CHAPTER VI.
NEUROLOGY.
The most simple condition of the nervous central organ known in Vertebrates is found in Branchiostoma. In this fish the spinal chord tapers at both ends, an anterior cerebral swelling, or anything approaching a brain, being absent. It is band-like along its middle third, and groups of darker cells mark the origins of the fifty or sixty pairs of nerves which accompany the intermuscular septa, and divide into a dorsal and ventral branch, as in other fishes. The two anterior pairs pass to the membranous parts above the mouth, and supply with nerve filaments a ciliated depression near the extremity of the fish, which is considered to be an olfactory organ, and two pigment spots, the rudiments of eyes. An auditory organ is absent.
The spinal chord of the Cyclostomes is flattened in its whole extent, band-like, and elastic; also in Chimæra it is elastic, but flattened in its posterior portion only. In all other fishes it is cylindrical, non-ductile, and generally extending along the whole length of the spinal canal. The Plectognaths offer a singular exception in this respect that the spinal chord is much shortened, the posterior portion of the canal being occupied by a long cauda equina; this shortening of the spinal chord has become extreme in the Sun-fish (Orthagoriscus), in which it has shrunk into a short and conical appendage of the brain. Also in the Devil-fish (Lophius) a long cauda equina partly conceals the chord which terminates on the level of about the twelfth vertebra.
