278 THE DOG-FISH. 



its whole length by a narrow central canal, continuous 

 in front with the fourth ventricle of the brain. 

 2. The spinal nerves arise in pairs from the sides of the 

 spinal cord. Each nerve arises by two roots, a 

 dorsal or sensory root, and a ventral or motor one : 

 of these the dorsal root is some distance in front of 

 the corresponding ventral one, the successive dorsal 

 and ventral roots alternating at about equal intervals 

 along the sides of the spinal cord. 



Each dorsal root arises from the spinal cord as a 

 single trunk with a ganglionic enlargement on it, and 

 passes out from the neural canal through a notch in 

 the hinder margin of the intervertebral neural plate. 

 The corresponding ventral root arises by three or 

 more rootlets from the spinal cord, and passes out 

 from the neural canal through a smaller notch in the 

 hinder margin of the vertebral neural plate. Out- 

 side the vertebral column the two roots unite to form 

 the spinal nerve. 



IX. DISSECTION OF THE SENSE OEGANS. 



A. The Nose. 



Dissect out one of the olfactory organs from its capsule : 

 cut it across transversely about the middle ; wash it, and 

 examine its structure. 



The olfactory organ is a sac, the walls of which are thrown 

 mto a large number of transverse folds attached in pairs to 

 a median septum. Each fold is covered by a soft mucous 

 membrane, the surface of which is increased by secondary 

 foldings. The olfactory nerves enter the organ from the 

 dorsal surface, and spread out in the folds. 



The great size of the olfactory organ, and of the nerves it 

 receives, is very characteristic of Elasmobranch fishes. 



B. The Eye. 



Bemove one of the eyes by cuttiiig across the eye muscles 

 and the optic nerve. Snip off the remains of the muscles, and 



