g6 ELEMENTARY BIOLOGY. [CHAP. 



a. The encephalon or brain ; that portion which 

 lies within the cranium, its constituents in 

 part paired. 



j3. The myelon or spinal cord; axial and lying 

 within the neural canal of the vertebral 

 column. 



Its form : wide in front, but narrowing ra- 

 pidly about the 5th or 6th vertebra, and 

 thence continued back as a slender tapering 

 filum terminate. 



The groove {dorsal fissure) running along 

 its middle line. 



b. The investing membranes and the origin of the 



spinal nerves. 



«. The pta mater ; a delicate densely pigmented 

 membrane, investing the whole nervous axis. 

 It is thickened and highly vascular above the 

 hind portion of the brain (choroid plexus). 



/?. The dura mater • a similar but denser mem- 

 brane lining the wall of the neural canal. 



y. The roots of the spinal nerves ; conspicuous in 

 the case of the second and eighth and ninth 

 pairs of nerves. The former are very stout, 

 and pass out from the myelon at right angles 

 to the axis, to reach the arm : the latter are 

 long and delicate, passing back on opposite 

 sides of the hinder half of the myelon. 



c. Obtain a second Frog (preferably one hardened 

 in alcohol) and make a transverse section across 

 the entire nervous axis and neural canal im- 



