DISSECTION OF THE HEART. Remove the heart together with the ventral aorta from the body and fasten it, dorsal side up, under water. Open the sinus venosus with scissors, wash it out, and observe the vertical slit-like opening into the auricle and the two membraneous valves which guard it. Continue the cut through the _sinu-auricular aperture_ along the median dorsal line of the auricle; observe the thin walls of the auricle and their strengthening by an irregular mesh of muscles, the _musculi pectinati_; the shape and position of the _auriculo-ventricular aperture_; the flaps of the _auriculo-ventricular valve_. Press upon the sides of the ventricle and, if possible, observe the mode of action of the valve. Cut across the ventricle from the auriculo-ventricular aperture. Carry another incision from this along the dorsal side of the conus arteriosus. Observe the small size of the cavity of the ventricle, the thickness of its walls, and the projecting network of muscles, the _columnae carneae_, some of which are attached to the edges of the auriculo-ventricular valves. In the conus arteriosus observe the rows of three pocket-like valves each around the proximal end (_semilunar valves_), and a single row of three similar but larger valves at the junction of the conus and ventral aorta. There is some variability in the number of rows of valves in the conus of Squalus; there are always three rows of three valves each in that of Eugaleus. In the aorta notice the apertures without valves which lead into the afferent branchial vessels. HEPATIC PORTAL SYSTEM. The _hepatic portal vein_ is the large vein entering the liver alongside the hepatic artery and bile duct. It receives branches from the stomach, pancreas, spleen, intestine, and rectal gland. At the surface of the liver it divides into two branches, which enter the two lobes of this organ. Within the liver the hepatic portal veins branch until a capillary system is formed from which the blood is collected by the hepatic veins and carried into the sinus venosus. In general, the branches of origin of the hepatic portal vein follow closely the arteries of the digestive organs. Trace the following parts of the system: A _posterior intestinal vein_, from the rectal gland and rectum, the large intestine and spiral valve, across to the end of the pancreas, along the pancreas to the hepatic portal vein; an _anterior intestinal vein_, from the large intestine and spiral valve, along the duodenal lobe of the pancreas; _gastric_, _duodenal_, and _pyloric veins_ joining the veins already traced; a _splenic vein_ joining the posterior intestinal vein. The liver, with the bile duct, may now be removed from the body if it is desired to trace the bile duct into the bladder or to trace the hepatic ducts. This can be done best by gently scraping away the soft liver tissue until the bladder and ducts are exposed. THE NERVOUS SYSTEM.