NERVOUS SYSTEM OF FISHES. 



171 



nation with the retractores pubis, tend to compress the abdomen, to 

 constrict the anus, and to depress the tail. 



The muscles of the pectoral fins, though, compared witli those of 

 the homologous members in higher vertebrates, they are very small, 

 few, and simple, yet suflice for all the requisite movements of the 

 fins ; elevating, depressing, advancing, and again laying them pi'one 

 and flat, by an oblique stroke, upon the sides of the body. The rays 

 or digits of both pectorals and ventrals, as well as those of the 

 median fins, can be divaricated and approximated, the intervening 

 webs spread out or folded up, and the extent of surface required 

 to react upon the ambient medium in each change and degree of 

 motion, can be duly regulated at pleasure. 



LECTURE VIIL 



NERVOUS SYSTEM OF FISHES. 



The neural axis is a simple continuous chord in the Lancelet 

 {Branchiostoma. Jig.AQ, md.), of opaline sub-transparency, ductile 



Diagram of Anatomy of (he Lancelet, Srunchiosluyna. 



and elastic, flattened, composed entirely of nucleated cells •• showino- 

 a feeble indication of a median linear arrangement, which becomes 

 more general and distinct at the anterior end, where the axis becomes 

 cylindrical and terminates obtusely : the nerves, trigeminal {ob), and 

 optic {op), in connection with this slightly modified part of the axis, 

 indicate it to be the brain. This is the most simple persistent con- 

 dition of the central organs of the nervous system known in the 

 vertebrate sub-kingdom : it is typified by that of the Entozoa in the 

 articulate sub-kingdom. In all other Fishes the fore part of the 

 neural axis receives the vagal, trigeminal, and special-sense nerves, 



I'lof. Goodsir, Ixsxix. 



