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or beak, or fhears, or pump, it is the fame part diverfified and it is alfo remarkable, that under all the varieties of configuration with which we are acquainted, and which are very great, the organs of taste and smelling are fituated near each other.

III. To the mouth adjoins the gullet: in this part alfo, comparative anatomy difcovers a difference of fru&ture adapted to the different neceffities of the animal. In brutes, because the posture of their neck conduces little to the paffage of the aliments, the fibres of the gullet, which act in this business, run in two clofe fpiral lines, croffing each other: in men, thefe fibres run only a little obliquely from the upper end of the cefophagus to the ftomach, into which, by a gentle contraction, they easily tranfmit the descending morfels; that is to say, for the more laborious deglutition of animals, which thruft their food up inftead of down, and also through a longer paffage, a proportionably more powerful apparatus of mufcles is provided; more powerful, not merely by the ftrength of the fibres, which might be attributed to the greater exercife of their force, but in their collocation, which is a determinate circumftance, and must have been original.

IV. The

IV. The gullet leads to the inteftines: here, likewife, as before, comparing quadru peds with man, under a general fimilitude we meet with appropriate differences. The valvulæ conniventes, or, as they are by fome called, the femilunar valves, found in the human inteftine, are wanting in that of brutes. Thefe are wrinkles or plaits of the innermoft coat of the guts, the effect of which is to retard the progress of the food through the alimentary canal. It is eafy to understand how much more neceffary fuch a provision may be to the body of an animal of an erect posture, and in which, confequently, the weight of the food is added to the action of the inteftine, than in that of a quadruped, in which the course of the food, from its entrance to its exit, is nearly horizontal: but it is impoffible to affign any caufe, except the final cause, for this distinction actually taking place. So far as depends upon the action of the part, this ftructure was more to be expected in a quadruped than a man. In truth, it must, in both, have been formed, not by action, but in direct oppofition to action, and to preffure: but the oppofition, which would arise from preffure, is greater in the upright trunk than

in any other. That theory therefore is pointedly contradicted by the example before us. The ftructure is found, where its generation, according to the method by which the theorist would have it generated, is the most difficult; but (obferve) it is found, where its effect is most useful.

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The different length of the inteftines in carnivorous and herbivorous animals has been noticed on a former occafion. The shortest, I believe, is that of some birds of prey, in which the intestinal canal is little more than a ftraight paffage from the mouth to the vent. The longest is in the deer kind. The intestines of a Canadian ftag, four feet high, measured ninety-fix feet*. The inteftine of a sheep, unravelled, measures thirty times the length of the body. The inteftine of a wild cat is only three times the length of the body. Univerfally, where the substance upon which the animal feeds, is of flow concoction, or yields its chyle with more difficulty, there the paffage is cir cuitous and dilatory, that time and space may be allowed for the change and the abforption which are neceffary. Where the food is foon diffolved, or already half affimilated, an un

*Mem. of Acad. Paris, 1701, p. 170.

neceffary,

neceffary, or, perhaps, hurtful detention is avoided, by giving to it a shorter and a readier

route.

V. In comparing the bones of different animals, we are struck, in the bones of birds, with a propriety, which could only proceed from the wisdom of an intelligent and defigning Creator. In the bones of an animal which is to fly, the two qualities required, are ftrength and lightnefs. Wherein, therefore, do the bones of birds (I fpeak of the cylindrical bones) differ, in these respects, from the bones of quadrupeds? In three properties: first, their cavities are much larger in proportion to the weight of the bone, than in those of quadrupeds: fecondly, thefe cavities are empty: thirdly, the fhell is of a firmer texture, that is the substance of other bones. It is easy to obferve these particulars, even in picking the wing or leg of a chicken, Now, the weight being the fame, the diameter, it is evident, will be greater in a hollow bone than a folid one; and, with the diameter, as every mathematician can prove, is increased, cæteris paribus, the ftrength of the cylinder, or its resistance to breaking. In a word; a bone of the fame weight would not have been fo ftrong

in any other form; and, to have made it hea vier, would have incommoded the animal's flight. Yet this form could not be acquired by use, or the bone become hollow and tubular by exercise. What appetency could exca, vate a bone?

VI. The lungs alfo of birds, as compared with the lungs of quadrupeds, contain in them a provision, diftinguishingly calculated for this fame purpose of levitation; namely, a communication (not found in other kinds of animals) between the air-veffels of the lungs and the cavities of the body: fo that by the intromiffion of air from one to the other, at the will, as it should feem, of the animal, its body can be occasionally puffed out, and its tendency to defcend in the air, or its specific gravity, made lefs. The bodies of birds are blown up from their lungs, which no other animal bodies are; and thus rendered buoyant.

VII. All birds are oviparous. This, like wife, carries on the work of geftation, with as little increase as poffible of the weight of the body. A gravid uterus would have been a troublesome burthen to a bird in its flight. The advantage, in this respect, of an oviparous procreation is, that, whilft the whole brood

are

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