222 NERVOUS SYSTEM. 



and the cerebral and cerebellic hemispheres,, form the most 

 constant elements of this axis, but their relative and actual 

 developments vary in the different classes. They are com- 

 posed of minute neurilematous tubular filaments which form 

 two posterior contiguous sensitive or ganglionic columns and 

 two anterior motor columns, the filaments and nerves of which 

 are not interrupted by ganglionic enlargements. Though 

 much varied in the extent of its development in the different 

 classes, there is great similarity in the successive stages of the 

 development of this system in the embryos of all the verte- 

 brated animals and great uniformity of plan in all its adult 

 forms. Beginning with the two columns of the axis, like the 

 two chords of a worm, it becomes reinforced by filaments 

 from every part of the periphery, and gradually receives its 

 ganglionic enlargements, as in all the inferior tribes, where 

 they are most required by the developing organs of the 

 body. The great sympathetic, or nervous system of 

 organic life, which is extended along the upper or dorsal 

 side of the symmetrical axis in the inverted bodies of 

 the articulata is here developed along the ventral or 

 under surface of the spino-cerebral axis, and like the sym- 

 pathetic system of the highest entomoid classes it is en- 

 closed with the viscera, in a cavity distinct from that 

 which envelopes the nervous axis of animal life. 



In the long vermiform bodies of the lowest cyclo- 

 stome fishes, as the lamprey, the pride, and the gastro- 

 branchus, the two slender columns extended along the 

 back and scarcely protected by a cartilaginous sheath, 

 are nearly without cerebellum, and destitute of gangli- 

 onic enlargements in their course to the head, where 

 the minute cerebral elements are enclosed, like the 

 ganglia of a cephalopod, in a cartilaginous tube, con- 

 sisting of a single piece. This simple condition of the 

 axis presented by the lowest fishes, resembles the pri- 

 mitive embryo-state of this system in the highest ver- 

 tebrata before the extremities begin to shoot from the 

 sides of the trunk. In fishes, as in cephalopods, where 

 a large exterior surface of the skull is required for 

 muscular attachments, the minute brain does not fill 

 the cavity of the cranium, and the space between the 

 dvra mater which lines the skull and the pia mater 



