xiii.] THE FROG. 241 



8. The point of union of the roots to form a 

 nerve trunk, in the intervertebral foramina. 



Draw the exposed parts of the brain and spinal 

 cord. 



4. Divide the olfactory lobes, and raise the front end 

 of the brain; turning it back gradually, divide with 

 a sharp scalpel any nerves that are seen running 

 from it to the cranial walls: most of the nerves 

 being small, they will probably be torn across unob- 

 served, but the large optic nerves will at any rate be 

 seen: next divide the nerve-roots of the spinal cord; 

 remove it and the brain together and place them 

 with the ventral side upwards. 



a. On the under surface (base) of the brain will be 

 seen 



a. The optic commissure or chiasma opposite the 

 posterior end of the cerebral hemispheres; 

 with the optic nerves diverging from its ante- 

 rior end and the optic tracts entering it poste- 

 riorly. 



/3. Lying behind the optic commissure, between 

 the optic tracts, is a small eminence the 

 pituitary body. 



y. On each side of the two last-mentioned struc- 

 tures and arched over in front by the optic 

 tracts, are the crura cerebri. 



b. Divide the cerebral hemispheres horizontally with 

 a sharp scalpel : in each will be found a cavity 

 the lateral ventricle. Each lateral ventricle com- 

 municates with the third ventricle. The optic 

 lobes, divided in the same way, will be seen to 



M. 1 6 



