Pagina-afbeeldingen
PDF
ePub
[ocr errors][merged small][ocr errors][merged small][ocr errors]

I

BELIEVE that all the inftances which I fhall collect under this title, might, confiftently enough with technical language, have been placed under the head of Comparative Anatomy. But there appears to me an impropriety in the ufe which that term hath obtained: it being, in fome fort, abfurd, to call that a cafe of comparative anatomy, in which there is nothing to "compare;" in which a conformation is found in one animal, which hath nothing properly answering to it in another. Of this kind are the examples which I have to propose in the present chapter; and the reader will fee that, though fome of them be the ftrongest, perhaps, he will meet with under any divifion of our fubject, they must neceffarily be of an unconnected and miscellaneous nature. To difpofe them, however, into fome fort of order, we will notice, first, particularities of ftructure which belong to quadrupeds, birds, and fish, as fuch, or to many of the kinds

S 2

kinds included in these claffes of animals; and then, fuch particularities as are confined to one or two fpecies.

I. Along each side of the neck of large quadrupeds, runs a ftiff robuft cartilage, which butchers call the pax wax. No perfon can carve the upper end of a crop of beef without driving his knife against it. It is a tough, ftrong, tendinous fubftance, braced from the head to the middle of the back: its office is to affift in fupporting the weight of the head. It is a mechanical provision, of which this is the undifputed ufe; and it is fufficient, and not more than fufficient, for the purpose which it has to execute. The head of an ox or a horse is a heavy weight, acting at the end of a long lever, (confequently with a great purchase,) and in a direction nearly perpendicular to the joints of the fupporting neck. From fuch a force, fo advantageoufly applied, the bones of the neck would be in conftant danger of dif location, if they were not fortified by this ftrong tape. No fuch organ is found in the human fubject, becaufe, from the erect pofition of the head, (the preffure of it acting nearly in the direction of the spine,) the junc tion of the vertebræ appears to be fufficiently

fecure

fecure without it. The care of the Creator is seen where it is wanted. This cautionary expedient is limited to quadrupeds.

II. The oil with which birds prune their feathers, and the organ which supplies it, is a specific provision for the winged creation. On each fide of the rump of birds is obferved a fmall nipple, yielding upon preffure a butterlike fubftance, which the bird extracts by pinching the pap with its bill. With this oil or ointment, thus procured, the bird dreffes its coat; and repeats the action as often as its own fenfations teach it that it is in any part wanted, or as the excretion may be fufficient for the expense. The gland, the pap, the nature and quality of the excreted fubftance, the manner of obtaining it from its lodgment in the body, the application of it when obtained, form, collectively, an evidence of intention, which it is not eafy to withstand. Nothing fimilar to it is found in unfeathered animals. What blind conatus of nature should produce it in birds; should not produce it in beasts?

[ocr errors]

III. The air bladder also of a fish, affords a plain and direct inftance, not only of contrivance, but ftrictly of that fpecies of contrivance,

S3

1

trivance, which we denominate mechanical; It is a philofophical apparatus in the body of an animal. The principle of the contrivance is clear: the application of the principle is also clear. The ufe of the organ to fuftain, and, at will, also to elevate, the body of the fish in the water, is proved by obferving, what has been tried, that, when the bladder is burft, the fish grovels at the bottom; and also, that flounders, foles, fkates, which are without the air bladder, feldom rife in the water, and that with effort. The manner in which the purpofe is attained, and the fuitableness of the means to the end, are not difficult to be apprehended. The rifing and finking of a fish in water, fo far as it is independent of the ftroke of the fins and tail, can only be regulated by the fpecific gravity of the body. When the bladder, contained in the body of the fish, is contracted, which the fish probably poffeffes a mufcular power of doing, the bulk of the fish is contracted along with it; whereby, fince the abfolute weight remains the fame, the specific gravity, which is the finking force, is increafed, and the fifh defcends on the contrary, when, in confequence of the relaxation of the muscles, the elafticity

4

elafticity of the inclosed, and now compreffed air, restores the dimenfions of the bladder, the tendency downwards becomes proportionably lefs than it was before, or is turned into a contrary tendency. These are known properties of bodies immersed in a fluid. The enamelled figures, or little glafs bubbles, in a jar of water, are made to rife and fall by the fame artifice. A diving machine might be made to afcend and defcend upon the like principle; namely, by introducing into the inside of it an air veffel, which by its contraction would diminish, and by its diftenfion enlarge, the bulk of the machine itself, and thus render it fpecifically heavier, or specifically lighter, than the water which surrounds it. Suppofe this to be done; and the artist to folicit a patent for his invention. The infpectors of the model, whatever they might think of the use or value of the contrivance, could, by no poffibility, entertain a question in their minds, whether it were a contrivance or not. No reafon has ever been affigned, no reason can be affigned, why the conclufion is not as certain in the fish, as in the machine; why the argument is not as firm, in one cafe as the other.

« VorigeDoorgaan »