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Procedure(i) Pin the rat to a dissecting board, ventral surface upwards and head pointing away from you. Make a mid- ventral incision through the skin and cut forward as far as the lower jaw, and backwards to the anus. Cut either side of the urino-genital openings as shown in Fig. 9.1. Free the skin from the underlying body wall.

(2) Pin back the skin and cut through the body wall as shown in Fig. 9.2.

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Identify all the structures shown in Fig. 9.2.

(3) Wet your fingers so they slide easily between the organs. Look between the stomach and the liver and find the lower end of the oesophagus where it joins the stomach. Using your fingers to move the organs this way and that, and without cutting the mesentery by which the gut is suspended in the abdominal cavity, follow the alimentary canal in the abdomen from oesophagus to rectum.

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Identify stomach (note spleen clinging to stomach),, duodenum, ileum, caecumwith short appendix, colon and rectum. The duodenum and ileum together make up the small intestine the colon and rectum make up the large intestine.

(4) Push the liver upwards, the stomach to your right and the ileum to your left. Deflect the duodenum downwards towards you as shown in Fig. 9.3. This will enable you to see:the pancreas within the loop of the duodenum, and the bile duct (white tube) running from liver to duodenum (there is no gall bladder in the rat).Numerous small pancreatic ducts open into the bile duct as it runs towards the duodenum. Running inthe same mesentery as the bile duct is the main trunk of the hepatic portal vein which conveys blood from the gut to the liver.

(5) Without breaking the mesentery, spread out the ileum to your left and notice numerous branches of the hepatic portal vein in the mesentery (Fig. 9.4).Running alongside the hepatic portals are branches of the anterior mesenteric artery but these are generally obscured by fat.

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(6) With the liver pushed forward, deflect the whole of the stomach and intestines to your left and stretch the mesentery. Running in the mesentery are three median (unpaired) arteries to the gut (Fig. 9.5): coeliac, anterior mesenteric, and posterior mesenteric arteries. All three are branches of the dorsal aorta, the first two arising at the level of the kidneys, thc last where the aorta splits into the leg arteries.Pluck away the fat clinging to th proximal end of these three arterieS to show their origin from the dors. aorta. Trace them to their destinationS What structures do they serve?

(7) Open the mouth and examine th buccal cavity. What special adaptation are shown by the teeth?

(8) Arrange the viscera in such a way as to display the whole of the alimentary canal and its blood supply:maximum advantage.

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(9) If the rat is to be used for furthcr dissection, remove the alimentary canal as follows. Ligature the main trunk of the hepatic portal vein, cut through the oesophagus where it enters the stomach, and cut through the rectum where it disappears undcr the urino-genital organs. Cut through the suspensory mesenteries so as to remove the whole of the gut, spleen. and pancreas. Leave the liver.

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