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teeth, could be answered by an instrument which had none of these; could be supplied, and that with many additional advantages, by the hardness, and sharpness, and figure of the bills of birds.
Everything about the animal mouth is mechanical. The teeth of fish have their points turned backward, like the teeth of a wool or cotton card. The teeth of lobsters work one against another, like the sides of a pair of shears. In many insects, the mouth is converted into a pump or sucker, fitted at the end sometimes with a wimble, sometimes with a forceps; by which double provisions, viz. of the tube and the penetrating form of the point, the insect first bores through the integuments of its prey, and then extracts the juices. And, what is most extraordinary of all, one sort of mouth, as the occasion requires, shall be changed into another sort. The caterpillar could not live without teeth; in several species, the butterfly formed from it could not use them. The old teeth therefore, are cast off with the exuviæ of the grub; a new and totally different apparatus assumes their place in the fly. Amid these novelties of form, we sometimes forget that it is, all the while, the animal's mouth; that, whether it be lips, or teeth, or bill, or beak, or shears, or pump, it is the same 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 aliment, 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 æsophagus to the stomach, into which, by a gentle contraction, they easily transmit the descending morsels; that is to say, for the more laborious deglutition of animals, which thrust their food up instead of down, and also through a longer passage, a proportionably more powerful apparatus of muscles is provided; more powerful, rot 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 gene.
ral similitude we meet with appropriate differences. The valvulæ connivenles, or, as they are by some called, the semilunar valves, found in the human intestine, are want. ing 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 canai. 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.* [Pl. XXIII, fig. 2.] So far as depends upon the action of the part, this structure was more to be expected in a quadruped than in a man. 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 difficult; 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
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 stag, four feet high, measured ninety-six feet.† The intestines of a sheep, unravelled, measures thirty times the length of the body The intestines of a wild cat is only three times the length of the body. Universally, where the substance upon which the animal feeds is of slow concoction, or yields its chyle
* It may be questioned, whether these extremely soft rugæ or folds of the villous coat of the intestine can in the least retard the passage of the ford through its canal ; nor does the erect attitude of man require them ; for since there are as many of the convolutions of the intestines ascending as there are descending, the weight of the food can have no influence in the action of the intestine: it is certain, however, that this arrangement of the internal coat, affords a more extensive surface for the lacteals and secreting vessels ; and this appears to be the real use of the valvule conniventes.-Paxton.
+ Mem. of Acad. Paris, 1701, p. 170
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 unnecessary, 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 designmg 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 than 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 same, the diameter, it is evident, will be greater in a hollow bone than in 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 same weight would not have been so strong in any other form; and to have made it heavier, 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?
VÍ. The lungs also of birds, as compared with the lungs of quadrupeds, contain in them a provision, distinguishingly calculated for this same 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 animal bodies are,) and thus rendered buoyant.
VII. All birds are oviparous. This likewise 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 burden to a bird in its flight. The advantage, in this respect, of an oviparous procreation is, that whilst the whole brood are hatched together, the eggs are
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excluded singly, and at considerable intervals. Ten, fif. teen, or twenty young birds may be produced in one cletch or covey, yet the parent bird have never been encumbered by the load of more than one full-grown egg at one time.
VIII. A principal topic of comparison between animals, is in their instruments of motion. These come before us under three divisions; feet, wings, and fins. man to say, which of the three is best fitted for its use; or whether the same consummate art be not conspicuous in them all. The constitution of the elements in which the motion is to be performed, is very different. The animal action must necessarily follow that constitution. The Creator, therefore, if we might so speak, had to prepare for duľerent situations, for different difficulties; yet the purpose is accomplished not less successfully in one case than in the otner; and, as between wings and the corresponding limbs of quadrupeds, it is accomplished without deserting the general idea. The idea is modified, not deserted. Strip a wing of its feathers, and it bears no obscure resemblance to the fore leg of a quadruped. • The articulations at the shoulder and the cubitus are much alike; and, what is a closer circumstance, in both cases the upper part of the limb consists of a single bone, the lower part of two.
But, fitted up with its furniture of feathers and quills, it becomes a wonderful instrument, more artificial than its first appearance indicates, though that be very striking: at least, the use which the bird makes of its wings in flying is more complicated, and more curious, than is generally known. One thing is certain, that if the flapping of the wings in flight were no more than the reciprocal motion of the same surface in opposite directions, either upwards and downwards, or estimated in any oblique line, the bird would lose as much by one motion as she gained by another. The skylark could never ascend by such an action as this; for, though the stroke upon the air by the underside of her wing would carry her up, the stroke from the upper side, when she raised her wing again, would bring her down. In order, therefore, to account for the advantage which the bird derives from her wing, it is necessary to suppose that the surface of the wing, measured upon the same plane, is contracted whilst the wing is drawn up; and let out to its full expansion, when it descends upon the air for the purpose of moving the body by the reaction of that element. Now, the form and structure of. the wing, its external convexity, the disposition, and particularly the overlapping,
of its larger feathers, the action of the muscles, * and oints of the pinions, are all adapted to this alternate adjustment of its shape and dimensions. Such a twist, for instance, or semirotatory motion, is given to the great feathers of the wing, that they strike the air with their flat side, but rise from the stroke slantwise. The turning of the oar in rowing whilst the rower advances his hand for a new stroke, is a similar operation to that of the feather, and takes its name from the resemblance. I believe that this faculty is not found in the great feathers of the tail. This is the place also for observing, that the pinions are so set upon the body, as to bring down the wings, not vertically, but in a direction obliquely tending towards the tail; which motion, by virtue of the common resolution of forces, does two things at the same time; supports the body in the air, and carries it forward.
The steerage of a bird in its flight is effected partly by the wingut in a principal degree by the tail.. And herein we meet with a circumstance not a little remarkable. Birds with long legs have short tails, and in their flight place their legs close to their bodies, at the same time stretching them out backwards as far as they can. In this position the legs extend beyond the rump, and become the rudder; supplying that steerage which the taii could not.
From the wings of birds, the transition is easy to the fins of fish. They are both, to their respective tribes, the instruments of their motion; but in the work which they have to do, there is a considerable difference, four.ded on this circumstance. Fish, unlike birds, have very nearly the same specific gravity with the element in which they move. In the case of fish, therefore, there is little or no weight to bear up; what is wanted, is only an impulse sufficient to carry the body through a resisting medium, or to maintain the posture, or to support or restore the balance of the body, which is always the most unsteady where there is no weight to sink it. For these offices the fins are as large as necessary, though much smaller than wings,
* There are three powerful muscles (the fleshy part of the breast) called pectoral muscles, which, with other smaller on the bones of the wing which are analogous to the arm, press with vigor on the air, the elasticity of which gives support.
“ And it is remarkable that the general resemblance which the best form of windmill sails bears to the feathers of the wings of birds is striking, and one of those beautiful instances of truly mathematical principles on which the works of creation are constructed."--Paxton.