Animals with membranous stomachs, such as frogs, newts, snakes, fishes, rumi nating animals ; carnivorous birds and beasts, as the eagle, falcon, man, dog, cat, &c. this class is infinitely more numerous than the two former: it comprehends nearly all the quadrupeds, fishes, reptiles, birds of prey, and the greater part of in sects.
From Spallanzani's experiments it ap pears, that carnivorous birds do not dis solve vegetables, and throw up the indi gestible part every twenty-four hours ; that nature in these animals, whose diges tion depends on the gastric juice alone, without any previous mastication or tri turation, has provided them with a much larger quantity of it than the other classes; that digestion is in proportion to the quantity of this fluid ; that the gastric juice of the ruminating class has no effect in dissolving plants, unless they have been previously macerated, and ground by the teeth ; that its colour in sheep is green, and yellow in cows ; that owls digest flesh and bones, but not grain ; that their gastric juice evaporates sooner than wa. ter ; that that of the eagle dissolves bread and bone ; its colour is cineritious, and it digests animal and vegetable mat ters out of the body ; that a wood-pigeon may be brought by degrees to live on flesh ; that the owl and falcon do not di gest bread; that the gastric juice of the dog dissolves the enamal of the teeth, ; and that trituration is necessary in the ruminating order and man, which is produced by the teeth, as in gallinaceous fowls by the gizzard ; but in other ani mals, as in the, frog, newt, serpents, and birds of prey, trituration does not contri bute to digestion : hence, in every order of animals, the gastric juice is the princi. , pal cause of digestion, and it agrees in every class in many properties, and dif fers in others. In the frog, the newt, scaly fishes, and other cold animals, it produces digestion in a temperature near ly equal to that of the atmosphere. In warm animals it is incapable of of dissolv ing the aliment in a degree of heat lower than that of these animals. In warm ani mals the food is digested in a few hours, whereas in the opposite kind it requires several days, and even weeks, particular ly in serpents; likewise, the gastric juice of the gallinaceous class can only dissolve bodies of a soft and yielding texture, and previously triturated ; whilst, in others, as serpents, the heron, birds of prey, and the dog, it decomposes substances of great tenacity, as ligaments and tendons ; and even of considerable hardness, as the most compact bone : man belongs to this class, but his gastric juice seems to have no action on the hardest kind of bones. Some species likewise are incapable of digesting vegetables, as birds of prey ; but man, the dog, cat, crows, &c. dis solve the individuals of both kingdoms alike, and are omnivorous, and in general their gastric juices produce these effects out of the body : hence, the dissolving power of this fluid depends on the dif ference of the nourishment, and by some authors it has been said to be the cause of hunger, and of the difference in the choice of the particular aliment; by which pow er the carnivorous only enjoy flesh ; the granivorous and ruminating, only vegeta ble aliments, and no flesh ; but man and ,the omnivorous, both vegetable and ani mal substances. It is, however, asserted
by Carradori, as decided, that nocturnal birds of prey are capable of digesting ve getables: it results from his experiments, that they also support themselves very well with this nourishment, in spite of their repugnance to it. If this be the case, the opinion is erroneous, that the gastric juice of these birds has only an affinity with animal substances ; and what he has established, viz. that carnivorous animals find a nutriment ih the products of plants, was already rendered probable ' by the discovery of Fourcroy, of the ex istence of gluten, albumen, and gelatin, in the tribes. Spallanzani, how , ever, proves the insufficiency of Cana dori's experinients, as the owl died when confined to vegetable food. The time, moreover, requisite for digestion is dif ferent in different animals; in many it does not exceed five or six hours, and in some it is much shorter.
From the numerous experiments of Gosse of Geneva upon digestion, and the action which the gastric juice Las upon different substances, great light has been afforded us upon this interesting subject. He informs us, that in about one hour and a half after the food is taken into the stomach, it is changed into a pultaceous mass; the gastric juice, likewise, renders it fluid without altering its nature ; and when digestion is properly carried on, there is no appearance of acidity or alka lescence ; the food does not ferment, and the process of digestion is not completed, until the space of between two and three hours has elapsed. The chyme which arises from aliments taken either from the animal or vegetable kingdom is the same; they both are by the gastric fluid convert ed into the same substance, which is in consequence, most probably ,of their both containing gelatin, &c.: if, however, the digesting solvent is not in sufficient quan tity, or is in a diseased state, the acetous fermentation will take place in vegetable, and the putrid in animal matter ; hence milk, vegetable matter containing sugar, wine, and even spirits, will degenerate, when left to their spontaneous changes in the stomach, to a very strong acid, and sooner, sometimes, than out of the body, perhaps from the heat, &c. ; all oily sub stances likewise become rancid, and flesh meat putrid, producing acid and putrid eructations, which is never the case in a state ofhealthy digestion ; whilst, in many animals, the digestion is finished before the acetous or putrid fermentation can begin.