INTRODUCTION. 65 



or downwards with equal facility; whereas the 

 natural tendency of the cetaceous tribes and of 

 birds being always to the top, and that of the rest 

 of the raammiferous animals and of reptiles to the 

 bottom, the two former experience comparative 

 difficulty in sinking, and the two latter, equal diffi- 

 culty in rising in the fluid. Independently, then, 

 of any other causes, they cannot, on this account, 

 be said to be so much in their natural element, 

 when surrounded by this medium, as fishes are, 

 nor to be at all upon a par with them, in their 

 claim to be considered inheritors of the waters. 



We alluded just now to the possession, by most 

 fishes, of an organ called the air or swim-bladder, 

 sometimes familiarly known by the name of the 

 sound. Every body must have noticed, near the 

 back-bone of the herring, and other fishes, a shin- 

 ing, pearly-looking membrane, almost enveloped 

 by the roe or milt of the animal. This is the 

 organ in question ; and it is of this organ, as found 

 in the sturgeon, the carp, the ling, the burbot, and 

 many more fishes, when dried and prepared by 

 certain processes, that the substance called isinglass 

 is manufactured ; and the same part of the cod, 

 when salted or cui'ed, forms a well-known favourite 

 dish for the table. The air-bladder consists of a 

 membraneous pouch, more or less tubular, situated 

 along the lower part of the spinal column. It is 

 simple in the majority of fishes which possess it, 

 fig. 1. of the salmon, but in some, as among the 

 Cyprinidce^ it is double, fig. 2. of the chub ; that is 



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