46 THE SKATE 



medulla J its roots are mingled with those of the fifth and 

 seventh nerves. 



i. The ninth or glossopharyngeal nerve passes through 

 the floor of the auditory capsule. It is attached to the 

 medulla posterior to the auditory. It has several branches 

 to the first functional branchial arch, to the pharynx and 

 to a few sense organs on the mid-dorsal line of the head. 



j. The tenth or vagus nerve is apparently made up of 

 several cranial nerves. It has numerous roots and divides 

 into six main ganglionated portions which supply the four 

 posterior clefts and arches, and the heart and stomach. 



Drawing 25. Add the cranial nerves to the drawing of the brain. 



3. The Spinal Cord. 



Expose about an inch of the spinal cord by shaving away 

 the neural arches through which it runs. It is divided by 

 deep dorsal and ventral fissures. The dorsal half of the 

 system is sensory and is concerned with the bringing into 

 the system of stimuli. The ventral half is concerned with 

 the initiation and conduction of impulses, exciting activity 

 of muscles, etc. 



4. The Ventral Aspect of the Brain. 



Free the brain from the chondocranium by cutting 

 through the olfactory tracts and lifting up the anterior 

 end of the brain. Cut through the two optic nerves and 

 pare away the wall of the orbit on one side. Certain of 

 the structures attached to the ventral surface of the dien- 

 cephalon extend ventrally into a pit in the floor of the 

 cranial cavity. Cut through the cranial nerves and across 

 the spinal cord and lift out the brain intact. Study the 

 ventral surface in light of the previous study of the brain. 

 Note the distribution of the internal carotid artery. 



Several additional structures are present on the dien- 



