136 



it occnpies. This is effected by incieasiug the sku of the fish without any 

 appreciable addition to its weight, and thus creating a displacement of water 

 equal in weight to that »f the fish. The body without the air-bladder weighs say 

 10 oz., and the water ilisjilaced weighs only 9 oz. ; hence the fish must sink, 

 unless it used incessant mxiscular exertion ; but the presence of the air-bladder 

 removes this difference, and thus the great end is obtained of enabling the 

 fish to poise or balance itself at varying depths of water, or according to its 

 temperatiu-e, and thus to remain at rest mth the slightest amount of muscular 

 exertion. 



This use is of a passive kind ; another of a more active kind is attributed 

 to it. This is : — 



To enable the fish to raise or depress itself in the water at pleasiu-e. 

 Opinions of learned naturalists differ semewhat on this point. Lacejicde asserts, 

 as a fact beyond question, that fish raise or depress themselves in the water by 

 the contraction or exjiansion at will of this vessel. He says "that the gas, 

 when transmitted thi'ough the pneumatic canal to the swimming bladder, which 

 is called the aerial bladder, swells and extends that vessel, renders it much, 

 lighter than water, and gives to the fish the facility of raising itself in this 

 liquid. AVhen, on the contrary, the animal wishes to go down, it compresses its 

 swimming bladder by means of the muscles which sviiTound this organ ; the gas 

 escapes by the pneumatic tube, and the weight of the solid parts of the fish drags 

 down the animal more or less rapidly to the bottom of the water. This effect of 

 the swimming bladder on the rising and descent of fish cannot be questioned, 

 since, independently of other reasons, and, as Aitedi has announced, any one 

 can prove the fact by piercing with skill, by means of a suitable nee.Ue, the 

 air-vessel of a living fish, when it will be found that the fish can no longer rise 

 in- the water." Lacepede further states "that this fact is well known in 

 countries where the art of fishing has been much cultivated, and that there 

 the fishermen, in order to prevent the fish leaping over the sides of the troughs 

 in which they are placed, pierce the air-bladders, and then the fish lie quietly at 

 the bottom." Wood, in liis Zoography, mentions this as being commoulj' done 

 by the Cod fishermen at the Newfoundland fisheries. 



Yarrell says " that one use of these air-bladders to the fishes possessing 

 them is to enable them to alter their specific gravity wiih reference to that of the 

 fluid they inhabit seems almost certain. We see the gold-fishes in our orna- 

 mental vases ascend and descend in the water without making any visible 

 external muscular effort. In this respect their action is to be understood and 

 explained by the well-known hydi-ostatic toy of the philosophical instrument 

 makers, in which a small glass balloon, or other figure, confined in a column o^ 

 water, has its weight, by the introduction of a small quantity of air, so nicely 

 b;ilanced in reference to the specific gravity of the water, that it is made to 

 ascend or descend accortUng to the degree of pressure made by the finger on the 

 clastic cover of the top." Talcy, in his "Natuial Theology," says: "The use 



