INTRODUCTION. 81 



spine and the fins, including, under the latter 

 term, the tail ; hut, to understand clearly the func- 

 tions and motions of these parts, it will be necessary, 

 first, shortly to notice the mechanism and structure 

 of the skeleton and muscles. The spine, or verte- 

 bral column, to the extremity of which the tail, or 

 caudal fin, is attached, is by far the most important 

 organ in the locomotion of fishes. The other fins, 

 analogous to the extremities of the higher animals, 

 being used, and only much developed under parti- 

 cular circumstances, never possess the firm and 

 sturdy, or active structure, which are requisite, 

 and enables birds and mammalia firmly to support 

 themselves in another medium. Those limbs, or 

 fins, then, in fishes, are used more for the purpose 

 of direction than of progression ; and even in the 

 prodigious manner in which we shall see that some 

 of them are developed, we do not find that there is 

 a corresponding power imparted. The spinal co- 

 lumn, then, is expanded upwards and downwards ; 

 and the muscles of the trunk, which almost all 

 assist in its movements, are placed in numerous 

 transverse strata along the sides, with strong ten- 

 dinous fascia between, and the whole are disposed in 

 longitudinal layers, directed alternately in different 

 directions. 



The spine, in general, consists of numerous 

 small irregularly shaped bones or vertebrae, of a 

 rounded form, from which proceed several projec- 

 tions or processes ; and they are familiarly known 

 to present the appearance of a shallow cup, with 



p 



