VII. THE NERVOUS SYSTEM 



The nervous system is composed of four parts: (i) the ce7itral 

 nervous system, composed of brain and spinal cord; (2) the 

 -peripheral nervous system, consisting of the cranial and spinal 

 nerves; (3) the sympathetic nervous system, which regulates 

 involuntary activities; (4) the sense organs. The brain, cranial 

 nerves, and certain of the sense organs will be dissected. 



SENSE ORGANS 

 Ear of the Shark 



Locate again the openings of the endolymphatic ducts on the 

 dorsal side of the head between the spiracles. It will be remem- 

 bered from the study of the shark skull that the endolymphatic 

 ducts connect with the inner ear. Remove the skin from the 

 head except for a small patch around the openings of the 

 endolymphatic ducts. This patch will aid as a landmark 

 in locating the position of ear structures. Clean away the 

 muscles and connective tissue of one side until the cartilage of 

 the skull is exposed in the region posterior to the eyes. 



Examine a prepared skull of a shark and locate the position 

 of the endolymphatic fossa and the semicircular canals of the 

 ear. Turn again to your own specimen and carefully shave 

 off the cartilage between the endolymphatic ducts and the 

 spiracle. Be careful not to injure the brain which lies in 

 the mid-dorsal region, or the nerves that penetrate the cartilage. 



Watch for the appearance of the semi-circular canals through 

 the cartilage. The first canals to be seen will be the anterior 

 vertical and the posterior vertical canals. Leave them in place, 

 but dissect the cartilage from around them. Do not injure 

 the structures at the ends of the canals. Continue the dissec- 

 tion, watching for the horizontal canal, which lies lateral to and 

 deeper than the others. The three semi-circular canals connect 

 with a thin-walled sac, the vestibule, lying beneath them. Leave 

 the organs embedded in the cranium, but continue remov^al 



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