THE BEAIN 



71 



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 





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 pneumogastrio nerves. 



left in the roof of the thalamencephalon. The 

 vessels of the plexus hang into the third ven- 

 tricle, 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 



