232 ELEMENTARY BIOLOGY. [XIII. 



/3. Each arising by two roots {anterior and poste- 

 rior) : these are most easily seen in the 7th, 

 8th and 9th nerves, where they are much 

 longer than in the others. Sometimes the an- 

 terior root is double. 



7, The direction of the roots : directly outwards 

 in anterior nerves ; obliquely backwards in the 

 4th, 6th and 6th; almost directly backwards 

 for a considerable distance in the neural canal 

 in the 7th, 8th, 9th and 10th. 



S. The point of union of the roots to form a nerve- 

 trank, 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 unobserved, 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 anterior 

 end and the optic tracts entering it posteriorly. 



/3. Lying behind the optic commissure, between 

 the optic tracts, is a small eminence — the 

 pituitary body. 



