258 ANATOMY OF VERTEBRATES. 



port and 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 faculty and an intel- 

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

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

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

 nay, to creatures that can only attain their prey, or escape their 

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

 trunk, the attachment there of Ions; flexible limbs would be a 



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

 the developement and connections of the hind limbs of Fishes, 

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

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

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

 more important parts of the locomotive machinery. 



The plane of each ventral fin being 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 effort. 



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 



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particular habits of certain species have previously been pointed 

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

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

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

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

 the absence of any connection with a sacral part of the vertebral 

 column, permits their transference forward, to aid the pectoral 

 fins in raising the head. 



The planes of the dorsal, figs. 24, 39 ? D, and anal, ib. A, fins 

 are in that of the mesial longitudinal section, and their movements 

 are usually restricted to elevation and depression. They accord- 

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

 recting any tendency to oscillate laterally, or to turn upside 

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

 in the ordinary posture, back upward, the centre of gravity lies 

 above the centre of figure. When the fins collapse and the mus- 

 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, 

 by lateral undulating movements, in the locomotion of the fish. 



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



