DISSECTION OF A DOGFISH (SHARK). 19 



sides of the body, just beneath the skin, find the canal of 

 the lateral line (p. 10). 



Split the skin in the median line on top of the head and 

 pull it off. On its under surface find the branching canals 

 of the lateral line system. 



Now carefully slice off the top of the skull, exposing the 

 brain. Enlarge the opening and compare the brain with 

 that of the bony fish. Notice especially the difference in 

 relative size of parts. Draw the whole brain. 



Cut away carefully the side walls of the cranium, expos- 

 ing the nerves coming from the brain. In this process you 

 will lay open the semicircular canals of the ear, behind the 

 spiracle, and, deeper down, the vestibule (p. 16). In this 

 last will be found a granular mass. Examine some of 

 it under the microscope in a drop of water and notice the 

 character of the small particles (otoliths). The principal 

 nerves that you will find will be the olfactory, going to the 

 nose; the optic, arising from the lower surface of the brain 

 and going to the eye ; the trigeminal, arising from the 

 anterior sides of the medulla and passing forward to supply 

 the "face." Just behind this is the combined auditory 

 and facial supplying the ear and face, and still farther 

 back the large vagus, which goes back to the gills, the 

 lateral line, and the viscera. Trace these nerves as far as 

 possible, and insert them in your sketch of the brain. 



Cut off the snout by an incision passing through the nos- 

 tril, and in the cut surface see the folds of the olfactory 

 membrane. 



Have you found bone in any part of the dogfish ? 



