SPINAL COED 259 



e. The fourth ventricle is the cavity in the medulla, 

 beneath and behind the cerebellum : its sides 

 and floor are thick, its roof very thin. 



E. The Spinal Cord and Nerves. 



SUce away the roof of the neural canal horizontally, so as 

 to expose the spinal cord from the dorsal surface. 



Press away the cord gently from the side of the canal, to 

 expose the roots of the spinal nerves, 



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 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 as a single trunk from 

 the spinal cord, and passes out from the neural 

 canal through a notch in the hinder margin of the 

 intervertebral neural plate. The corresponding ven- 

 tral 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 imite to form the spinal nerve. 



IX. DISSECTION OP THE SENSE-OBGANS. 



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. 



s 2 



