CHAPTEK VI. 



NEUEOLOGY. 



The most simple condition of the nervous central organ 

 known in Vertebrates is found in Brancliiostoma. 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 sjjots, 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 Chimwra it is 

 elastic, but flattened in its posterior portion only. In all 

 other fishes it is cylindrical, non-ductile, and generally ex- 

 tending 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 shorten- 

 ing 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 tlie Devil-fish 

 {LojjMus) a long cauda equina partly conceals the chord which 

 terminates on the level of about the twelfth vertebra. 



