286 THE DOG-FISH 



median longitudinal grooves or fissures along its 

 dorsal and ventral surfaces ; and traversed through- 

 out 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 ganglionated sensory root, and a ventral or 

 motor one : of these, the dorsal root is some distance 

 in front of the corresponding ventral one, the succes- 

 sive dorsal and ventral roots alternating at about 

 equal intervals along the sides of the spinal cord. 



Each dorsal root passes out from the neural canal 

 through a notch in the hinder margin of the interverte- 

 bral neural plate. The corresponding ventral root 

 arises by three or more rootlets, 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. 



Make out the pectoral plexus arising from the first thirteen 

 spinal nerves ; and the pelvic plexus arising from nerves 25-35. 



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. 



