the general propositions which he lays down.

In the body of the memoir he treats successively of the anatomi▾ cal structure of the air-bladder, of the nature of the sources of the air which it contains, and of the functions which it exercises.

He speaks, in the first place, of its existence, and gives a list of those fishes which have it, and of those in which it is wanting. The results of this list, which adds several species to those which had already been adduced with respect to this subject, are nearly the same which had been already drawn; namely, that the existence or nonexistence of the bladder does not correspond with the other affinities of organization which connect fishes with each other.

He afterwards speaks of the various situations of the bladder, of its variation in size, and in the configuration of its tunics (an article in which he compares the internal membrane to the serous membranes); and finally, of the particular muscles which it has in several fishes; and he gives a more detailed description of these muscles than is to be found in the comparative anatomy of M. Cuvier. What he says on the subject of the canal of communication also presents a great number of novelties. On this head he has made some very acute remarks, and has ascertained that this canal is wanting in the greater part of sea fishes. He did not find it in any of the jugular or thoracic classes, which compose nearly three-fourths of the total species of fishes with which we are acquainted. The lectures on comparative anatomy had assigned this canal to the uranoscope, which is a jugular; but according to M. Delaroche, the

authors of this work have made new inquiries, and found that they were mistaken.

M. Delaroche has studied in a particular manner the red bodies with which certain bladders are furnished. He found them, like Perrault and Monro, in all those which want the canal of commenication, and in the anguilla genus although furnished with this canal.

Our author gives a very detailed description of these bodies, in the gadi, the trigli, the perches, some labri and holocentres, as well as in the atherina rhepsetus, the bienurus physis, the orphius or esox belonus, and lastly in the eel and the conger.

We have verified that part of the descriptions which refers to the species with which we are familiar, or could procure, and have found them generally correct.

It appears to us, however, that M. Delaroche grants too great a homogeneity to the inner texture of these bodies. One of our number, who, along with M. Duvernoy, recently made some inquiries in or der to verify this point of anatomy, found these bodies in the larger fishes formed of lobes flattened like ribands, placed almost parallel on each other, very distinct from one another, by clearly marked intervals, and proceeding obliquely in various directions from the proper membrane to the internal membrane of the bladder.

The distribution given by M. Delaroche of the vessels which is sue from the red bodies of the eel, and from those which return to it, has also been verified, and found correct; but he passes rather too hastily over the red body itself, which is also divided into flakes, separated by intervals, which are frequently found filled with blood. In short, Messrs. Cuvier and

Duvernoy

Duvernoy think they have found strongly marked relations in the red bodies of fishes with the cavernous bodies; but their inquiries posterior to the memoir of M. Delaroche, are only brought forward here that the Institute may not be ignorant of what has been done on this interesting subject. A full account of their experiments will shortly appear.

The author of the present Memoir speaks only from the lectures in comparative anatomy as to certain branching air-bladders, entirely peculiar to one species of fish. M. Cuvier, who had described them when on the sea-coast, where he had no books from which to determine the species of the fish, thought it was the perca labrax ; but other naturalists, besides himself, have since sought for it in vain in the fish so called in the systems of ichthyology. By unexpected good fortune, the true fish which was the subject of observation was brought to Paris some time ago, and proved to belong to the rare species denominated by M. Lacepede cheilodiptera, or sea eaglet, but which ought to be placed among the centropommes, beside the labrax.

The bladder of this fish, unique of its kind, will be presented to the Institute along with a description by M. Duvernoy, and which will be more minute than any hitherto given under less advantageous circumstances.

In his analysis of the air contained in the bladder, M Delaroche confirms, in general, the experiments of M. Biot; adding, that besides the various degrees of depth at which fishes live, there are other causes which concur to vary the proportions of the gases in their air-bladders. Thus, of

two fishes caught on the same spot, one has given 5.00 and the other scarcely 40 of oxygen. M. Delaroche also rectifies the idea that M. Biot had given of the eruption of the bladder from the mouth, in fishes drawn up suddenly from great depths, when he says that a rupture of the bladder then takes place, and that it is the air which forces up the stomach to the mouth. As to the source of this air, our author (like Needham, Perrault, Monro, Kohlreuter, Duvernoy and Cuvier,) thinks it is produced in the interior of the bladder by a secretion of an unknown nature, of which the red bodies seem to be the organs in such fishes as have these bodies.

It is unnecessary to ask for a proof of this opinion in fishes which have no exterior canal, for in them it is demonstrated by itself. We might also fairly extend it to those which have a canal and red bodies, like the eel.

But in those which want the red bodies, as we must admit a new kind of exhalation, the analogy no longer takes place completely; and perhaps many persons would be equally willing to bave recourse to the aerial canal, insomuch as it always exists in this description of fishes. As fishes of the same family frequently have the airbladder, and others want it, it is probable that its functions may be supplied by different means.

