356 THE NERVOUS SYSTEM 



DISSECTION OF THE HEAD OF THE DOGFISH 



1. The dogfish is the smallest of the sharks. Either the spiny dogfish (Squalus 

 acanthias) or the smooth dogfish (Mustelus canis) may be used for dissection. 



2. The special sense organs include the olfactory organs, the eyes, the ears, and 

 certain sense organs in the skin, known as the lateral line canals, and the ampullae of 

 Lorenzini. 



3. Locate the position of the lateral line canal which produces a light colored ridge 

 in the skin extending from head to tail along either side of the body. The line may be 

 recognized by the presence of numerous small pores which open into the canal. It 

 extends on to the head and there forms the supraorbital, infraorbital, and hyoman- 

 dibular canals. The ampulla of Lorenzini are bulb-shaped bodies connected by long 

 canals with pores in the skin. They are irregularly arranged and are most numerous 

 on the snout. 



4. Locate the olfactory organs or nasal capsules which have their openings on the 

 ventral surface of the snout in front of the mouth. 



5. Note the gills and spiracles (Fig. 12). Find two minute apertures near the 

 midline between the spiracles. These are the openings of the endolymphatic ducts. 



6. The internal ear, a membranous labyrinth inclosed in a cartilaginous capsule, 

 should be exposed on the left side. Shave off the cartilage in thin slices in the region 

 between the spiracle and the median plane. The membranous labyrinth can be seen 

 through the translucent cartilage, and care should be exercised to avoid injuring it while 

 the cartilage is being removed. It consists of a spheric sac, the utriculosaccular chamber, 

 to which there are attached three semicircular canals (Fig. 12). The endolymphatic 

 duct is a small canal, which extends from this chamber through the roof of the skull to 

 the small opening in the skin, which has previously been identified. Note the enlarge- 

 ment at one end of each semicircular canal, known as the ampulla, and observe that 

 each of these canals lies in a plane at right angles to the planes of the other two. 



7. The Brain and Cranial Nerves. Remove the remainder of the roof of the skull 

 and expose the brain, eyes, and cranial nerves. 



8. Examine the brain as seen from the dorsal surface. Note the continuity of the 

 medulla oblongata with the spinal cord. Identify the cerebellum, the thalamus, epiphysis, 

 habenula, cerebral hemispheres, and olfactory bulbs (Fig. 8 and pp. 26-31). 



9. By dissection display on the left side the eye-muscles and the nerves which in- 

 nervate them, as well as the optic nerve (Fig. 12). 



10. Find the nervus terminalis (Fig. 8). Now locate each of the cranial nerves 

 from the second to the tenth inclusive, and trace them from the brain as far as possible 

 toward their peripheral terminations (Figs. 12, 13). Note particularly that Nn. VII 

 and X each have an extra root, indicated in black in Fig 13, which carries fibers from 

 the lateral line organs to the acusticolateral area of the medulla. 



11. Attention should now be paid to the functional types of nerve-fibers which 

 compose each of the cranial nerves (see pp. 168-170 and Figs. 119, 120). The ac- 

 companying table shows in which of the cranial nerves of the dogfish each of the four 

 principal functional groups of fibers are to be found (Herrick and Crosby, 1918). 



