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in his work on the air-bladder the following passage occurs. " With 

 all the pains that I took to inject the vessels on every side, and even 

 with the assistance of ray friends, Herr Benad and Herr Steiniger, 

 both very dexterous in dissecting, all our endeavours were fruitless. 

 These vessels are so minute, even in fish of a considerable size, that 

 the patience of CarorTni was even insufficient to follow them." 



Before entering on this subject, it will be necessary to make some 

 remarks on a few of the many kinds of air-bladders met with in fishes 

 generally ; and for my present purpose it will be merely requisite to 

 divide them into three classes. 



1. Into those which are simple musculo- membranous sacs, having 

 no duct of communication either with the alimentary canal, or in fact 

 any external opening. This kind of bladder is found in the cod tribe, 

 in the gurnard, sword-fish, haddock, perch, and many other fish. 



2. Into those that are single, but have a duct of communication 

 which is termed (always when it is found) ductus pneumaticus or air- 

 duct. This opens sometimes into the stomach or oesophagus or some 

 part of the alimentary canal. Instances of this kind are found in the 

 pike, salmon, sturgeon, trout, and many others. 



3. Into those which are double, the two compartments being kept 

 together, or rather communicating with each other by a narrow tube, 

 and the ductus pneumaticus in all cases proceeding from the posterior 

 compartment. We have numerous examples of this kind of air-blad- 

 der in most of our fresh-water fishes, as the carp, barbel, tench, roach, 

 chub and gold fish. 



The first class of air-bladders, or those which have no duct of com- 

 munication, are generally found in those fish which frequent deep 

 water ; and in the interior of the bladder is situated a highly vascular 

 body, termed by authors the gland for secreting the air. Of this kind 

 of air-bladder we have an excellent example in the common cod ; in 

 this fish it is a thick muscular bag, of a pyriform figure, indented at 

 the margins, and firmly attached to the bony framework of the verte- 

 brae and ribs, and having two tubular processes, about three inches in 

 length, one on each side, proceeding from the upper part of the blad- 

 der, and provided with blind extremities. They are curved upon 

 themselves, and the curved portion lies in a cavity in the head of the 

 fish, near to the labyrinth of the ear. From this circumstance, the 

 air-bladder has been supposed, by Professor E. H. Weber and others, 

 to be concerned in the function of hearing. The outside of the blad- 

 der is invested with a thin membrane, continuous with that of the 

 peritonaeum, which lines the whole abdominal cavity ; and beneath 



