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But the species of bill which belongs to birds that live by suction, deserves to be described in its relation to that office. They are what naturalists call serrated or dentated bills; the inside of them, towards the edge, being thickly set with parallel or concentric rows of short, strong, sharp-pointed prickles. These, though they should be called teeth, are not for the purpose of mastication, like the teeth of quadrupeds : nor yet, as in fish, for the seizing and retaining of their prey ; but for a quite different use. They form a filter. The duck by means of them discusses the mud; examining with great accuracy
the puddle, the brake, every mixture which is likely to contain her food. The operation is thus carried on :--The liquid or semi-liquid substances, in which the animal has plunged her bill, she draws, by the action of her lungs, through the narrow interstices which lie between these teeth; catching, as the stream passes across her beak, whatever it pen to bring along with it, that proves agreeable to her choice, and easily dismissing all the rest. Now, suppose the purpose to have been, out of a mass of confused and heterogeneous substances, to separate for the use of the animal, or rather to enable the animal to separate for its own, those few par
ticles which suited its taste and digestion ; what more artificial, or more commodious, instrument of selection, could have been given to it, than this natural filter? It has been observed, also (what must enable the bird to choose and distinguish with greater acuteness, as well, probably, as what greatly increases its luxury), that the bills of this species are furnished with large nerves,—that they are covered with a skin, and that the nerves run down to the very extremity. In the curlew, woodcock, and snipe, there are three pairs of nerves, equal almost to the optic nerve in thickness, which
pass first along the roof of the mouth, and then along the upper chap down to the point of the bill, long as the bill is.
But to return to the train of our observations. The similitude between the bills of birds and the mouths of quadrupeds, is ex-" actly such, as, for the sake of the argument, might be wished for. It is near enough to show the continuation of the same plan: it is remote enough to exclude the supposition of the difference being produced by action or
A more prominent contour, or a wider gape, might be resolved into the effect of continued efforts, on the part of the species, to thrust out the mouth, or open it to the
stretch. But by what course of action, or exercise, or endeavour, shall we get rid of the lips, the gums, the teeth ; and acquire in the place of them, pincers of horn? By what habit shall we so completely change, not only the shape of the part, but the substance of which it is composed? The truth is, if we had seen no other than the mouths of quadrupeds, we should have thought no other could have been formed : little could we have supposed, that all the purposes of a mouth, furnished with lips, and armed with 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. Every thing 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 whimble, sometimes with a forceps ; by which double provision, 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, 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
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 tne æsophagus to the stomach, into which, by a gentle contraction, they easily transmit the descending morsels ; that is to say, for the more laborious deglu
tition 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, 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 valvula 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 plates 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