THE FROG. 187 



3. Behind these are a pair of conspicuous rounded 

 eminences, well marked off from each other, and from the 

 other parts. These are the optic lobes. The large optic 

 nerves arise from these below, and cross each other before 

 going to the eyes. 



4. Behind the optic lobes, and on the median line, is a 

 deep depression, the fourth ventricle. It is covered over 

 with a thin plexus of blood vessels, which is easily stripped 

 off, and is often torn away with the cranial wall. In front 

 of the ventricle, and behind the optic lobes, is a single 

 transverse band of nervous tissue. This is the cerebellum. 

 The nervous mass which surrounds the ventricle on the 

 sides and below is the medulla. It tapers posteriorly 

 into the spinal cord. 



5. There remains a portion of the brain between the 

 cerebral hemispheres and the optic lobes which has not 

 yet been mentioned. It is the midbrain. From its dor- 

 sal surface, between the posterior ends of the cerebral 

 hemispheres, and on the median line, arises a single 

 rounded prominence which is not composed of nervous 

 tissue. It is the pineal gland. 



6. About eight pairs of nerves arise from the ventral 

 surface of the medulla : these are small, and exceedingly 

 difficult to trace. The fifth and tenth cranial nerves are 

 largest, and have each a ganglion not far out from their 

 origin. These ganglia are each connected with forward 

 prolongations from the longitudinal commissures of the 

 sympathetic system. 



7. The fact that the single cavity that has been seen in 

 the brain thus far is called the fourth ventricle, will sug- 

 gest that there are others. Two others will be exposed 

 by cutting a horizontal slice from the top of the cere- 

 brum : these are the two lateral ventricles of the fore- 

 brain. They occupy the cerebral hemispheres, and unite 

 posteriorly around a median septum. The cavity formed 



