2 4 o ELEMENTARY BIOLOGY. [CHAP. 



c. The optic lobes (tnesencephalon) : a pair of 

 rounded eminences lying behind the thalamen- 

 cephalon. 



d. The cerebellum (metencephalon): a narrow trans- 

 verse band lying behind the optic lobes. 



e. The medulla oblongata (myeloncephalon)-. the por- 

 tion of the brain lying behind the cerebellum. 



a. Lying on the medulla oblongata is a trian- 

 gular depression (4th ventricle) with its apex 

 turned backwards. 



3- The spinal cord. 



a. Its form: wide in front, but narrowing rapidly 

 about the 5th or 6th vertebra, and thence con- 

 tinued along the neural canal as a slender 

 tapering filament. 



b. The groove (posterior fissure) running along its 

 middle line. 



c. The spinal nerves arising from it. 

 a. Ten on each side. 



ft. Each arising by two roots (anterior and poste- 

 rior): these are most easily seen in the yth, 

 8th and pth nerves, where they are much 

 longer than in the others. Sometimes the 

 anterior root is double. 



y. The direction of the roots : directly outwards 

 in anterior nerves; obliquely backwards in 

 the 4th, 5th and 6th almost directly back- 

 wards for a considerable distance in the neu- 

 ral canal in the 7th, 8th, gth and loth. 



