and motion. For whether this principle refides in the head, as in its citadel; or whether it has no fixed place, but is difperfed all over the body, according to the opinion of Xenocrates, a difciple of Plato *; it commands within us, and directs the whole. Now, our motions, as well as our fenfes, are twofold, internal and external: the internal have not only the vital parts, as the heart, lungs, ftomach, and intefines, fubject to them, but likewife all the nervous membranes.

Medical writers generally make a wide difference between the motions of the vitals, and those of the reft of the parts; imagining that the former, after they are once begun in the firft formation of the individual, neceffarily continue even against our will; but that the latter are performed at the difcretion of the mind, as circumftances require. But this is a falfe notion, into which they are deceived by obferving, that these natural actions continue without any fenfible intermiffion, even when we are little or not at all conscious of them, through the whole course of life: whereas, if the thing be narrowly looked into, it will manifeftly appear, that the reafon why thefe vital motions feem not to be governed by the mind, is, becaufe being inceffantly accuftomed to them, we perform them fo quick, and without attention, that we cannot eafily check and hinder them from performing their refpective offices. Something of this kind we experience, when, looking at the fun, or any body that strikes the eye too forcibly, we wink whether we will or not: and yet nobody doubts, but that those motions are effected by * See Lactantius de opificio Dei, cap. 16.

the

the direction of the mind. I could bring feveral other examples to confirm and illuftrate this fentiment; but to avoid being tedious, I chufe to refer the reader to a differtation of Dr Porterfield, who has handled this fubject with great perfpicuity *, as I have faid in another place †.

But this power of the mind is most remarkably perceptible in those fevers, which are called peftilential. For in these one may observe, that the mind hurries to the affiftance of the labouring frame, fights against the enemy, and, with the aid of the animal fpirits, excites new motions in the body, (though we may be infenfible of it), in order to expel the poison, which is blended with the humours, through every outlet. And hence very great physicians have defined a disease to be a struggle of nature in defence of the health of the individual.

In this manner it is, that care is taken of the whole machine, when in danger. But it it likewife neceffary fometimes to have a regard to fome particular part; nor does the mind fail of executing its office in this cafe. Thus, if a topical diforder has happened any where; to prevent the part from being overloaded and pained, nature has provided a paffage for the blood and humours by the adjacent veffels. This is effected by that admirable difpofition of the body, whereby the blood-veffels are interwoven and spread throughout every part; fo that the blood may pass, not only from vein to vein, but alfo from the fmall arteries into others. And this mechanism is peculiarly apparent where obftructions are moft to be feared;

See medical effays, published at Edinburgh, vol. 3. effay 12. and vol. 4. essay 14. † Difc. on the smallpox, vol. 2. pag. 112.

as

as in the head, abdomen, and the long winding ducts of the organs of generation

Now, fuch a construction of our frame is the more neceffary, becaufe, even when a difeafe is not in the cafe, the very actions and customs of the body fometimes require the humours to be conveyed with greater freedom through fome paffages than through others. Hence the fame blood veffels become wider or narrower in different perfons, according as their manner of living hath occafioned thefe veffels to be more or less dilated by the perpetual motions of the Aluids. Thus in perfons addicted to drinking, the arteries in the head, in people given to venery, thofe in the genitals, are bigger than in the fober and continent.

And this additional remark may not be improper here, that it is almoft impoffible, but that the very make of the animal parts, though extremely convenient for the purposes of life, must be attended with fome inconveniencies in particular cases; as in the fabric of the univerfe, thunder, ftorms, inundations, plagues, and other fuch calamities neceffarily happen fometimes in fome places. But as the divine Creator of the microcofm has given us natural means of guarding against these evils; fo he has afforded proper affiftance against those which affect our microcofm.

Geometricians have been long intent on contriving a machine, that may be endued with perpetual motion; but have conftantly loft their labour. For in handy-works of this kind fome portion of the moving power is loft every inftant, by reafon of the friction of the parts, whereby it requires to be perpetually renewed. Wherefore it is God alone who can

complete

complete fuch a machine; and was pleafed that our body should be a fabric of that fort, by difpofing all its powers in fuch a manner, that they fhould form a kind of circle, in which, at the fame time that they perform their refpective functions, they fhould conftantly and mutually repair each other.

Hence it manifeftly appears, that the animal machine is made, not by parts, but all together; fecing it is impoffible, that a circle of motions, fome of which depend on others, can be completed without all their inftruments being in their proper places. For example, how can the heart contract, to push the blood forward, without the affiftance of the animal. fpirits; or the fpirits be fecreted without the brain? And fo of all the other principal parts. Wherefore the animalcula, which by the help of microfcopes we discover swimming in the femen mafculinum, are really little men; which being received into the womb, are there cherished as in a neft, and grow in due time to a proper fize for exclufion. Therefore Hippocrates. faid very juftly, In the body there is no beginning, but all the parts are equally the beginning and end*.

To what has been hitherto faid let me add, that every animal machine is of fuch a nature, that there is a fort of infinity in its conftituent parts; by which expreffion I mean that their fibres are fo extremely fmall, that we cannot discover the ultimate ftamine, even by the affistance of the best microscopes. Had it been otherwife, aliment would not be conveyed to every individual part of the body, nor could the neceffary functions of life be performed.

* Ἐμοὶ δοκέει ἀρχὴ μὲν οὖν ἐδεμία είναι τα σώματα, ἀλλὰ πάντα ὁμοίως ἀρχὴ καὶ πάντα τελευτή. De locis in homine, initio.

VOL. III.

с

Wherefore,

Wherefore, upon the whole, health confifts in regular motions of the fluids, together with a proper state of the folids; and diseases are their aberrations: which as they are numberlefs, and one often produces another, it is next to a miracle, that the animal body fhould be able to hold out to extreme old age. And hence indeed plainly appears the extensiveness of the art of medicine, and how much fuperiour it is to all other arts.

Now, this machine, the only one that is endued with perpetual motion, was formed by the almighty geometrician to last a longer or shorter space of time, according to the different geniuses of living creatures. For a body, fuch as ours, cannot poffibly retain life for ever; which is not difficult to account for. Because the membranous fibres of the blood-veffels, which were made elastic, in order to drive their included juices forward, become gradually harder, and at length rigid; whence they are rendered incapable of executing their offices, and the fecretions of the feveral parts are diminished by degrees. And that the useless juices are not fufficiently carried off by perfpiration in old age, (a bufinefs very material to the continuance of life), manifeftly appears from diffections of the bodies of very old people; the infides of their arteries being fometimes found offified here and there, whereby they had almost entirely lost their springinefs; and the orifices of the natural ducts are often obferved to be quite cartilaginous.

In confirmation of this truth I fhall give two remarkable examples; the firft of which is taken from our own hiftory. Thomas Parr, a poor labouring man, was born in a healthful part of Shropshire,

where