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tion alone ? 2. Is it brought about by the joint operations of trituration and dissolvents ? Experiments alone must settle the controversy; and birds, I think, for many reasons, are, of all animal subjects, by far the properest to try them upon.
The structure of the stomach in birds, is as various as their outward form. In some it is very fleshy, thick, and of a close texture, called gizzards; others very thin, though of a much larger capacity, in proportion to the body, being a sort of membranous pouch : in some, the stomach is partitioned into gizzard, and membrane; and lastly in others it is all over of a middle texture and thickness, between the one and the other.
The gizzard is the stomach which seems the most favourable to the system of trituration. Its thickness, solidity, and compact texture, lead us to think it destined to act with a mighty force; and birds that have it are known to swallow sand, gravel, and small flints, with other little stones, some of which are always found within them. Such stomachs therefore seem fitted as mills for grinding and braying the grain they eat for food. The experiments of the Florentine academy, repeated by Redi and Borelli, have further confirmed this plausible notion. Hollow particles of glass, which they gave to chickens, ducks, and turkeys, were found reduced to a fine powder. However Valisnieri, famous for a multitude of fine observations in natural history, and ever ready to oppose popular prejudices, could not rest satisfied in these facts. He looked upon the resemblance between a stomach and a mill as chimerical; he could not but think, with a great many others, that a stomach thus capable of grinding corn, must also grind itself away. He considered the reduction of glass to powder as the effect of a powerful dissolvent, and found proofs thereof in the stomach of an ostrich, which he judged incontestable. I shall name one in particular; he there met with bits of glass perforated with a vast number of holes more minute than those of the finest silver wire plates.
Having myself experienced how easily small glass beads of various sizes and shapes were powdered, without at all excoriating the gizzard, I caused chickens, ducks, and turkeys to swallow short tubes of glass, which were about five lines in length, and four in diameter, of which the bore was about two lines. These, after the death of the fowls, I found no longer to retain their former shape, for they were all split asunder lengthwise. They had resisted the pressure which acted upon them inwards, from without, which must have been prodigious to have broken them; but they yielded
to that from within, ontwards; for it is certain that their splitting was the effect of the pressure of some small stones acting as wedges against their extremities. No dissolvent by any conceivable action upon a tube, could have divided iť into two nearly equal parts.
I then got a parcel of tin tubes made, whose small diame: ter rendered them capable of sustaining much stronger efforts, the diameters of their hollow being, at most, but a line and three quarters; and, to add to their strength, they were covered a line and an half thick with solder, which also closed up their orifices; they were about seven lines long. I caused a turkey to swallow one of them; to another
gave two; ant to a third six at once. When I opened these birds, 24 hours, and two days after, I could not find a single tube whose solidity had been proof against the force that attacked it. That which had suffered the least was grooved from end to end on two opposite sides ; most of the rest were absolately flatted, and some of them in part unrolled; the little plates that had been firmly soldered to their ends, were forced away, some of them being driven into the tubes, and others pushed outwards.
What a mighty resistance must the gizzard have been able to overcome in flattening these tubes, and producing the other remarkable alterations in their figure! The result of some different trials I made upon the like tubes, may serve in some measure to convey some idea of it: several of these, for instance, I squeezed between the cheeks of a vice, by hanging weights on the end of the handle; and it required about +37 pounds to flatten them as the gizzard had done.
A large nyt with its shell is easily ground to pieces in a turkey's gizzard ; and the recipe for fattening them by giving thein one or two a day, is not so ridiculous as may at first appear. I have carried the point so far as to force four and twenty large ones into a turkey's craw at once, where they might be heard to rattle, upon fillipping the neck on the outside ; next morning they were all gone, having undergone the operation of grinding, in the gizzard.
But notwithstanding this aniazing force which the gizzard exerts, in grinding the aliment it receives, does it not require the assistance of a dissolvent? And is not such a one actually to be found there?-The force of the gizzard, great as it is, has its limits; and I have cast lead in moulds, whose shape it could not alter. Into one leaden tube, left open
at each end, I introduced a grain of raw barley, with its husk on; into another a grain of the same husked, and into a third a grain boiled till it was ready to crack. These
tubes -continued at least twenty-four hours, and sometimes double that space, in a turkey's gizzard; and the experiment was repeated divers times : -The grain, whether raw and in its husk, or without the husk, or boiled, perfectly retained its figure and solidity, without any sensible alteration, except perhaps a little swelling, such as it would have acquire ed in any moist place.
It is well known how quickly ducks digest flesh food. I caused one to swallow several leaden tubes, in each of which was a bit of raw beef or veal, of the size of a barley corn. When these tubes were taken out of the gizzard, several hours after, the bits of flesh extracted from them, were found no ways altered either in smell, consistence, weight, or colour.
