THE FROG 73 



cavity is known as the third ventricle; the roof of the latter 

 is very thin and vascular and forms the anterior choroid 

 plexus. Near the center of this arises a slender lobe, the 

 pineal body or epiphysis, which extends forward over the 

 thalamencephalon ; a small parietal nerve runs along the 

 dorsal surface of the epiphysis and passes through the skull 

 to the brow spot. Both epiphysis and parietal nerve are 

 frequently torn away from the brain when the roof of the 

 cranium is removed. In front of the epiphysis and between 

 the posterior ends of the cerebral hemispheres is the para- 

 physis, a vascular and probably glandular outpocketing of 

 the roof of the thalamencephalon. 



(?) The cerebral hemispheres are two anterior swell- 

 ings separated from the forward-projecting olfactory lobes 

 by a slight constriction. The olfactory lobes are continued 

 forward as the olfactory nerves (first pair of cranial 

 nerves). 



Draw a dorsal view of the central nervous system, twice 

 natural size. Make this drawing in the center of a clean 

 page, so that other features may be added later. 



Remove the brain and spinal cord along with the roots of 

 all the nerves so far as possible. Begin at the anterior end; 

 cut through the olfactory nerves, then turn the brain back, 

 cutting the nerves close to the bony wall. Great care is 

 necessary to avoid tearing off the exceedingly delicate cranial 

 nerves and the roots of the spinal nerves. Immerse the 

 nervous system, ventral surface up, in water, and observe : 



(a) The ventral longitudinal fissure of the spinal cord. 



(b) On the ventral side of the thalamencephalon the 

 optic chiasma formed by the crossing of the optic nerves 

 (second pair of cranial nerves) ; the slightly bi-lobed infun- 



