258 ANATOMY OF VERTEBRATES. 



port find progression in Man, whose pectoral members or arms 

 are liberated from that office, and made entirely subservient to 

 the varied purposes to which an inventive facidty and an intel- 

 ligent will would apply them. To wliat purpose, then, encumber 

 a creature, always floating in a medium of nearly tlie same specific 

 gravity as itself, with hind limbs ? They could be of no use : 

 nay, to creatures that can only attain their prey, or escap>e their 

 enemy, by vigorous alternate strokes of the hind part of the 

 trunk, the attachment there of long flexiljle limbs would be a 

 grievous hindrance, a very monstrosity. So, therefore, we find 

 the developement and connections of the hind limbs of Fislies, 

 figs. 29, 34, .38, 64, v, restricted to the dimensions and form 

 which, whilst suited to the limited functions they are capable of 

 in tliis class, would prevent their interfering with the action of 

 more important parts of the locomotive machinery. 



Tlie plane of each ventral fin Ijeing horizontal, at right angles 

 to that of the caudal fin, their action serves to balance the body, 

 to incline it on either side when one fin only acts, and to elevate 

 or depress the fish by their joint eftbrt. 



In most fishes the ventral fins merely combine with the pectoral 

 fins in raising the body, and in preventing, as outriggers, the roll- 

 ing movement : but some interesting modifications in relation to 

 particular habits of certain species have previously been pointed 

 out (p. 180). In the long-ljodied and small-headed abdominal 

 fishes, the ventrals are situated near the anus, where they best 

 suliserve the office of accessory balancers ; in the large-headed 

 thoracic and jiigular fishes, the loose suspension of these fins, and 

 the alisence of any connection with a sacral part of the vcrteliral 

 column, permits their transference forward, to aid the pectoral 

 fins in raising the head. 



The planes of the dorral, figs. 24, 39, D, and anal, ib. a, fins 

 are in that of the mesial longitudinal section, and their mo-\ements 

 are usually restricted to elevation and depression. They acciird- 

 ingly increase or diminish the lateral siirfaces of the fish, cor- 

 recting any tendency to oscillate laterally, or to turn ujisido 

 down, as the Ijody would do without some muscular effort, since 

 in the ordinary posture, back ujnvard, the centre of ixra\ ity lies 

 a1)ove the centre of figure. When the fins collapse and the nuis- 

 cular action ceases, as in death, the fish floats belly upward. 

 However, in some singular exceptions, e. g. the Sun-fish, the dorsal 

 and anal fins are unusually extended, and take a more direct share, 

 liy lateral undulating movements, in the locomotion of tlie fish. 



In ordinary shaped Osseous Fishes, if the dorsal and ana! fins be 



