may

proach of their enemy from behind, when he may fteal upon them unseen. This is a critical diftinction; and is mechanical: but it be suggested, and, I think, not without probability, that it is the effect of continued habit.

The eyes of animals which follow their prey by night, as cats, owls, &c. poffefs a faculty, not given to those of other species, namely, of clofing the pupil entirely. The final caufe of which seems to be this. It was neceffary for such animals to be able to defcry objects with very fmall degrees of light. This capacity depended upon the fuperior fenfibility of the retina; that is, upon its being affected by the most feeble impulfes. But that tenderness of ftructure, which rendered the membrane thus exquifitely fenfible, rendered it also liable to be offended by the accefs of ftronger degrees of light. The contractile range therefore of the pupil is increased in these animals, so as to enable them to close the aperture entirely; which includes the power of diminishing it in every degree; whereby at all times fuch portions, and only fuch portions of light are admitted, as may be received without injury to the fense.

There appears to be alfo in the figure, and in fome properties of the pupil of the eye, an appropriate relation to the wants of different animals. In horfes, oxen, goats, sheep, the pupil of the eye is elliptical; the tranfverfe axis being horizontal: by which ftructure, although the eye be placed on the fide of the head, the anterior elongation of the pupil catches the forward rays, or those which come from objects immediately in front of the animal's face.

CHAP

CHAPTER XIII.

PECULIAR ORGANIZATIONS.

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 propofe in the prefent chapter; and the reader will fee that, though fome of them be the strongest, perhaps, he will meet with under any division 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, firft, particularities of structure which belong to quadrupeds, birds, and fish, as fuch, or to many of the kinds

S 2

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

1. Along each fide of the neck of large quadrupeds, runs a ftiff robuft cartilage, which butchers call the pax wax. No person 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 use; and it is fufficient, and not more than fufficient, for the purpose which it has to execute. The head of an ox or a horfe 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 advantageously 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, because, from the erect pofition of the head, (the preffure of it acting nearly in the direction of the spine,) the junction of the vertebræ appears to be fufficiently

fecure

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

II. The oil with which birds prune their feathers, and the organ which fupplies it, is a fpecific provifion 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 substance, 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; fhould not produce it in beafts?

III. The air bladder also of a fish, affords a plain and direct inftance, not only of contrivance, but ftrictly of that species of conS 3 trivance,