678 



ICHTHYOLOGY. 



Air-bag. 



structure bladder of urine ; but the observations of Willougliby 

 and others have demonstrated their existence. The 

 kidneys of fishes are uniform in their substance, and 

 of a reddish brown colour. They are in general long 

 and narrow, and apparently united into one mass. 

 The peritoneum covers their under surface, and they 

 are placed longitudinally under the spine. The ure- 

 ters begin by numerous roots, and run along the under 

 surface of the kidney. They terminate either in a ve- 

 sica urinaria, or a cloaca ; or unite together to form a 

 dilation, which supplies the place of a bladder of urine. 



In the chondropterygii, the ureters terminate in the 

 cloaca, but in the other cartilaginous fishes the bladder 

 of urine is present, although very small and thin in its 

 coats. The urethra in most fishes is short, and com- 

 monly opens behind the anus by an orifice which also 

 gives issue to the sexual evacuations. Renal glands 

 are wanting in this class. 



5. Air-bag. This organ is called by some the swim- 

 ming bladder, by others the air bladder. It is the ve- 

 sica natatoria of Willoughby, and the vesica aerea of 

 Artedi. In this country it is called the sound. When 

 present, it is situated in the anterior part of the abdo- 

 minal cavity, and adheres to the spine. It is wanting 

 in the chondropterygii, and even in some of the osseous 

 fishes, as the flounder and mackrel. 



It is very different in shape according to the species. 

 In the herring and some other fishes it is oblong and 

 pointed at both ends. In the salmon it is obtuse at 

 both ends. In the burbot it is obtuse in the lower end, 

 and bifid at its superior extremity. In the carp it is 

 divided transversely, and in the silurus longitudinally, 

 into two lobes. 



In general there is a duct (duclus pneumaticus), by 

 means of which this air bag communicates with the 

 ossophagus, or the stomach. In the sturgeon there is a 

 round hole, nearly one inch in diameter, in the upper 

 and back part of the stomach, communicating with the 

 air bag. The hole is surrounded by thin muscular fi- 

 bres placed between the membranes of the stomach and 

 air bag, which decussate at opposite sides of the hole. 

 These are considered by Monro as having the effect of 

 a sphincter muscle. In the salmon, the last quoted au- 

 thor found a hole so large as to admit readily the largest 

 sized goose quill, leading directly through the coats of 

 the ffisophagus into the air bag. The ossophagus in 

 this fish has a thick muscular coat, but the fibres of 

 that coat do not seem to form a distinct sphincter 

 around the hole. In other fishes the duct of .commu- 

 nication is of considerable length. In the common her- 

 ring the under part of the stomach has the shape of a 

 funnel; and from the bottom of the funnel a small duct 

 is produced, which runs between the two milts, or the 

 two roes, to its termination in the middle of the air bag. 

 In some fishes, as the cod and haddock, Monro could 

 not perceive any ductus pneumaticus, or opening into 

 any of the abdominal viscera. The air bag was not 

 enlarged by blowing into the alimentary canal, nor 

 could he empty the air bag without bursting it. 



In the air bag of the cod and haddock, the same 

 acute observer examined the red coloured organ noticed 

 by VVilloughby, and considered by him as a muscle, the 

 surface of which is very extensive, as it is composed of 

 a vast number of leaves or membranes doubled. In 

 those fishes, however, in which the air bag communi- 

 cates with the alimentary canal, this red body is either 

 very small and simple in its structure, as in the conger 

 eel, or entirely wanting, as in the sturgeon, salmon, 

 herring, and carp. 



Naturalists, in general, are disposed to regard the Structure 

 air bag as accessory to the organs of motion. Having ami 

 observed that flat fish, which reside always at the hot- 1 " U " - V' : 

 torn, are in general destitute of this organ, they have ^ 

 assigned to it the office of accommodating the specific 

 gravity of fishes to the density of the surrounding cle- 

 ment, and thus enabling them to suspend themselves 

 at any depth. A very simple experiment has likewise 

 countenanced the opinion. When the air bag of a fish 

 is punctured, the animal immediately falls to the bot- 

 tom, nor is it able, by any exertion of its fins, to elevate 

 itself again. When in a sound state, the external skin 

 of the air bag (regarded as possessing strong muscular 

 power) is supposed capable of contraction, so as to 

 condense the air, and enable the animal to sink, or of 

 extension, so as to allow the air to expand, and aid the 

 animal in rising in the water. 



The air bag of some fishes soon loses its muscular 

 power, in consequence of the air being expanded by 

 the action of the sun, when the fish has remained too 

 long at the surface. In this situation the fish continues 

 at the surface. When some fish are suddenly brought 

 up from deep water, the diminished pressure occasions 

 the expansion of the air contained in the bag. The or- 

 gan sometimes bursts in such cases, and the contents, 

 rushing into the abdomen, push the gullet sometimes 

 out of the mouth of the fish. We have witnessed this 

 effect produced in the cod fish. 



The above theory fails in explaining all the pheno- 

 mena. The eel, which resides alwa3 r s at the bottom, 

 is yet possessed of an air bag; while the sharks, which 

 roam about in all depths, and the mackrel, which pur- 

 sues its prey at the surface, are destitute of thus re- 

 puted organ of equilibrium. 



Various opinions have been advanced with regard to 

 the manner in which this air bag is filled. By some it 

 has been supposed, that a portion of the air, which 

 fishes are capable of abstracting from the water, is 

 transmitted through the gullet and stomach into the air 

 bag when necessary, and expelled and renewed at the 

 pleasure of the animal. Needham long ago considered 

 that the air, or, as he termed it, a vaporous exhalation 

 contained in the air bag, was generated in the blood, 

 secreted into this organ, to be afterwards thrown into 

 the stomach or intestines, to promote the digestion ef 

 the food. 



The nature of the air contained in the air-bag, was 

 never investigated until pneumatic chemistry had open- 

 ed up new fields of discovery. In 1774, Dr Priestley 

 turned his attention for a short time to the subject ; 

 and in the air-bag of the roach he found azote in one 

 instance unmixed, and in another in company with oxy- 

 gen. Fourcroy afterwards examined the gaseous con- 

 tents of the air-bag of the carp, and found them to 

 consist of almost pure azote. 



The most accurate and extended experiments on this 

 subject are those of M. Biot, published in the Mem. 

 d'Arcueil, i. 1 252. and ii. 8. He found the proportion 

 between the oxygen and the azote (for he was unable 

 to detect the presence of hydrogen, or any sensible 

 quantity of carbonic acid) to vary according to the spe- 

 cies, as may be seen in the following table. 



Names of the Fish. Proportion of Oxygen. 



Mugil cephalus quantity insensible. 



Ditto ditto. 



Muraenophis heiena . . very little. 



Spams annularis, female 0.09 



Ditto, male 0.08 



