THE NECTURUS 65 



Spinal Nerves 



Follow the cord back to the spinal nerves and dissect 

 out a pair to see their structure. 



It will be noted that there are two branches, a dorsal 

 and a ventral. The dorsal is sensory and supplied with a 

 ganglion while the ventral is motor, and not supplied with 

 an exterior ganglion. The two branches of the spinal nerve 

 unite to form a single, short branch, which in turn 

 divides into three subdivisions, djorsal, ventral and 

 visceral. 



Ventral Surface of the Brain 



Carefully dissect out the brain by loosening the cranial 

 nerves. Take as much of the cranial nerves with the bram 

 as possible so that they may be used as landmarks. Place 

 the removed brain in a watchglass with alcohol or water 

 for study. A small hand lens of 3 diameters magnification 

 will be of service for this work. 



Drawing 24. Make a drawing of the brain X 6, labeling all 

 structures. 



Show the origins of the cranial nerves as they come off 

 from the different regions. 



The olfactory and cerebral lobes show no unusual char- 

 acteristics. The diencephalon is not structurally separated 

 from the mesencephalon. The optic nerve comes out on 

 the ventral, midline of this part of the brain. The pituitary 

 body is a large structure that extends well back towards the 

 medulla. If carefully dissected out, all of this structure 

 will remain attached, as it rests on the floor of the brain 

 case and is not in a depression as in the higher vertebrates. 

 There is no sella turcica in Necturus. The metencephalon 

 and mesencephalon are closely connected with no external 

 marks to separate them. The medulla is separated from 

 the metencephalon by the enlarged anterior end of the 



