LESSON 82.] NERVOUS SYSTEM IN THE PISHES. 269 



of all the vertebrated 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 enlarge- 

 ments, 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 (spine, brain) axis, and like the sympathetic system of the 

 highest articulata, it is enclosed with the viscera, in a cavity distinct 

 from that which envelops the nervous axis of animal life. 



1206. In the long, worm- like form of the lowest fishes, as the 

 Lamprey, the Pride, and the Gastrobranchus, the two slender col- 

 umns extending along the back, and scarcely protected by a cartilag- 

 inous (cartilage, gristle) sheath, are nearly without cerebellum, and 

 destitute of ganglionic enlargements in their course to the head, 

 where the minute cerebral elements are .enclosed, like the ganglia of 

 a Cephalopod, in a cartilaginous tube, consisting of a single piece. 



1207. This simple condition of the axis, presented by the lowest 

 fishes, resembles the primitive development of this system in the 

 highest vertebrata before the extremities began to shoot from the 

 sides of the trunk. 



1208. In fishes, as in Cephalopoda, 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 

 dura mater, which lines the skull, and the pia mater, which invests 

 the cerebral organs, is occupied by the soft, transparent, semi-fluid 

 cellular tissue of the arachnoid coat, which passes down likewise 

 through the vertebral canal, enveloping the spinal chord. 



1209. The spinal chord is nearly equal in its development 

 throughout the vertebral column, even in many of the osseous fishes, 

 from the smallness of the arms and legs not requiring those enlarge- 

 ments which we observe in most higher animals, where the nerves 

 are larger and more powerful extremities are given off. 



1210. In species which have the arms of great magnitude, as 

 Rays, and Flying-fishes, there is a proportionate development of the 

 upper enlargements of the spinal chord. 



