252 THE DOG-FISH 



1. The spinal cord lies in the neural canal of the vertebral 



column. It is flattened dorso-ventrally ; marked by 

 median longitudinal grooves or fissures along its 

 dorsal and ventral surfaces ; and traversed throughout 

 its whole length by a narrow central canal, continuous 

 in front with the fourth ventricle of the medulla. 



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 as a single trunk from 

 the spinal cord, and passes out from the neural canal 

 through a notch in the hinder margin of the inter- 

 vertebral 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. Outside the vertebral column the two 

 roots unite'to form the spinal nerve. 



IX. DISSECTION OF THE SENSE-ORGANS. 



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 

 into 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. 



