THE BRAIN 71 



iii. The thalamencephalon is a lozenge-shaped portion 

 lying immediately behind the hemispheres and 

 between their diverging posterior ends : it is 

 covered by a thick vascular membrane the 

 choroid plexus over which passes the stalk of 

 the pineal body, a small body adherent to and 

 generally removed with the roof of the skull. On 

 removing the choroid plexus a slit-like hole is 



-I 



FIG. 13. The brain of the frog: dorsal surface, x 4. 



FIG. 14. The brain of the frog : ventral surface. X 4. 



C. cerebellum ; CH, cerebral hemisphere ; CP, choroid plexus of 

 third ventricle ; F, fourth ventricle; IN, tuber cinereum ; M. medulla 

 oblongata ; O, olfactory lobe; OC, optic chiasma ; OL, optic lobe; 

 P, stalk of pineal body ; PB, pituitary body ; T, thalamencephalon. - 



*I, olfactory nerve ; II, optic nerve ; III, third or motor oculi nerve ; 

 IV, fourth nerve; V, fifth or trigeminal nerve; VI, sixth nerve; 

 VII and VIII, combined root of facial and auditory nerves ; IX and 

 X, combined root of glossopharyngeal and pneumogastric nerves. 



left in the roof of the thalamencephalon. The 

 vessels of the plexus, covered by a thin layer of 

 nervous matter, hang into the third ventricle, or 

 cavity of the thalamencephalon. The thickened 

 sides of the thalamencephalon are the optic 

 thalami. 

 iv. The optic lobes are a pair of prominent ovoid bodies 



