6o8 



NA TURE 



\_April 2^, 1881 



Coming off from the underside of the sac and communi- 

 cating with it by a narrow opening, there is an elongated 

 tubular organ, the walls of which are so thick and muscular 

 that it might almost be compared to a gizzard. It is 

 directed forwards, and opens by a narrow prominent 

 aperture into the intestine, which runs straight back to 

 the vent. Attached to the commencement of the intes- 

 tine, there is a score or more of larger and shorter tubular 

 organs which are called the pyloric csca. These open 

 into the intestine, and their apertures may be seen on one 

 side of it, occupying an oval space, in the middle of 

 which they are arranged three in a row. 



The chief food of the herring consists of minute Crus- 

 tacea, some of them allied to the shrimps and prawns, 

 but the majority belonging to the same division as the 

 common Cyclops of our fresh waters. These tenant 

 many pai'ts of the ocean in buch prodigious masses that 

 the water is discoloured by them for miles together, 

 and every sweep of a fine net brings up its tens of 

 thousands. 



Everybody must have noticed the silvery air-bladder 

 of the herring, which lies immediately under the back- 

 bone, and stretches from close to the head to very near 

 the! vent, being wide in the middle and tapering off to 

 each end. In its natural state, it is distended with air ; 

 and, if it is pricked, the elastic wall shrinks and drives the 

 air out, as if it were an indiarubber ball. When the con- 

 nexions of this air-bladder are fully explored it turns out 

 to be one of the most curious parts of the organisation of 

 the whole animal. 



In the first place, the pointed end of the sac or crop 

 into which the gullet is continued runs back into a very 

 slender duct which turns upwards and eventually opens 

 into the middle of the air-bladder. The canal of this 

 duct is so very small and irregularly twisted, that, even if 

 the air-bladder is squeezed, the air does not escape into 

 the sac. But, if air is forced into the sac by means of a 

 blowpipe, the air passes without much difficulty the other 

 way, and the air-bladder becomes fully distended. When 

 the pressure is removed, however, the air-bladder di- 

 minishes in size to a certain extent, showing that the air 

 escapes somewhere. And if the blowing up of the air- 

 bladder is performed while the fish is under water, a fine 

 stream of air-bubbles may be seen to escape close to the 

 vent. Careful anatomical investigation, in fact, shows 

 that the air-bladder does not really end at the point 

 where its silvery coat finishes, but that a delicate tube is 

 continued thence to the left side of the vent, and there 

 ends by an opening of its own. 



Now the air-bladder of all fishes is, to begin with, an 

 outgrowth from the front part of the alimentary canal, 

 and there are a great many fishes m which, as in the 

 herring, it remains throughout life in permanent com- 

 munication with the gullet. But it is rare to find the 

 duf t so far back as in the herring ; and, at present, 

 I am not aware that the air-bladder opens externally in 

 any fishes except the herring and a few of its allies. 



There is a general agreement among fishermen that 

 herrings sometimes make a squeaking noise when they 

 are freshly taken out of the water. I have never heard this 

 sound mjself, but there is so much concurrent testimony 

 to the fact that I do not doubt it ; and it occurs to mc 

 that it may be produced when the herrings are quickly 

 brought up from some depth by means of this arrange- 

 ment. For under these circumstances the air, which the 

 air-bladder contains, expands to such a degree, on being 

 relieved from the pressure of the water, that deep-sea 

 fishes with a closed air-bladder which are brought to the 

 surface rapidly are sometimes fairly turned inside out by 

 the immense distension, or even bursting, of the air-bladder. 

 If the same thing should happen to the herring the like 

 misfortune would not befall it, for the air would be forced 

 out of the opening in question, and might readily enough 

 produce the squeak whicli is reported. The common 



Loach ' is said to produce a piping sound by expelling the 

 air which this fish takes into its intestine for respiratory 

 purposes. 



At the opposite end of the air-bladder there is an even 

 more curious arrangement. The silvery coat of the air- 

 bladder ends in front just behind the head. But the air- 

 bladder itself does not terminate here. Two very fine 

 canals, each of which is not more than a two-hundredth 

 of an inch in diameter, though it is surrounded by a 

 relatively thick wall of cartilage, pass forward, one on each 

 side, from the air-bladder to the back of the skull. The 

 canals enter the walls of the sl.ull, and then each divides 

 into two branches. Finally, each of these two dilates into 

 a bag which lies in a spheroidal chamber of corresponding 

 size and form ; and, in consequence of the air which they 

 contain, these bags may be seen readily enough shining 

 through the side walls of the skull, the bone of which has 

 a peculiar structure where it surrounds them. Now 

 these two bags, which constitute the termination of the 

 air-bladder on each side, are in close relation with the 

 organ of hearing. Indeed, a process of that organ pro- 

 jects into the front chamber on each side, and is separated 

 by only a very delicate partition from the terminal sac of 

 the air-bladder. Any vibrations of the air in these sacs, 

 or any change in the pressure of the air in them, must 

 thus tell upon the hearing apparatus. 



There is no doubt about the existence of these struc- 

 tures which, together with the posterior opening of the air- 

 bladder, were most accurately described, more than sixty 

 years ago, by the eminent anatomist Weber, but I am 

 afraid we are not much wiser regarding their meaning 

 than we were when they were first made known. In fishes 

 in general, there can be little doubt that the chief use of 

 the air-bladder is to diminish the specific gravity of the 

 fish and, by rendering its body of nearly the same weight 

 as so much water, to render the business of swimming 

 easier. In those fishes in which the passage of communi- 

 cation between the air-bladder and the alimentary canal is 

 closed, the air is no doubt secreted into the air-bladder 

 by its vessels, which are often very abundant. In the 

 herring, the vessels of the air-bladder are very scanty ; and 

 it seems probable that the air is swallowed and forced 

 into the air-bladder just as the loach swallows air and 

 drives it into its intestine. And, as I have already 

 suggested, it may be that the narrow posterior canal 

 which leads from the air-bladder to the exterior is a sort 

 of safety-valve allowing the air to escape, when the fish, 

 rapidly ascending or descending, alters the pressure of 

 the water upon the contained air. 



This hypothesis may be put forward with some show of 

 probability, but I really find it difficult to suggest any- 

 thing with respect to the physiological meaning of the 

 connection between the air-bladder and the ear. Never- 

 theless such an elaborate apparatus must have some 

 physiological importance ; and, this conclusion is strength- 

 ened by the well-known fact that there are a great many 

 fishes in which the air-bladder and the ear become con- 

 nected in one way or another. In the carp tribe, for 

 example, the front end of the air-bladder is connected by a 

 series of httle bones with the organ of hearing, which is, 

 as it were, prolonged backwards to meet these bones in 

 the hinder end of the skull. But here, the air-bladder, 

 which is very large, may act as a resonator ; while, in 

 the herring, the extreme narrowness of the passages which 

 connect the air-bladder with the ear renders it difficult to 

 suppose that the organ can have any such function. 



In addition to the singular connection of the ear with 

 the exterior by the roundabout way of the air-bladder, 

 there are membranous spaces in the walls of the skull 

 by which vibrations can more directly reach the herring's 

 ear. And there is no doubt that the fish is very sensitive 



■ See Mttller, " Ueber Fische welche To 

 Pitysiologie, 1857, p. 267. The herring is 

 vocal fishes. 



