OP FISH IN GENEHAL, 11 



Facts communicated by Dr. Munro. 



'shaped fish, as in the mackerel, he has also found 

 it wanting. It is likewise known, that in many 

 fish the air-bag communicates by a duct with 

 the oesophagus." 



On examining this matter, he found in a stur- 

 geon a round hole, nearly an inch in diameter in 

 the upper and back part of the stomach, by 

 which it communicates with a very large air-bag. 

 In the salmon, he found a hole so large as to 

 admit readily the largest size goose-quill, leading 

 directly through the coats of the oesophagus into 

 the air-bag; and if, as in the carp, there are 

 two air-bags, the duct leads to the posterior bag, 

 from which there is a passage into the anterior. 

 From these circumstances he concludes, that 

 the air found in the swimming bladder, passes 

 into it through the above-mentioned ducts, and 

 they seem well suited for this purpose ; for, as in 

 the common horizontal situation of the fish, their 

 beginning is at the upper part of the stomach, it 

 is easy to conceive that the air which they take 

 in at their mouth, when they ascend, or that 

 which may by some more latent process be dis- 

 engaged from the water, is applied to the ducts ; 

 and that the fish, by an instinct of nature, distin- 

 guishes the irritation of air from that of water, 

 and propels the air into the air-bag, but excludes 

 the water. But in the cod and haddock, though 

 the air-bag is very large, and its sides remarkably 

 strong, yet the doctor was not able to discover 

 any communication of it with the mouth, oeso- 



