Spallanzani made his experiments by introducing certain substances, such as raw vegetables, &c. enclosed in small perforated tubes, and causing animals to swallow them ; he then either destroyed the animal, in order to examine it, or waited until it was vomited up. The animal kingdom may be divided into three kinds ; containing stomachs muscular, intermediate, or membranous ; the last class is infinitely more numerous than the two former. Of animals with mus cular stomachs, such as fowls, turkeys, ducks, geese, doves, pigeons, &c. the food is seeds, such as wheat, barley, pease, &c. ; when it is taken spontane ously by these birds, it remains some time in the craw, where it is macerated and becomes softer ; it is then conveyed into the stomach or gizzard, which is composed of very strong muscles, capa ble of grinding not only the grain it re ceives, but is of such force as even to re duce small pieces of glass, and blunt the points of needles : by this means the food is triturated and reduced very small ; it is then converted, by the gastric juice it meets with in this viscus, into a pulta ceous mass, called chyme. Spallanzani found, that the gastric juice of this class digests flesh, and that the animals are for the most part both frugivorous and gra nivorous. Ile found it dissolved raw flesh, when bruised, in about two days ; but when entire, four and sometimes five days were necessary : it dissolves grain only when bruised ; hence, in the gallina ceous class, trituration and the gastric fluid in the gizzard, although Reaumur was of opinion it contained no menstru um, mutually assist each other ; the former, by breaking down the aliments in a mechanical way, prepares it for the latter, which penetrates it, destroys its texture, dissolves its particles, and dis poses them to change their nature and become animalized. Spallanzani thinks that this gastric fluid traced in the giz zard proceeds chiefly from the cesopha gas ; the chyme he found to be a semi fluid pultaceous mass, of a whitish yellow colour ; the transparency of this gastric juice, in a state of purity, is little inferior to water; it is fluid, and a little bitterish and saline ; it retains, out of the body, when warm, the power of dissolving ani mal and vegetable substances ; but it must be fresh, for if kept in vessels, par ticularly if open, it loses its efficacy ; it must nothave been used for experiments; and likewise a heat equal to that of the bird, is necessary, otherwise it has no more effect than water.
The ruminating animals of the third class, such as sheep, oxen, &c. very much resemble this class of birds in their man ner of digesting substances : in both, the gastric fluid requires an agent capable of breaking down the food, before it can dissolve it. The hay and grass, in the ruminating tribe, descend immediately into the first and second stomachs, in nearly the same state as when they first bronzed ; here they are softened by the juices, as seeds are in the craws of birds with gizzards ; but as the stomachs of these quadrupeds have no triturating power, and the aliment requires tritura tion, it ascends, in consequence of a gen tle stimulus to vomit, into, the cavity of the mouth, where, by means of rumina tion, it is put into the same state, previous to being digested by the gastric fluid, as happens to the food in the stomachs of granivorous fowls, after they have been properly triturated by the gastric mus cles.
Animals with intermediate stomachs, such as ravens, crows, herons, &c. have muscular stomachs, which are by no means equal in force to the stomachs of the first class, but much more so than those of the third class. These animals possess the privilege of returning sub stances they are incapable of digesting, at least every nine, and in general every two or three hours ; they are omnivorous. Their gastric juice does not dissolve whole seeds, they therefore bruise them with their beak and feet, and they are dissolved in twenty-four hours ; it soon dissolves flesh andcartilage, but not bone. The fluid in the Spallanzani found inconsiderable as a menstruum, when compared with that of the stomach, since the first was six hours in dissolving two parts of flesh, and the second one hour only in dissolving six parts ; conse quently, the cesophagal liquor in the craws of thegallinaceous is different from that in class. The resemblan ces between the gastric fluids of these two classes may be reduced to five : 1st. These fluids, besides being alike in co lour, are always salt and bitter, which bitterness proceeds from the bile regurgi tating through the pylorus into the sto mach. 2dly, They are the immediate agents of digestion, both in the muscular and intermediate stomachs, independently of .rituration. idly, The fluids act in the stomachs of these two classes of birds in the same manner, in the solution of the food; they first soften, and next convert the surface into a jelly, then produce the same effect on the intermediate parts, insinuating themselves gradually into its substance until it is completely dissolved. 4thly, They do not entirely lose their solvent power as soon as taken out of the stomach, provided they be heated to a proper degree. 5thly, The origins whence these fluids spring are nearly the same, viz. the follicular glands with which their organs abound. With respect to the dif ferences, they are in part reducible to the inferior efficacy of the gastric fluid in muscular, to that of the same fluid in in termediate stomachs. The gastric juice of the first-is incapable of dissolving the .; same aliment that the latter readily dis. solves ; likewise, the food, which each kind of gastric juice decomposes and di gests, is sooner subject to this change from that which belongs to intermediate. stomachs : hence, artificial digestion suc ceeds much sooner with the first than the second. The same inefficacy that the gastric juices of birds with muscular sto machs spews in the solution of aliments of a firm texture, extends also to their cesophagal juices in the solution of soft substances, notwithstanding the latter are tolerably well decomposed by the cesophagal juice of birds with interme diate stomachs. Another very striking difference is, the prodigious force of trituration in muscular stomachs, and the weakness of the other, which greatness of strength was necessary in birds whose food is of considerable firmness, as seeds.