It is therefore fairly demonstrated, that if the aliment were not comminuted in the gizzard, by grinding, it would not be digested, since no dissolvent exists there, capable of dividing it; and that the comminution it undergoes in the gizzard, equal at least to that of corn in a mill, is entirely owing to the action of that viscus. I do not however imagine that a simple trituration of aliments, like corn milled, is a perfect digestion : to that, I think, a seasoning of certain liquids is requisite; but I have not now time to explain my idea, much less to recite the facts on which it is founde ed: nor have I leisure to remove the principal difficulty, that has been opposed to trituration, namely, that a stomach which can divide exceeding hard substances, must destroy itself; nor to explain the nature of the seeming callous membrane which lines the gizzard; nor even to say any thing of the gizzard of a species of Indian pigeons, arned with two mills, thought to be stone, but are really hard horn. I basten therefore to the experiments I have made on the membranous stomachs of birds, of a texture differing in all respects from that of gizzards.
The partizans of trituration, believing they had suffici. ently proved from the texture of a bird's gizzard, that diges, tion was effected by it; did likewise insist that it was performed purely by the same means, in the stomachs of other animals, even in the simply membranous, as well as in the most fleshy. On the contrary, those who could not be brought to conceive how membranous stomachs fould be capable of grinding, would have it, that, in such, a dissolvent was alone sufficient for the business of digestion; and that it was also accomplished the same way in the inost fleshy stomachs. It is too common a presumption to imagine the laws of nature
more uniform than they really are: and notwithstanding it has been well proved, that digestion is done by trituration in gizzards, yet it remains to be made appear that it is brought about by the same mechanism in membranous stomachs.
Those of birds of prey are the most proper for affording us information in this matter, especially as they are of the kind nearest in relation to our own. It is true they swallow large morsels at a time, without teeth to chew; in which they differ greatly from the human species. I was therefore desirous of trying them with variously conditioned tubes; and this I was aware I might do without difficulty, and even without taking away the life of any one of these murderers.
Whoever has looked into books of falconry must know, that carnivorous birds have a faculty of readily rejecting by vomit, any thing that their stomach cannot digest. If they swallow feathers, as they very commonly do, these never digest, nor are they evacuated through the passage of the excrements, but vomited up. I therefore provided myself with a parcel of tin tuves, of about ten lines long, and seven in bore.
A young buzzard of the largest size, from whom I had plucked some of the quills of his wings-tó confine him to my garden, was destined to undergo the several trials I judged fit to be made on the carnivorous kind: and the first that was resolved upon was to make him swallow one of the said tubes, open at the ends. Their size rendered them incapable of any great resistance, insomuch that they might be squeezed together by the bare pressure of a finger and thumb. The gizzard of a turkey would not have only flatted such a tube, but broken it in pieces. The buzzard, then confined under a hen coop, vomited it up, after about 24 hours, exactly in the same condition it was swallowed, without the least distinguishable mark of any friction it had undergone. This experiment, which had convinced me, that if the bird's stomach had any force of trituration, it was extremely weak in comparison of that of a gizzard, induced me'to satisfy myself forthwith if a dissolvent might not there be employed instead of it. I shall mention the precautions I made use of in order incontestably to prevent deceptión.
At each end of the tube I fastened a kind of grate, by means whereof the included food was secured from any motion which could possibly be produced in the stomach; so that nothing but a dissolvent could act upon it. All that seemed liable to doubt was, that if a dissolvent did really reside in the stomach, whether it might not be in too small a quantity for enough of it to get into the tube, so as to act successfully?
The very slender opinion that my first experiment had impressed upon me of a force of pressure in the buzzard's stomach, made me imagine that a grating of fine linen thread might be sufficient for my purpose; and therefore I took a thread, and applying it according to the length of the tube, wound it round, so that at every turn it passed over the centres of the open ends, and formed meshes, which instead of squares, as usual, were circular sectors, pointed at the centre, and widest at the circumference of the ends of the tubé; yet so close there, as not to allow a free passage to any thing more than of half or three quarters of a line in diameter. The turns which formed the grates, and which I call longitudinal, were kept tight with transversal ones which formed a sort of girdle about the middle part
of the length of the tube.
The buzzard which I had made to swallow the first tube filled with flesh, and grated at the ends, disgorged it in about 24 hours, without one of the threads being broken or put in the least out of its place, or any of the meshes stretched wider. As soon as I cast my eyes on one end of the tube, I was fully convinced of the reality of a dissolvent in this bird's stomach, capable of itself to effect digestion : for, at first view, I perceived, within, a very soft greyish white paste, a little of which I took out with a point of a penknife, and squeezing it between my fore finger and thumb found it esceeding tender, resembling the finest clay, inoistened to an equal degree. I could observe nothing sromous, nor any diversity of colour or consistence, that had the least resemblance of the fragments of fleshy fibres. Having, by degrees, gotten it all out, I found there was as much as about half filled the tube, and I discovered therein somewhat of a slightly, reddish substance, which had rather a more solid consistence than the rest. It surrounded the remains of the bit of beef, and was nearly of the natural colour. The remaining portion of the beef I washed gently to clear it from the reddish paste; and then it did not appear to be aboye an eighth part of what I had put into the tube.
In making a second experiment I took care to weigh exactly the bit of beef to be inclosed in the tube, which was more than 47, but not quite 48 grains. This tube, which had a grating at the ends, like the first, remained in the buzzard's stomach almost as long again as any of the others; for it was not rejected in less than 14 or 45 hours. The dió