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or beak, or shears, or pump, it is the fame part diversified: and it is also 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 situated near each other.
III. To the mouth adjoins the gullet: in this part also, comparative anatomy discovers a difference of structure adapted to the different necessities of the animal. In brutes, because the posture of their neck conduces little to the passage of the aliments, the fibres of the gullet, which act in this business, run in two close spiral lines, crossing each other: in men, these fibres run only a little obliquely from the upper end of the esophagus to the stomach, into which, by a gentle contraction, they easily transmit the descending morsels; that is to say, for themore laborious deglutition of animals, which thrust their food up instead of dovon, and also through a longer passage, a proportionably more powerful apparatus of muscles is provided; more powerful, not merely by the strength of the fibres, which might be attributed to the greater exercise of their force, but in their collocation, which is a determinate circumstance, and must have been original.
IV. The gullet leads to the intestines: here, likewise, as before, comparing quadrupeds with man, under a general similitude we meet with appropriate differences. The valvulæ conniventes, or, as they are by some called, the semilunar valves, found in the human intestine, are wanting in that of brutes. These are wrinkles or plaits of the innermost coat of the guts, the effect of which is to retard the progress of the food through the alimentary canal. It is easy to understand how much more necessary such a provision may be to the body of an animal of an erect posture, and in which, consequently, the weight of the food is added to the action of the intestine, 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 impossible to assign any cause, except the final cause, for this distinction actually taking place. So far as depends upon the action of the part, this structure 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 opposition to action, and to pressure: but the opposition, which would arise from pressure, is greater in the upright trunk than in any other. That theory therefore is pointedly contradicted by the example before us. The structure is found, where its generation, according to the method by which the theorist would have it generated, is the most difficulty but (observe) it is found, where its effect is most useful.
The different length of the intestines in carnivorous and herbivorous animals has been noticed on a former occasion. The shortest, I believe, is that of some birds of prey, in which the intestinal canal is little more than a straight passage from the mouth to the vent. The longest is in the deer kind. The intestines, of a Canadian flag, four feet high, measured ninety-fix feet', The intestines of a sheep, unravelled, measures thirty times the length of the body. The intestine 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 passage is circuitous and dilatory, that time and space may be allowed for the change and the absorption which are necessary. Where the food is soon dissolved, or already half assimilated, an ua,-.
* Mem. of Acad. Paris, 1701, p. 170.
necessary. necessary, 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 designing Creator. In the bones of an animal which is to fly, the two qualities required, are strength and lightness. Wherein, therefore, do the bones of birds (I speak 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: secondly, these cavities are empty: thirdly, the shell is of a firmer texture, that is the substance of other bones. It is easy to observe 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 solid one; and, with the diameter, as every mathematician can prove, is increased, cæteris paribus, the strength of the cylinder, or its resistance to breaking. In a word; a bone of the fame weight would not have been so strong .% R 4 in 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 excavate a bone?
VI. The lungs also of birds, as compared with the lungs of quadrupeds, contain in them a provision, distinguishing! y calculated for this fame purpose of levitation; namely, a communication (not found in other kinds of animals) between the air-vessels of the lungs and the cavities of the body: so that by the intromission of air from one to the other, at the will, as it should seem, of the animal, its body can be occasionally puffed out, and its tendency to descend in the air, or its specific gravity, made less. The bodies of birds are blown up from their lungs, which no other ani,jnal bodies are; and thus rendered buoyant.
VII. All birds are oviparous. This, like, wise, carries on the work of gestation, with as little increase as possible 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, whilst the whole brood