232 ELEMENTARY BIOLOGY. [XIII. 



f3. 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, 5th 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- 

 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 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. 



