal mesenteric a. (See DG Figs. 4-29 and 4-31) Another branch from the middle colic a. passes to the right side along the transverse colon and will anastomose with a branch from the right colic a.

• The right colic a. is typically the second branch leaving the cranial mesenteric a. (or common trunk if there is one). (See DG Figs. 4-29 and 4-31) The right colic a. courses through the mesocolon to the right colic flexure where it will give branches to both the transverse and ascending colon. (These will anastomose with branches from the middle colic and ileocolic aa. respectively)

• The ileocolic a. is typically the third branch leaving the cranial mesenteric a. (or common trunk if there is one). (See DG Figs. 4-29 and 4-31) The ileocolic a. will give off several branches that supply the ileum, cecum, and ascending colon.

  • Dissection Note: In the cat you may see a variation of the ‘common trunk’ from the cranial mesenteric a. that gives off both the right colic and ileocolic aa.

• The next branch from the cranial mesenteric a., after the ileocolic a. is the caudal pancreaticoduodenal a. The caudal pancreaticoduodenal arises from the cranial mesenteric a. just distal to the common trunk (or the separate branches supplying the colon). (See DG Figs. 4-29 and 4-31) and runs alongside the descending duodenum in the mesoduodenum. (As mentioned previously, this artery anastomoses with the cranial pancreaticoduodenal along the descending duodenum.)

• The cranial mesenteric gives off numerous jejunal arteries that cascade caudally through the mesentery to supply the jejunum. Identify a few of these readily visible arteries in your specimen. (See DG Figs. 4-29 and 4-31)

• The cranial mesenteric a. terminates as the ileal arteries that supply the ileum; these terminal branches course through the mesentery toward the ileum. (Note that the final branch anastomoses with the mesenteric ileal branch from the ileocolic a.) (See DG Fig. 4-31)

Move further caudally, following the abdominal (descending) aorta. Identify the origins of the renal arteries (left and right) supplying each respective kidney (see DG Figs. 4-12, 4-22, 4-27, and 4-31) and the testicular aa. (male) (left & right) or ovarian arteries (female) (left & right) (dependent on the sex of the animal). Also, note the paired lumbar arteries branching off the dorsal aspect of the descending aorta as you continue caudally. (See DG Figs. 4-27 and 4-28)

• Dissection Note: The testicular/ovarian arteries may be small and easily broken, so dissect with care. (See DG Figs. 4-12, 4-27, 4-28, 4-29, and 4-31)

• Females: Attempt to trace at least one of the ovarian aa. to the respective left or right ovary (if present).

• Males: Attempt to trace at least one of the testicular aa. to the region of the left or right vaginal ring.

• Comment: Note that you will see ovarian/testicular veins that satellite these arteries.

Continue tracing the descending aorta until you come to an unpaired branch from the aorta, which is the caudal mesenteric artery. Identify this artery, and wrapped around the artery, identify the caudal mesenteric plexus & ganglion. The ganglion will appear to be a small enlargement in the nerve and will have numerous nerves branching from it as the plexus. (See DG Fig. 4-27)

Branching from the caudal mesenteric ganglion caudally, identify the left & right hypogastric nerves. These run in the mesocolon and enter the pelvic canal to connect with the pelvic plexus on either side (left and right).

• Dissection Note: Be careful as you trace the hypogastric nerves, they tend to break easily!

  Related Learning Objective

  • D5.6

   

Return to the caudal mesenteric artery and trace it through the mesocolon to identify its two terminal branches, the left colic and cranial rectal aa. Generally speaking, the caudal mesenteric a. supplies the terminal portion of the gastrointestinal tract, the descending colon and rectum.

• Identify the left colic a. as it courses cranially in the mesocolon along the descending colon.

  • Comment: As previously mentioned, this artery anastomoses with the middle colic artery.

• Identify the cranial rectal a. coursing caudally along the terminal colon and rectum.

  • Dissection Note: The cranial rectal a. may be difficult to trace distally as it courses through the pelvic canal.

Identify the (paired, left & right) deep circumflex iliac a. arising from the abdominal aorta near, or just caudal to, the caudal mesenteric a. The deep circumflex iliac aa. will supply the skin of the caudal abdomen, the flank and the cranial thigh. (See DG Figs. 4-12, 4-27, 4-28, and 4-33)

Identify the abdominal region of the caudal vena cava coursing through the abdomen alongside the abdominal aorta. (See DG Figs. 4-12, 4-27, and 4-28)

Reflect the caudate process of the caudate lobe of the liver cranially. Near the cranial duodenal flexure, identify the portal vein in the hepatoduodenal ligament. Note that the portal vein WILL NOT contain blue latex, instead it will simply be very dark in color and congested with clotted blood. The portal vein carries venous blood to the liver from the abdominal viscera, including blood from the stomach, small intestine, cecum, colon, pancreas, and spleen. (See DG Figs. 4-17 and 4-34)