COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 321 



side the frontal bones. The maxillary nerve runs along the margin of the upper 

 jaw. The mandibular nerve passes laterally to the angle of the jaws and then 

 turns forward along the lower jaw. 



e) The seventh or facial nerve: This arises just behind the trigeminus and 

 sends a branch forward to join the latter at the semilunar ganglion. This branch 

 of the facial passes out with the ophthalmic nerve as the superficial ophthalmic 

 branch of the facial and with the maxillary nerve as the buccal branch. Both of 

 these go to lateral line organs. The greater part of the facial arises from the 

 medulla in common with the auditory nerve ventral to the above-named branch. 

 This common acustico-facial trunk of facial and auditory passes into the anterior 

 part of the otic capsule. From here the main trunk of the facial or hyomandib- 

 ular may be followed laterally. It branches to muscles, lateral line organs, etc. 



/) The eighth or auditory nerve: This arises in common with the facial and 

 is distributed to the internal ear. Its branches are readily noted in the otic 

 capsule. 



g) The ninth or glossopharyngeal and tenth or vagus nerves: These arise 

 together from the medulla posterior to the acustico-facial trunk, by three roots. 

 The common trunk passes along the posterior margin of the ear capsule and 

 enters a large ganglion. From this several nerves arise which may be traced 

 into the external gills (these nerves are not homologous with the pre- and post- 

 trematic branches of elasmobranchs), and to the visceral arches. The most 

 posterior branch of the vagus gives off a lateral branch which passes to the lateral 

 line which it accompanies. The vagus also supplies the viscera. 



Add the cranial nerves to your drawing of the brain, as far as seen. 



6. Ventral aspect of the brain. Remove the brain by cutting across the 

 spinal cord, the olfactory, and other nerves. The ventral view reveals some 

 additional parts of the diencephalon. At the anterior end of the ventral surface 

 of the diencephalon is the small optic chiasma formed by the optic nerves. Pos- 

 terior to this is the large infundibulum from the posterior end of which projects 

 the hypophysis. The ventral surface of the midbrain consists of the two cerebral 

 peduncles. 



Draw the ventral view. 



D. THE NERVOUS SYSTEM AND SENSE ORGANS OF THE TURTLE 



i. The spinal nerves, the sympathetic system, and the spinal cord. Remove 

 all of the viscera from the pleuroperitoneal cavity, leaving the large neck muscles 

 in the mid-dorsal region undisturbed. Identify the spinal nerves as the white 

 cords passing along the sutural lines of the costal plates. The sympathetic 

 chain should be identified as a white cord or cords located on the sides of the 

 mass of neck muscles. 1 



1 The fact that the sympathetic system was not described in elasmobranchs and Necturus should 

 not be taken to indicate that the system is absent in those forms. It is present but impractical to 

 dissect out. 



