OF FISHES IN GENERAL. 
Section II. 
Of tie external Parts and Motion of Fifhes. 
The external form of the greater part of fifties, tends 
greatly to the eale and celerity of their motion. It is 
lliarp at either end, and fwelling towards the middle ; 
and is modelled by nature after that fliape, which we en- 
deavour to imitate in thole veflels, that are intended lor 
the greatefl defpatch. livery human contrivance, hotv- 
ever, falls far ftiort of the rapidity of the natives of the 
lea. All the larger fifties can eafily overtake the bed; 
confirufled veflel while in full fail ; and play around it, 
without any apparent effort. 
The principal inftrument of this great velocity in fifties, 
is the tail, aided by the ftrength and flexibility of the 
hack-bone. The other fins are too fmall, {lender, and 
flexible, compared with the weight of the animal, to im- 
pel it through the water with i'uch vaft rapidity and 
force. Their principal ufe is to direft and moderate the 
movement, communicated by the impulfe of the tail. 
A fifth when deprived of thele, and put into a pond, 
darts upwards, downwards, and laterally, with all its 
wonted velocity, but without being able to direct its 
couvfe *. The ventral and dorfal fins, ferve the pur- 
pofe of keeping the fifties in an ercel and perpendicular 
pofition ; and, perhaps, the firit contributes to raife or 
fiuk them in the water. The peftoral fins regulate and 
affift progreffive motion. When fvvimming rapidly for- 
ward. 
* Berlin apud Will, de motu anim. 
