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.
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 required 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 connection with such a review.
If a line be drawn over the dorsal surface of the head connecting 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 depression of the skull below them. These tubes are the ducti endolymphatici, 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 internal 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 also be cut away from above the portion of the spinal cord next the skull. Gently wash away any coagulated lymph.
The brain and spinal cord are invested by two membranes (meninges). The tough dura mater lines the cavity in which they lie, clinging closely to the cartilage; in fact it forms the perichondrium of the internal surface of the cranium. The pia mater envelops closely the brain and cord, and contains numerous blood vessels. Between the two is the arachnoid space, traversed by occasional fine threads of connective tissue and filled with lymph.
As the spinal cord passes forward into the skull it enlarges and merges with the posterior portion of the brain, the medulla oblongata (myelencephalon). The roof of the medulla is extremely thin, and is broken if the cartilage has not been removed with extreme care, exposing a cavity within, the fourth ventricle.