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and meeting in the middle of its cavity, shut up the channel. Can any one doubt of contrivance here; or is it poffible to shut our eyes against the proof of it?

This valve also, is not more curious in its ftructure, than it is important in its office. Upon the play of the valve, even upon the proportioned length of the ftrings or fibres which check the afcent of the membranes, depends, as it should seem, nothing less than the life itself of the animal. We may here likewife repeat, what repeat,what we before observed concerning fome of the ligaments of the body, that they could not be formed by any action of the parts themselves. There are cases, in which, although good uses appear to arife from the shape or configuration of a part, yet that fhape and configuration itfelf may seem to be produced by the action of the part, or by the action or pressure of adjoining parts. Thus the bend, and the internal fmooth concavity of the ribs, may be attributed to the equal preffure of the foft bowels ; the particular shape of fome bones and joints, to the traction of the annexed mufcles, or to the position of contiguous mufcles. But valves could not be fo formed. Action and preffure are all against them. The blood, in its proper

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courfe, has no tendency to produce fuch things; and, in its improper or reflected current, has a tendency to prevent their production. Whilst we fee, therefore, the ufe and neceffity of this machinery, we can look to no other account of its origin or formation than the intending mind of a Creator. Nor can we without admiration reflect, that fuch thin membranes, fuch weak and tender instruments, as these valves are, fhould be able to hold out for seventy or eighty years.

Here also we cannot confider but with gratitude, how happy. it is that our vital motions are involuntary. We fhould have enough to do, if we had to keep our hearts beating, and our ftomachs at work. Did these things depend, we will not fay upon our effort, but upon our bidding, our care, or our attention, they would leave us leifure for nothing else. We must have been continually upon the watch, and continually in fear: nor would this conftitution have allowed of fleep.

It might perhaps be expected, that an organ fo precious, of fuch central and primary importance, as the heart is, fhould be defended by a cafe. The fact is, that a membranous purfe or bag, made of ftrong tough materials,

is provided for it; holding the heart within its cavity; fitting loofely and easily about it; guarding its fubftance, without confining its motion; and containing likewise a spoonful or two of water, just fufficient to keep the furface of the heart in a state of fuppleness and moisture. How fhould fuch a loose covering be generated by the action of the heart? Does not the inclosing of it in a fac, answering no other purpose but that inclosure, fhew the care that has been taken of its preservation ?

ONE USE of the circulation of the blood (probably amongst other ufes) is to distribute nourishment to the different parts of the body. How minute and multiplied the ramifications of the blood-veffels, for that purpose, are; and how thickly fpread, over at least the fuperficies of the body, is proved by the single obfervation, that we cannot prick the point of a pin into the flesh, without drawing blood, i. e. without finding a blood-veffel. Nor, internally, is their diffufion lefs univerfal. Bloodveffels run along the furface of membranes, pervade the fubftance of muscles, penetrate the bones. Even into every tooth, we trace, through a fmall hole in the root, an artery to feed the bone, as well as a vein to bring back

the fpare blood from it; both which, with the addition of an accompanying nerve, form a thread only a little thicker than a horsehair.

WHEREFORE, when the nourishment taken in at the mouth, has once reached, and mixed itself with, the blood, every part of the body is in the way of being fupplied with it. And this introduces another grand topic, namely, the manner in which the aliment gets into the blood; which is a fubject diftinct from the, preceding, and brings us to the confideration of another entire fyftem of veffels.

II. For this neceffary part of the animal œconomy, an apparatus is provided, in a great measure, capable of being, what anatomists call, demonftrated, that is, fhewn in the dead body; and a line or course of conveyance, which we can purfue by our examinations.

First, the food defcends by wide paffages into the intestines, undergoing two great preparations on its way, one, in the mouth by mastication and moisture, (can it be doubted with what design the teeth were placed in the road to the ftomach, or that there was choice in fixing them in this fituation ?) the other, by digestion in the ftomach itself. Of this laft furprising

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furprising diffolution I fay nothing; because it is chymistry, and I am endeavouring to difplay mechanifm. The figure and pofition of the ftomach (I speak all along with a reference to the human organ) are calculated for detaining the food long enough for the action of its digeftive juice. It has the fhape of the pouch of a bagpipe; lies across the body; and the pylorus, or paffage by which the food leaves it, is fomewhat higher in the body, than the cardia or orifice by which it enters; fo that it is by the contraction of the muscular coat of the ftomach, that the contents, after having undergone the application of the gastric menftruum, are gradually preffed out. In dogs and cats, this action of the coats of the ftomach has been difplayed to the eye. It is a flow and gentle undulation, propagated from one orifice of the ftomach to the other. For the fame reason that I omitted, for the prefent, offering any obfervation upon the digestive fluid, I fhall fay nothing concerning the bile or the pancreatic juice, further than to obferve upon the mechanifm, viz. that from the glands in which these fecretions are elaborated, pipes are laid into the firft of the inteftines, through which pipes the product of each gland flows

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