A Letter Written to the Publisher by the Learned Mr. Ray, Containing Some Considerations on the Conjecture in Numb. 114. of These Tracts, about the Swiming Bladders in Fishes

A Letter written to the Publisher by the Learned Mr. Ray, containing some Considerations on the Conjecture in Numb. 114. of these Tracts, about the swimming Bladders in Fishes. This Letter is not mentioned in the Contents, because it was thought at the time when the first sheet was a printing, the Discourse about the precedent Instrument would have taken up all the room of this Tract. SIR, I was much pleased and satisfied with the ingenious conjecture I found in your Transactions of May last, pag. 310. concerning the swimming Bladders of Fishes; and persuade myself that the Author thereof hath hit upon their true use, viz. to sustain or keep them up in any depth of water. For it hath been observed by some, and I find it in Mr. Willughby's general notes of Fishes, that if the swimming bladder of any Fish be pricked or broken, such a Fish sinks presently to the bottom, and can neither support nor raise itself in the water. 2. Flat Fishes, as Soles, Plaice, &c. which lie always groveling at the bottom, have no swimming bladders that I could ever find. 3. In most Fishes there is a manifest channel leading from the gullet or upper orifice of the stomach to the said bladder, which without doubt serves for conveying air thereinto, as may easily be tried by any one that pleases. But though air may be received into the bladder, yet is there a value or some other contrivance to hinder the egress of it; for you shall sooner break the bladder than force any air out by this channel. Yet in Sturgeons Mr. Willughby hath observed, that pressing the Bladder, the stomach presently swelled: So that it seems in that Fish the air passes freely both ways. Possibly, the Fish while alive may have an ability to raise up this valve, and let out air upon occasion, which yet I doubt of, because other Animals have no such faculty of opening any valves made to stop the reflux of fluids. But I verily think, there is in the coat of this bladder a muscular power to contract it when the Fish lists: For, in many Fishes it is very thick and opake, like the coat of an Artery (which hath, as Dr. Willis observes, such a muscular faculty) as for example in all the Cod-kind; in some, v.g. the Hake, called in Latin Merluccius, it is inwardly covered with a red carious substance, which I take to be muscular flesh; in others, it is forked at the top, and to each horn hath a muscle affixed. Now the muscular force need not be great, being still assisted by the water as the Fish descends; the pressure of the water being much greater at the bottom than at the top, as appears by the ascending bubble. But whereas it is said, Perhaps the Fish can by its sides or some other defence keep off the pressure of the water, and give the air leave to dilate itself: It may be objected, if it can do so, what needs then any air-bladder? the cavity of the abdomen may serve the turn. To which I answer, that this power of dilating the abdomen by the muscles may assist Fishes to rise, whose natural place is toward the bottom; and the Air compressed in the bladder dilating itself as the Fish ascends, facilitates the action of the muscles. But those Fishes that descend by contracting the bladder, letting the contracting muscle cease to act, will rise again of their own accord, the Air within dilating itself, as we see in glass bubbles by compression of the Air in them descending, which as soon as the force is removed ascend without more ado. Besides the flat Fishes I before mentioned, all the cartilaginous kind, as well flat as long, want swimming bladders: What course they use to ascend and descend in the water, I know not. Many of the Eel-kind (not all) have swimming bladders; yet can they hardly raise themselves in the water, by reason of the length and weight of their tails: I suppose, the Air-bladder being near their heads heads helps them to lift up their head and fore-part. Great diversity there is of swimming bladders in respect of figure, substance, situation and connexion in several Fishes: But not being able to give you any reasonable account of each, I shall forbear to add any thing further of them. SIR, Your very humble Servant Middleton June 22. 1675. John Ray.