16 



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 sytem 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 fol- 

 low closely the arteries of the digestive organs. Trace the follow- 

 ing 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 join- 

 ing 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. 



Only the head and anterior part of the trunk will be required for 

 the dissection of the nervous system. Cut across the body back of 

 the pectoral fins; the posterior part of the body will not be re- 

 quired further unless it is desired to study the muscles and skeleton. 



The manner of dissecting the brain depends somewhat upon 

 the specimens at the disposal of the student. If a large head is to 

 be used especially for the dissection of the cranial nerves, only the 

 brain, eye and ear need be studied in the present specimen. But in 

 most cases it will be found best for the student to dissect the first 

 dogfish as thoroughly as possible, working out the cranial nerves 

 as well as the brain, and reserving the second head for a thorough 

 review of the entire nervous system. Chapter III of Herrick and 

 Crosby's "Laboratory Outline of Neurology" should be used in con- 

 nection with such a review. 



If a line be drawn over the dorsal surface of the head connect- 

 ing the two spiracles, two small pores will be found near the middle. 

 These are the external apertures of the ducti lymphatici. Cut 

 carefully through the skin in a small circle around the pores, and 

 remove the skin from the remainder of the dorsal surface of the 

 skull without disturbing the small section containing the pores. 

 The latter part should now be lifted gently; beneath it will be 

 seen two delicate tubes passing from the pores to apertures in a de- 

 pression of the skull below them. These tubes are the ducti endo- 

 lymphatici, through which a passage exists between the internal ear 

 and the exterior. As they cannot be preserved in the subsequent 

 dissection, the pores by which they pass through the skull to the in- 

 ternal ear should be found now, and a memorandum-sketch made 

 of the ducts themselves. 



DORSAL SURFACE OF THE BRAIN. The roof of the skull should be 

 removed from over the brain. Use a sharp scalpel and take very 

 thin slices of cartilage. Do not cut beyond the brain at the sides. 

 No attempt should be made at this time to expose more than the 

 dorsal surface of the brain. 



Above the anterior end of the brain there is a small median 

 foramen through the skull, the epiphysial foramen. A strand of 

 tissue, the epiphysis, leading from this to the surface of the brain, 

 should be carefully observed and retained. The cartilage should 