M. Delaroche, without considering that question as at all decided, nevertheless supports the argument of analogy, from the difficulty which any given gas 'would have in many species, in penetrating into the bladder by the canal; from the still greater difficulty which it would have of arriving pure, particularly when it was re

quisite

quisite for it to pass through the substances contained within the stomach; and, lastly, from the difficulty of knowing, from whence, or by what mechanism, the fish could procure it from nature, in order to introduce it into its bladder at great depths, where it is so frequently and so long retained.

The habit in which physiologists are of seeing matters of every kind come out of the blood by secretions, renders them on the contrary very easy as to this kind of production; and in fact there is no real difficulty on the subject, since azote and oxygen, which compose the air in the bladder, exist abun dantly in the blood.

But it may be asked; if the gas be exhaled or separated from the blood, wherefore does it vary so much when the greater part of the other secretions are so constant in their nature? Above all, how can the animal body, so greedy of oxygen in general, exhale it so precisely at depths where it has the fewest methods of getting it from the external medium? M. Delaroche, who puts these questions, admits that it is difficult to answer them satisfactorily.

He afterwards proceeds to the uses of the air-bladder.

From its absence in many fishes taken indiscriminately from all classes, he concludes, with the authors of the comparative anatomy, that it cannot hold an important place in the vital functions; and this makes him reject all necessary connexion between the airbladder and respiration.

He would have even been in clined to conclude, from its solute stoppage in the greater number of fisbes that are furnished with it, that it could not in general be employed in the absorption of any

useful matter, in the excretion of any injurious substance, nor even in the production of a substance to be employed in some other part of the body; but that it is solely by itself as the air-vessel, and in its quality of considerably capacity, filled with a light elastic substance that it may be useful to the fish.

Now in this respect it can only have a mechanical use, either with respect to its station or movement.

M. Delaroche in the first place ascertains its use in the station, and admits that it serves to render the whole fish specifically lighter, and to place it in equilibrium with the water in which it is suspended.

This is one part of the most generally received opinion; but it is clear that the necessity of the bladder for this sole purpose is any thing but demonstrated, Nature would rather have made all fishes of the same gravity as the water, as she has done with those fishes that have no bladders: thus, the common opinion is also composed of two other integrant parts equally necessary with the former. The one is, that the fish can compress as it pleases, to a certain extent, its bladder, or dilate it; which we prove by the peculiar muscles with which the bladder is furnished in certain fishes, and by the mediate action which the sides and the muscles of the abdomen exercise on it in all those which have it.

M. Delaroche also adopts this second part of the common opinion.

He thinks even that it is in this way the fish supplies, when it rises, the pressure exercised on its bladder in deep water by the column of water above it. Were it otherwise, the air, which would be no longer compressed, would be too much dilated, and would render the fish

too

too light, or even produce some rupture, as happens to fishes drawn suddenly from great depths.

But who is there who is not aware, that this, on the part of nature, would be correcting very clumsily a defect which she might have refrained from introducing at all into her work? She had only to give no air-bladder at all to fishes; and we have seen that she need not to have done so to place them in equilibrium with the water in that case she would no longer have required the apparatus of compression, which has been supposed as serving only to correct the inconvenience of an useless bladder.

Thus we are of opinion that the third, and the chief part of the commonly received opinion, in reality resolves the problem: we mean that part of it which says, that the bladder is placed there to assist the fish in ascending and descending, according as it is compressed and dilated; and we confess that we do not see why M. Delaroche should reject this use of the bladder, to which the two others are, in our opinion, merely accessaries.

That the fish has strength sufficient to enable it to descend. clearly results from what M. DeJaroche himself admits; for if the fish, which ascends 30 feet for example (and it is difficult not to believe that many fishes can ascend that height without any accident), if, we say, such a fish has sufficient strength to compress its bladder, by means of its muscles, to the same degree that the 30 feet of water formerly did, it is evident that a similar fish, supposed to be in equilibrium at the height to which the former ascended, will also have sufficient strength to com

press its bladder, as much as would the addition of a weight of 30 feet, and that there would result from such a compression or diminution of volume more than sufficient to force it to descend.

M. Delaroche, against this most essential part of the vulgar opinion, advances only a single objection, which he borrows from M. Fischer: this is, that the variation of specific gravity which may result, with respect to the total body of fishes, from the variations of the volume of the bladder being very small, the ascents or descents, which are the consequences of it, could not but be very slow: but, besides, the circumstance of these variations never having been yet measured, no person has ever said that the bladder cannot be aided in this function by other organs. Those fishes which have no bladder, ascend and descend very well, although, other circumstances considered, rather more tardily. Now those which have a bladder have, in addition, all the organs employed by those which have none, and they can use them like the others,

One difficulty which we have sometimes heard started, is to ask how a fish, when it wishes to ascend from the bottom of the sea, can find strength to raise up by means of its sides, or generally of its integuments, the enormous column of water which lies upon it, in order to permit its bladder to dilate. But as this vessel is already, by its compression, in equilibrium with the water which presses it,. the least effort is sufficient; and even this effort, however small, is nevertheless necessary, that the fish may only rise a few feet by the means which are common to it with the fishes without a bladder : instantly its bladder, being less compressed,